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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.

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.

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.

blog GFCI 1c

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.

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.


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.


An ionization smoke alarm