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Electrical grounding

Electrical grounding is a very misunderstood topic.  Here’s a quick description of the major facts about electrical grounding.

There are two distinct grounding systems in your house.  First we have the grounding electrode system.  This is the system that connects one half of the electrical system directly to the ground.  We call this have of the system the “neutral” wire.

Grounding Electrode System

In the USA we use a grounded electrical system.  This means that what we call the neutral wire is physically connected to the ground at various points in the system.  It’s grounded at the utility transformer, at the electric meter on the side of your hose (through a buried ground rod), and at the metal water pipe coming into your house from the street.

Not all countries use a grounded system like this.  But since 1913 our National Electrical Code has required this.  It helps to protect us from high-voltage crossover problems at the transformer, and nearby lighting strikes.  It helps to discharge static electricity, and it provides protection in case the service neutral wire coming to the house from the utility ever gets broken.

A grounding electrode is a piece of metal that creates this direct connection to the ground.  In a typical house the grounding electrodes consist of the buried metal water pipe and a ground rod driven into the ground near the electric meter.

Any wire that helps to create this direct connection to the earth is called a grounding electrode conductor.  You should have such a grounding electrode conductor going from your main electrical panel over to where the metal water pipe enters your house from the street.

So the grounding electrode system connects much of the metal in your house together with the neutral wires and connects it to the ground.  Basically it helps to protect us from electricity that originates outside of the house – like lighting or transformer problems.

Equipment Grounding System

Equipment grounding is a very different part of the electrical system, although they are connected together.  When you plug in a device with a 3-prong plug that third prong is providing equipment grounding.  Equipment grounding is intended to protect us from electricity that comes from the appliances we use.  And the most important thing to understand about equipment grounding is that it has nothing to do with the ground, or the earth, or the dirt beneath our feet.

So the ground rod or metal water pipes mentioned as part of the Grounding Electrode System has nothing to do with equipment grounding.  The National Electrical Code (NEC) forbids using the earth as an equipment grounding path. NEC 250.4(A)(5) “. . . . The earth shall not be considered as an effective ground-fault current path.”

Here’s a schematic showing how electricity is wired from the utility transformer into an appliance in your house.

Electricity wants to complete a path, so in the image above electricity wants to complete the circuit from point A to point B (in the utility transformer, where the neutral is grounded through a driven copper ground rod).  The schematic below shows what happens when we close the switch (just to the right of the appliance in the schematic above) and turn on the appliance.  You can follow the electrical path — follow the little red lightning bolts.  Electrical current starts at Point A and goes into the house through the meter (where the neutral is connected to the ground through a driven copper ground rod), into the electrical panel (where the neutral is connected to the ground through metal water pipes), through a main breaker and a branch circuit breaker, and out to the appliance. The current goes through the appliance load and then returns through the neutral wire out to Point B at the transformer.

This is the way things were before about 1960, when there was no equipment ground.

Eventually we learned that an effective equipment grounding system could help to protect people, so we instituted that requirement. Here’s how we did NOT do that. This is a schematic showing a ground connection from the appliance case directly into the ground.

Now suppose that the hot wire has some torn insulation and the wire makes contact with the metal case of the appliance. That’s a ground fault. Now the case of the appliance is at 120 volts to ground and if you touch it you could be shocked. Where is the electrical current going? Remember that electricity wants to make a circuit, so the current wants to go back to point B in the utility transformer.  Follow the little red lightning bolts again to see the path that electrical current takes.

There are two paths for current to return to the transformer. One is through your hand, through your heart (that’s not good!), into the ground, and then over to the ground rod at the utility transformer. Another path is through the appliance’s ground rod and over to the utility transformer. Both paths have a lot of resistance, so neither will be favored much and current will flow through both paths. (When talking about alternating current the proper term is impedance, not resistance. But I’m going to keep using the term resistance.) So a lot of current will flow through you, certainly enough to kill you. But the resistance of the earth is too high so not enough current will flow to trip off the breaker. So you’re standing here as part of an electrical path that’s sending current through your body. Terrible.

Here’s another schematic showing the proper equipment grounding configuration. There’s simply a ground wire going from the appliance case back to the neutral bus bar in the main electrical panel.

Now if there’s a ground fault and you touch the appliance case, there are again two paths for current to take to get back to the utility transformer. One is through you and into the ground and back to the transformer. That’s a lot of resistance.

But another path is through the ground wire, back to the electrical panel, and then back out to the transformer through the service neutral. And this path has no load at all on it, so it has very low resistance. With low resistance this path will take most of the current — a lot of current. Electrical current favors the path of least resistance. Enough current will flow through the ground wire to quickly trip off the branch circuit breaker. And that’s what you really want — you want the breaker to trip off and de-energize the appliance.

This is the way that equipment grounding keeps us safe in the U.S.

Sediment Trap

A sediment trap is a short piece of gas pipe installed near an appliance that helps to collect debris in the gas line.  The idea is that any debris in the gas line will collect in the sediment trap before that debris can foul up the appliance’s gas valve and cause trouble.

The sediment trap on my water heater is nice and long.

You’ll often hear the term “drip leg” instead of sediment trap.  In fact that’s what I usually call it, but it’s not really the same thing.

A sediment trap is just a T fitting in the gas pipe with a capped pipe nipple at the bottom of the T.  It’s required at a furnace, boiler, and water heater, but not at a clothes dryer, a kitchen range, or a fireplace.

The sediment trap needs to be installed downstream of the appliance’s shutoff valve and as close to the appliance as practical.  And it needs to be installed so that the gas flow changes direction, so that the debris will fall into the trap.  If you install it so that the gas flows horizontally then the debris might just jump over the trap and get into the gas valve.

There’s no excuse for the lack of a sediment trap.  A competent plumber knows it’s required by all relevant codes and by the appliance manufacturer’s instructions.  And you don’t want an incompetent plumber installing your gas pipes.  So if it’s missing that’s one small strike against the plumbing system as we work on telling the story of the house.

But how bad is this problem, really?  Well, I’ve heard that this might have been an issue many years ago, but today the natural gas delivered to your house generally is clean and dry.  And I’ve never personally heard of a problem that occurred because of a missing sediment trap.

So have a sediment trap installed the next time the appliance is replaced.  In the meantime it’ll just be one of the many things in your house that’s not perfect.

Reversed Polarity Electrical Receptacles

A typical 120-volt electrical circuit has a hot wire and a neutral wire.  The hot wire has 120 volts to ground and so it can easily shock you.  But the neutral wire is physically connected to the ground, so it shouldn’t have any voltage present.  This makes it a lot safer — but there are plenty of things that can go wrong, so please don’t ever touch any wire.

Electrical devices are designed to take advantage of this difference between the hot and neutral wires.  Any appliance that has a switch is going to be safer if, when the switch is off, there’s no voltage inside the appliance.

The way to be sure that there’s no voltage inside the appliance when the switch is off is to put the switch on the hot wire.  So when the switch is off only the neutral wire is still connected back to the electrical panel, and since the neutral wire is connected to the ground there’s no voltage difference to push electrical current.

Here’s a simple schematic of an appliance showing the issue.  The switch is on the hot side of the appliance, so that when it’s switched off, the load is only connected to the neutral wire, which is connected to the ground and so it should have zero electrical potential.  The appliance is much safer this way.

A typical appliance plugged into a receptacle with correct polarity. When the switch is off there are no live electrical connections in the appliance.

One blade of the plug and one slot of the receptacle are wider (this is the neutral side), so that the plug can only be inserted with one orientation. This ensures proper polarization — as long as the receptacle is wired correctly.

To make sure this happens, we use polarized plugs and polarized receptacles.  Plugs on devices that need to be polarized have one blade wider than the other.  And all receptacles have one slot wider than the other, so that the plug can only physically be inserted in one way.  This makes sure that the appliance sees the proper polarity.

Of course the only way this works is if the wires feeding into the electrical receptacle are connected in the proper way, with the hot wire attached to its proper location on the receptacle (the narrow slot), and the neutral wire connected to its proper location (the wide slot).  If the receptacle is wired backwards, then we have a condition known as reversed polarity.

Here’s our appliance schematic showing what happens if it’s plugged into a receptacle with reversed polarity.  Now even when the switch is off the load inside the appliance is connected to the hot side of the wiring and all the electrical components are live all the time.  This dramatically increases the chance of electrical shock.

An appliance plugged into a receptacle with reversed polarity. Even when the switch is off all of the components of the appliance are energized.

One very important example of this situation is a lamp.  Consider the lamp holder — where you screw in the light bulb.  You want the threads of the shell (green arrow in picture below) to always be on the neutral side, because it’s fairly easy to touch this accidentally. You want the button at the bottom (red arrow) to always be on the hot side, because it’s hard to touch it.  If a lamp is plugged into a receptacle with reversed polarity then the shell of the lamp holder is always going to be live, even when the lamp is switched off, and so it’s easy to be shocked when you change the bulb.

If a receptacle has reversed polarity it’s probably just because the wires at the receptacle are reversed.  This is an easy fix.  But it might be the case that the reversal was done somewhere farther back upstream.  An electrician needs to find the source of this problem.

Decks built sideways

Decks are one of the most common do-it-yourself projects.  Unfortunately many deck builders fall into the category of shouldn’t-do-it-themselves.

It’s easy to hammer a few boards together, but it’s a lot harder to do with the best techniques, tools, equipment, and devices so that the deck has the best chance of safely lasting many decades.  Almost every deck that I see has some problem, and many are built so poorly that I advise my client to just tear it down and rebuild.

A deck has many details to inspect, but there’s one deck problem in particular that drives me crazy.  And it seriously weakens the deck structure so it’s worth looking at more closely.

Here’s the typical way to build a standard deck:

A ledger board is securely attached to the house, with bolts staggered up and down and spaced no more than about every 8 inches.  Then the floor joists are installed perpendicular to the house and ledger board, and they’re attached to the ledger using joist hangers.  At the far end the floor joists sit on a beam that’s supported on both ends by posts that sit on concrete footings.

Now let’s take a look at the load path takes to the ground.  When you’re standing on the deck, your weight is shared among several joists (but we’ll look at just one joist in this example), and that load (green lines) moves horizontally out to the beam and in to the ledger board.  When that load hits the ledger board it only has to move a very short distance horizontally before it hits the very secure connection of the ledger board to the house structure.  At least we hope that connection is secure, but that’s a topic for another day.

When your load hits the beam it then might have to move quite a long distance horizontally before it gets to the posts and into the ground.  Because the load might have to move a long distance horizontally the beam needs to be doubled – it needs to be made up of 2 pieces of 2x lumber.

Everything’s fine with this design.

But here’s how some decks are built.

 

There’s a ledger against the house, but then there are boards at both ends of the ledger coming out perpendicular to the house.  These boards are usually not attached very well to the ledger board — that’s the problem.

But now all the floor joists are attached to these side boards.  Now let’s take a look at the load path.  When you stand on the deck the load is shared among several joists, transferred horizontally through the joists over to the side board, and then horizontally again over to the ledger at the house and over to the post at the corner of the deck.  But because the load is moving a long way horizontally these side boards should be doubled – they should each be made up of 2 pieces of 2x lumber.  Otherwise they’re not strong enough to carry all that load.  (Remember that the several floor joists share the load so they’re OK as single pieces of lumber.)

But when the load gets to the ledger board, the only thing holding this deck up is the single connection between the side board and the ledger (red oval in the figure).  Often times this connection is only a bunch of nails.  Now think about this.  About half of the entire load on this deck – and that might be a whole deck-full of people if you’re having a party – is supported only by this bunch of nails.  And then suppose that this deck is 10 or 20 years old, and it’s been subject to rain, sun, hot, cold, over and over and over.  So that connection isn’t nearly as strong as it was when originally built.

This connection just isn’t strong enough, and this is the wrong way to build a deck.

Water, Water Everywhere

“Water, water, everywhere” goes the old saying.  And that might be a good thing for a sailor, but not so much for a homeowner.

I tell all my clients that water is their home’s number one enemy. If you keep your house dry, it’ll last for a long long time.  If it gets wet, it goes downhill fast.

Of course, I’ve never met anybody who’d be happy going outside to the well every time they wanted a drink of water or to take a shower.  So we bring water pipes into our houses.  And we don’t (at least none of my clients) live in the desert, so rain water (snow too!) will fall on and around our houses.  We can’t stop water, we just have to control it.

Too much water in the attic. Notice that only the roof sheathing is affected, and not the rafters. That’s because the sheathing is the coldest part of the attic, so that’s where the water vapor condenses.

It’s important to keep in mind that water comes in different forms.  We have to control liquid water of course, but we also have to control water vapor.  And that’s usually the hard part.

When I see mold in an attic I know that there’s too much water in the attic.  The problem might be a roof leak – liquid water getting in.  But the problem might also be water vapor coming up from a wet basement or crawl space.  I saw quite a few cases of moldy attics last year and in all but one case the culprit was pretty clearly a wet crawlspace.

 

When the basement or crawlspace is wet that moisture easily evaporates into the air.  And that moist air diffuses throughout the entire house.  Water vapor wants to move from areas of high concentration and warm temperature to areas of lower concentration and colder temperatures.  The coldest and driest part of most houses is the attic (at least in the winter) and so the water vapor is going to be driven into the attic by natural forces.

Water in the crawlspace eventually ends up in the attic. Count on it.

Many people seem surprised and even a little dubious when I tell them that a wet crawlspace is the source of their attic moisture and mold problems.  But consider that in a typical house in this area you can expect anywhere from about 0.3 up to 0.7 natural air changes per hour.  This means that the total of cold air leaking in and warm air leaking out of your house will equal somewhere between about 0.3 and 0.7 times the size of your house.  So in about 1.5 hours up to maybe 3 hours the air in your house completely changes over.  Air moves, pretty fast.  And the water vapor in the air diffuses even faster than that.

So in short order that water vapor finds its way up into the attic.  When it hits the cold surface of the roof sheathing the water vapor condenses, and you get just the right conditions for mold to grow.

There are three general ways to control water: source control, ventilation, and dehumidification.  Source control involves stopping water from getting into the attic in the first place, and so stopping it from getting into the crawlspace.  Or we can ventilate the attic to bring in dry outdoor air and remove the moist air.  Or we can install a dehumidifier in the attic to remove moisture and drain it away.

Ventilation is important in an attic, and most home inspectors make a deal out of a lack of attic ventilation.  But I’ve seen so many attics with bad or no ventilation that had no problems.  And really, why let water vapor get into the attic in the first place and then be forced to deal with it.  Why not keep it out in the first place?

By far the best option is source control.  It’s always best to avoid the problem completely if you can.  Source control is the easiest, most dependable, and most robust solution.  Keep water out of the attic – and that means keeping water out of the crawlspace.

The way to keep your crawlspace dry is for another post, but here are the highlights.  There should be a vapor barrier on the ground to keep water from coming up through the ground.  (The ground is very wet – that’s why trees have their roots there!)  The gutters and downspouts should be in good condition to move rain water away from the house.  And the ground around the house should be sloped away so that rain water flows away from the house.

Stay dry my friends.  Your house will appreciate it.

 

Water, water, every where,

And all the boards did shrink;

Water, water, every where,

Nor any drop to drink.

“The Rime of the Ancient Mariner”  by Samuel Taylor Coleridge

Galvanized pipes

Galvanized steel pipes were used for potable water distribution in single family homes as well as in apartment and condominium buildings until about the 1960’s.  It was the standard water pipe material; everybody used it pretty much exclusively.  They’re joined together with threaded fittings.

Galvanized steel pipes are just steel with a zinc coating for protection.  The zinc coating is the “galvanized” part.  But over time the zinc coating wears away, and you’re down to bare steel.  And of course steel rusts.

galv-pipe-rust

The inside of this galvanized steel water pipe is corroded.

The pipes will rust on the inside (obviously, since that’s where the water is), and this corrosion tends to clog and choke down the inside diameter of the pipe.  With the smaller inside diameter you’ll get less water flow and lower pressure at your fixtures.  Hot water pipes tend to corrode more quickly, because the heat accelerates the corrosion process.

 

Reduced water flow and pressure is the predominant problem, but the pipes can also rust through and start to leak.  And of course leaking water pipes can cause a tremendous amount of damage, especially if the leak is inside a wall.  The threads typically rust first, so this is where you want to start looking if you’re going to examine your galvanized pipes.

dscf8594

This pipe rusted all the way through and had to be repaired with a clamp.

 

 

 

 

 

 

 

 

The life expectancy of galvanized pipes varies – a lot.  I’ve seen galvanized steel water pipes in houses from the 1920’s and even the 1910’s that are in surprisingly good condition.  The water flow through these pipes is good, and there’s little sign of rusting.  Folks knew how to make pipes back then.  Still, these pipes are very old, certainly have some rust, and are probably near the end of their expected life.  And I’ve seen galvanized pipes from this era that are badly rusted and provide terrible pressure.  You should plan on needing to replace all galvanized pipes in the near future.  Also, most houses of this vintage have had all (or almost all) of their galvanized pipes replaced already.  So if your house still has galvanized pipes then it’s behind the curve in terms of being updated like it needs to be – something to keep in mind when making an offer.

dscf5829

This pipe fitting is rusted through and leaking, just a little bit. The leak will just get worse and worse, until . . . disaster.

 

On the other hand, some Chicago buildings from the 1950’s and 1960’s have already had to replace their galvanized pipes.  It seems that cheaper production methods and materials created an inferior product that quickly deteriorated.  And I’ve seen pretty much everything between these two extremes.  Either way, if you have galvanized steel water pipes in your house it’s probably near the end of its expected lifespan, and you should plan on needing to replace it in the near future.

Keep in mind that there’s no way to know for sure when galvanized pipes will fail.  You can have a team of scientists and engineers inside your house for a month, and you still can’t know.  You just have to follow the basic guidelines.

In Chicagoland, almost all jurisdictions require new water pipes to be copper.  But some cities will allow PEX (cross linked polyethylene) water pipes.  PEX is used almost exclusively throughout much of the rest of the country, so it has a long record of success, and the problems with fittings (the weak link in any water system) have been worked out by others.

Combustion air

I’m sure that we’ve all seen the elementary school experiment where there’s a lighted candle inside a jar, and a lid is put on.  After just a few moments the candle flame is extinguished.  Without oxygen the candle can’t burn.

Your appliances that burn natural gas (usually just called “gas”) are just the same.  They need oxygen to burn, and they get that oxygen from the air.  If there’s not enough air, then there’s not enough oxygen – and bad things will happen.

The air that your gas furnace (and water heater, and boiler) needs in order to burn safely is called combustion air.  In this post we’ll discuss combustion air, including what happens if we don’t have enough and what the requirements are for “enough.”

There are two basic ways for your furnace to get its combustion air.  Most high efficiency furnaces get their combustion air directly from the outside.  This is the best and most efficient way to do it.  If your furnace has two PVC pipes going to the outside, then one of those pipes is taking the flue gases out and one is bringing fresh air in for combustion.  This is called a direct vent furnace.  You have nothing to worry about in terms of combustion air for this furnace, and this post won’t deal with direct vent furnaces.

On the other hand, standard efficiency furnaces typically have a metal flue, and they get their combustion air from inside the house.  This post deals with this type of furnace.

When your furnace is running it’s pulling combustion air out of the room and through the flame, and then exhausting it out of the house through the flue.  These flue gases contain lots of bad things including carbon monoxide, which is deadly.  So of course it’s important that the flue gases be sent to the outside.

So while the furnace is running air is constantly being sucked out of the house and sent up the flue to the outside.  This is basically a big exhaust fan, pulling air out of the house.  Figure 1.

comb air fig 1 b

Figure 1: The gas appliances are pulling air out of the house for combustion, and sending those flue gases out of the house.

 

 

 

 

 

 

 

 

 

 

 

 

If the gas appliance is in a small closed space – a confined space – then the air pressure in that space can drop when air from the room is sucked out for combustion and sent up the flue.  Figure 2.

comb air fig 2 a

Figure 2: As the appliances suck air out of the house, the house comes under negative pressure with respect to the outside.

 

 

 

 

 

 

 

 

 

 

 

Imagine putting a straw into an empty plastic soda bottle.  If you simultaneously seal up the top around the straw then you can suck the air out and the bottle will even collapse because the air pressure inside is so much lower.  Same idea with your furnace.  It’s sucking air out of the room and lowering the air pressure in that room.

With standard efficiency appliances, flue gases rise up the flue and out of the house because they’re warmer than the surrounding air.  Warm air rises.  But if the air pressure in the furnace room is lower than the air pressure outside then those flue gases can be sucked back into the house instead.  This is called back drafting.  Remember, those flue gases are unsafe and are likely to contain carbon monoxide.  Figure 3.

comb air fig 3 a

Figure 3: When the room with the furnace comes under negative pressure the flue gases are sucked back into the house instead of going out.

 

 

 

 

 

 

 

 

 

 

 

So the flue gases can’t rise up and out of the house, because they are being pulled back into the house by the drop in pressure.  And the drop in pressure is caused by the furnace pulling combustion air out of the room and sending it up the flue.

So how do we prevent this problem?  That’s easy: we provide lots of combustion air to the gas appliances.  If the furnace is in a small room, then we need ventilation openings in the walls or door of that room to let in plenty of air to make up for the air that the furnace is using.  If we provide plenty of air to the room then the air pressure in the room won’t drop and the furnace will be able to send the flue gases out of the house.  It’s just like the plastic soda bottle with a straw.  If you don’t seal up the top around the straw then plenty of air can get back into the bottle to make up for all the air you’re sucking out.  In that case there is no drop in air pressure and you can’t get the bottle to collapse.

The size of the ventilation openings we need is a fairly straight forward problem, because requirements for combustion air are specific, and they are uniform across all the codes and manufacturer’s requirements that I’ve ever seen.

The first thing to determine is if the room with the gas appliance is big enough to supply all the combustion air.  If it’s not big enough then it’s called a confined space and we have to provide ventilation openings for more combustion air.

To determine if a room is a confined space you add up the gas input ratings of all the gas appliances in the space, measured in BTU’s per hour (BTU/hr).  (A BTU is a British Thermal Unit, and is a standard measure of the energy contained in natural gas.)  Then you calculate the volume of the appliance space.  If the volume of the space is less than 50 cubic feet per 1,000 BTU/hr of total input for the gas appliances then it’s a confined space and it’s too small for safe operation of the gas appliances.

For example, say your furnace and water heater are in a room together.  In this example we’ll say that the furnace has an input rating of 80,000 BTU/hr and the water heater has an input rating of 40,000 BTU/hr.  So the combined input rating is 120 when measured in 1,000 BTU/hr.  So the room needs to have a volume of 120 x 50, or at least 6,000 ft3.  If the room is smaller than this then it’s a confined space and is too small to be safe.  To put this size in perspective, with 9 feet ceilings this room would have to be 667 square feet, or almost 26 feet by 26 feet.  That’s a pretty big room, and there are an awful lot of gas appliances stuffed into small spaces that just don’t meet this requirement.

So what to do if the appliance space is too small?  We need to get more combustion air from somewhere, and there are plenty of options for doing that.

louver door

A fully louvered door.

The most common solution is to get combustion air from the rest of the house.  This is done simply by putting vent openings in the furnace room to connect it to other rooms in the rest of the house and allow air to get in.  This will work as long as the rooms combine to meet the 50 ft3 per 1,000 BTU/hr input requirement.

The most common way to add vents to the room is to install a louvered door.  (A louvered door has horizontal slats for venting.)  If the door is fully louvered over its entire surface then this will work as long as the combined input rating of the gas appliances is less than 175,000 BTU/hr.

If you don’t want to or can’t use a louvered door, here are your other options.

You can connect the furnace room to the rest of the house with two permanent vent openings.  Each of the two openings needs to be at least 1 in2 for each 1,000 BTU/hr of input of all the gas appliances combined, but each opening must be at least 100 in2.  One of these openings must be located within the top 12 inches of the appliance space, and one opening must be located within the bottom 12 inches of the appliance space.  The minimum dimension of each opening is 3 inches.  Figure 4.

comb air fig 4 a

Figure 4: Two permanent openings must be sized and located correctly.

 

 

 

 

 

 

 

 

 

 

 

Note that this requirement is for the free area of the opening, so if you put some sort of vent cover or grill over the openings you have to take that into account.  Usually a metal grill reduces the free area by 25%, and a wood grill reduces the free area by 75%.  So a 10 inch by 10 inch opening with a metal grill has a net free area of 75 in2, while a 10×10 opening with a wood grill has a net free area of only 25 in2.

For example, let’s look again at our situation above where we have a furnace with an input rating of 80,000 BTU/hr and a water heater with 40,000 BTU/hr.  And let’s say that we know the appliance room is too small.  So we can put openings in the wall of this room, so long as the openings lead directly into other rooms with a combined volume of 6,000 ft3.  We have to put two openings in the wall, one starting within 12 inches of the ceiling and one starting within 12 inches of the floor.  Each opening has to have a net free area of 120 in2, which meets the requirement of 1 in2 for each 1,000 BTU/hr.  So if the openings will have a metal grill we need to allow for that, so each opening needs to have a total area of 120/0.75, or 160 in2.  So each opening can be 13 inches by 13 inches square.  Or 18×9, or even 53.3×3.  But 3 inches is our minimum dimension, so the opening couldn’t be any more squished than that.

There are several other options for getting combustion air, including air from a crawl space, an attic, and from the outside.

A single opening in the room is OK if it connects to the exterior or a properly ventilated attic.  The opening must be in the upper 12 inches of the appliance space and it must have a net free area of at least 1 in2 for each 3,000 BTU/hr of appliance input.

If there are two permanent openings in the appliance space then one must start in the top 12 inches of the space and one must start in the bottom 12 inches of the space.  If the openings connect directly to the outside or connect to the outside through vertical ducts then each opening must have a net free area of at least 1 in2 for each 4,000 BTU/hr of appliance input.  If the openings connect to the outside through horizontal ducts then each opening must have a net free area of at least 1 in2 for each 2,000 BTU/hr of appliance input.

In general, ventilated attics and ventilated crawl spaces are considered equivalent to the outdoors.  Although ventilated crawl spaces are a bad idea (but that’s a topic for another post).

There are some other details that must be satisfied for combustion air.  Probably the most important detail to keep in mind is that it’s never acceptable to get combustion air from a bedroom.  If a gas appliance is in a bedroom closet then the closet door must be solid, self-closing, must be weather stripped to make it air tight, and the gas appliance must get all its combustion air from outside.

There are other details as well, but this gives the basic information.  Be sure to contact a qualified contractor when providing combustion air.  And of course if you have any questions feel free to contact Nations Home Inspections.

Ground Fault Circuit Interrupter (GFCI) explained

blog GFCI 2One of the most common electrical problems I find during home inspections is a lack of Ground Fault Circuit Interrupter (GFCI) receptacles.  You should see these devices in several places around the house, and you’ll recognize them as having a test and a reset button.  In this blog post I’m going to describe how GFCI’s work, why they’re important, and where they should be in your house.

There are two basic risks associated with electricity: fire and electrocution.  GFCI’s help to protect people against electrocution.  They do this by constantly monitoring the electrical current flowing through them.  If everything is working properly then the current flowing through the hot and the neutral conductors will be balanced and equal.  After all electricity makes a circuit, so what goes in must come out.

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But if the GFCI detects an imbalance in the currents flowing in and out then the assumption is that some of that stray electrical current is flowing out through you and you’re being shocked, so the GFCI shuts the circuit off and stops the flow of electricity.

GFCI’s do this by passing both the hot and neutral conductors through a current transformer (CT), as shown in the figure above.  If the CT detects balanced current then power is delivered to the receptacle.  But if a person touches a live conductor, or touches an electrical enclosure that is being faulted to a live wire, then some of the current goes out through that person.  The resulting current imbalance is picked up by the CT, which sends a signal through the sensor relay and the circuit is opened and the flow of current stops.

GFCI receptacles also have a test button.  When the button is pressed a little bit of current (limited by the resistor shown) is shunted around the CT, creating an imbalance that trips the GFCI off.

The UL standard that covers GFCI’s (UL 943) requires that they trip off within 1 second on a 6 mA (milliamp) fault.  Their actual performance is typically much better, tripping off in about 0.1 second and in less than 0.03 seconds on a 20 mA fault.  By way of comparison, a healthy adult can generally tolerate a shock of up to about 50-100 milliamps before death becomes a real likelihood.

Note that GFCI’s don’t have to be built into a receptacle where you’ll plug in your toaster or hair dryer.  GCFI protection can be built into the circuit breakers in your electrical panel, or they can simply be installed in-line with the wires of the circuit in the form of a GFCI switch.

In 1975 there were approximately 650 deaths by electrocution in a residential setting in the U.S.  In 2008 there were only 50, according to the National Center for Health Statistics.  That’s a dramatic drop, and there are several reasons, including newer housing electrical systems in general, the use of double-insulated power tools, and of course the introduction and expanded use of GFCI protection.  So even though you probably don’t know personally anybody whose life has been saved by a GFCI, and you may not have heard any stories about GFCI’s saving a life, there is no doubt that the use of GFCI receptacles has saved thousands of lives over the last decades and I urge you to consider them a very important safety component of your house.

The National Electrical Code (NEC) first required GFCI’s in 1968, for underwater pool lights.  In 1971 that was expanded to include all outdoor receptacles in residential use.  The locations requiring GFCI’s have been expanding ever since, and today GFCI protection is required for all receptacles in these locations, according to the NEC section 210.8(A) and the International Residential Code section 3902:

  • All receptacles in bathrooms.
  • All receptacles that serve a kitchen countertop.
  • Receptacles that are outdoors.
  • All receptacles that are in garages.
  • Receptacles that are in unfinished basements.
  • All receptacles in crawl spaces.
  • All receptacles that are within six feet of the outside edge of a sink other than the kitchen.

There is an exception in the basement for receptacles that serve a fire or burglar alarm as long as the receptacle isn’t accessible to the homeowner.  There’s also an exception outdoors for receptacles used for de-icing equipment, again as long as the receptacle isn’t readily accessible.

Garages and unfinished basements require GFCI protection not because of water, but because concrete is actually a pretty good conductor of electricity.  So if you’re standing on a large slab of concrete then electrical current will want to go through you and through the concrete to get to the ground.  So there’s an increased risk of shock and so GFCI protection is required there.

There is a myth that in the kitchen only receptacles within six feet of the sink require GFCI protection.  This used to be true, but in 1996 the NEC was changed to require GFCI protection for all receptacles that serve a kitchen countertop, regardless of how far they are from the sink.

Older GFCI electrical receptacles seemed to be more prone to nuisance tripping, which happened when the GFCI trips off even though there is no problem.  This was usually associated with issues of electric motor startup from a clothes washer, freezer, garage door opener, sump pump, or even a bathroom exhaust fan.  Newer GFCI’s have virtually eliminated nuisance tripping.  I occasionally get push-back from people who don’t want to install a GFCI receptacle because, “It might trip off when you don’t want it to.”  But that’s exactly the point: it might trip off.  It’s supposed to trip off, because that’s how it protects people.  And with newer GFCI equipment the chances of it tripping off when it shouldn’t – nuisance tripping – has been reduced to almost zero.  And isn’t that a good tradeoff?  A very small chance that the GFCI might trip off and inconvienience you versus the chance that the GFCI might save your life.  Or the life of someone you love.

If your home was built or remodeled before the codes required GFCI protection then strictly speaking you are not required to install them.  The codes aren’t retroactive.  But as your home inspector I’m going to recommend that you install them every place that the NEC currently requires them.  Let’s all be safe out there.

Insulation and your attic hatch

Since Autumn is fully upon us and it’s starting to get quite chilly out there, let’s talk about energy efficiency.  Over the years I’ve told thousands of clients that they should insulate their attic hatch.  I hope they’ve all taken my advice, but for anybody who still needs convincing let’s put some numbers to the problem.

First a quick insulation lesson.  Insulation is rated by its R-value, which is a measure of the insulation’s resistance to heat flow.  A higher R-value means better insulation because it provides more resistance to heat flow.  So we want a high R-value insulation at our exterior walls and ceilings.

In contrast, U-value is a measure of how well a material allows the flow of heat.  Windows and doors are rated by their U-value (sometimes called U-factor), and of course you want a low U-value which would mean very little heat flow through the door or window.

R-value and U-value are reciprocals: R=1/U  and U=1/R

The International Energy Conservation Code currently requires R-49 insulation in the attic in the Chicagoland area.  This is a fairly new requirement, however.  For many years only R-38 was required in the attic, so if that’s all you’ve got it’s still pretty good.

When you add layers of insulation you just add the R-values to determine what the new total insulating value is.  So if you have only R-20 insulation on your attic floor, and you blow in another R-29 of insulation, you now have R-49 insulation in your attic.

But when you have different areas of the attic with different levels of insulation, finding the total average R-value over the entire area is more complicated.  The formula for finding the total average R-value when different areas have different insulation levels is:

UtAt = U1A1 + U2A2 + U3A3 + . . . . You find Ut and then convert that to Rt with the formula above, R = 1/U.

Let’s do an example and see how important it is to insulate your attic hatch.

Let’s say your attic floor covers an area of 800 square feet, and you have R-38 insulation over that area.  But your attic hatch, which is 6 sqaure feet, is only covered with a piece of plywood.  That plywood only has an R-value of about R-1.  But it’s only a samll area, less than 1 percent of the total floor area of the attic, so it can’t make much of a difference, right?  Let’s see.

Here’s the formula again:

UtAt = U1A1 + U2A2 + U3A3 + . . .

In our example:

Total —  At = 800 square feet, and we’re trying to find Ut

Insulated area — A1 = 794 square feet,  R1 = 38, U1 = 1/38

Uninsulated area — A2 = 6 square feet, R2 =1, U2 =1

Ut*(800) = (1/38)*(794) + (1)*(6) = 26.8947

Ut = 26.8947/800 = 0.0336    and Rt=1/Ut, so

Rt = 29.7

Wow, a small little uninsulated area reduced the total average R-value of your attic insulation from R-38 to R-29.7, a reduction of 22%.  That’s a terrible waste.

If you put just a single layer of 1-inch rigid insulation on the hatch, with a value of R-5, the new number for total attic R-value would be R-36.5.  That’s a huge improvement.  Just two inches of rigid insulation and you’re at R-37.3.

So please insulate your attic hatch.  Save some energy, and save yourself some money.

Smoke Alarms Are Not All Created Equal

It’s no secret that smoke alarms save lives.  Between the mid-1970’s and today the percentage of homes with at least one smoke alarm has gone from about 10% to around 96%, according to a recent survey.  Having a working smoke alarm cuts the chances of dying in a reported fire in half, according to the National Fire Protection Association.

But there does seem to be a secret – a very well-kept secret – about the two types of smoke alarms that are available.

About 94% of the smoke alarms in U.S. homes are ionization type smoke alarms, while only about 6% are photoelectric.  These different types of alarms react very differently to different types of fires, and this difference can add up to precious minutes that you may need to escape from a fire.

A photoelectric smoke alarm, as indicated by the “P”

A smoldering fire will just create smoke for a long time, as much as two hours or more.  Then the fire will flash over into flames and quickly spread.  Most fire deaths are from smoke inhalation, so these types of fires are very deadly.  A smoldering fire is associated with newer synthetic materials.  The other type, a flaming fire more quickly flashes over into flames, and is associated with accelerants and highly combustible materials.  In tests, a smoldering fire is usually simulated by the cushions of an easy chair, while flaming fires are simulated by news print in a wicker basket.

In general, ionization alarms respond slightly faster to flaming fires – about 10 to 15 seconds faster than photoelectric alarms.  But when dealing with smoldering fires, ionization alarms respond as much as 30 to 60 minutes slower than photoelectric.  That’s right, 30 to 60 minutes – not seconds.  This drastic lag in responding to smoldering fires makes ionization alarms unacceptable to a growing chorus of fire protection specialists.

Smoldering fires are not only more deadly, they are very common and becoming more so as synthetic materials continue to be used more and more in our homes.  And yet the type of smoke alarm present in 94% of U.S. homes may be an hour late in responding to these types of fires.  It’s clear that photoelectric smoke alarms are the safer type.

A study at Texas A&M University from 1995 concluded that for a smoldering fire the probability of a fatality due to the failure of a photoelectric alarm  was 4%, while the probability of a fatality due to the failure of an ionization alarm was 56%.  For flaming fires the probability of a fatality due to the failure of a photoelectric alarm was again 4%, while the probability of a fatality due to the failure of an ionization alarm was 20%.

Underwriters Laboratory, which sets standards for smoke alarm performance, published the Smoke Characterization Study in 2007.  They tested both ionization and photoelectric smoke alarms using the two existing test standards and with traditional test materials.  They also tested the alarms using a variety of synthetic materials, and toast.  Ionization alarms failed the UL tests 20% of the time using the traditional test materials.  And remember, this is the same test that these alarms are supposed to pass just to be offered for sale in the U.S.  When tested using newer synthetic materials, ionization alarms had a failure rate of 100%.  On the other hand, photoelectric alarms has a pass rate of 100%.  There was only one material for which ionization alarms outperformed photoelectric – burnt toast.  Let me summarize: for the most common type of fire, ionization alarms had a 100% failure rate, while photoelectric alarms had a 100% pass rate.

This leads to the other problem with ionization alarms.  They are notoriously susceptible to nuisance tripping, from cooking, showering, and other common activities.  So they are commonly disabled.  In fact, ionization alarms are up to 8 times more likely to be intentionally disabled.  A disabled alarm of any type is of no value at all.

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Combination smoke alarm with both ionization and photoelectric sensors

Combination smoke alarms with both photoelectric and ionization detectors are available.  But there are no industry standards for these dual detector alarms, and the manufacturers can adjust the alarm points of the two detectors so that the unit behaves no better than a photoelectric-only alarm.  The International Association of Fire Fighters recommends against installing combination smoke alarms.

Finally, if you want to know what type you have, here are some guidelines.  If the label mentions radioactive material, Americium-241, or if the model number has an “i” in it, then it is almost certainly an ionization alarm.  If you’re not sure, then you should assume it’s ionization and replace it with a photoelectric type.  The life you may save is precious.

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An ionization smoke alarm