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Fire Sprinkler Systems

Fire sprinkler systems are becoming very common in new construction and major remodel projects, and so here’s a brief primer on residential fire sprinkler systems.

When it comes to enforcing life safety codes in commercial and multi-family buildings, local jurisdictions usually enforce some version of either the International Fire Code (part of the ICC family of codes) or the National Fire Protection Association (NFPA®) document NFPA 101® Life Safety Code®.  But both of these documents refer to NFPA® 13 as the standard for how to install sprinkler systems, so this is the definitive source.  A separate document, NFPA® 13D, applies only to one- and two-family dwellings and manufactured homes.

There are four basic types of water-based fire sprinkler systems.

  1. Wet-pipe sprinkler system – The sprinkler pipes are constantly full of water under pressure. When heat from a fire activates one or more sprinklers then water flows from only those heads until the system is shut off.  The NFPA® reports that with home sprinkler systems roughly 85 per cent of the time only one sprinkler will activate.  This is by far the most common type of sprinkler system in residential settings.
  2. Dry-pipe sprinkler system – The sprinkler pipes are only charged with compressed air, and there’s a dry-pipe valve somewhere not subject to freezing temperatures. When heat from a fire opens a sprinkler the compressed air is released and the dry-pipe valve senses the loss of pressure and opens to allow water flow until the system is shut off.  This type of system is typically used in areas where freezing temperatures are likely to occur, like open parking garages.
  3. Deluge sprinkler systems – This system consists of open sprinklers installed in unpressurized pipes, and a valve at some central location. When some thermally sensitive electronic device senses a fire it sends a signal to open the valve and water pours out of all the sprinklers simultaneously until the system is shut off.  This type of system is usually found in areas where even a small fire could very quickly spread due to the type of materials stored in the area.  This type of system is also often found in the movies – it’s certainly more dramatic to have all the sprinklers go off at once even if it’s not usually an accurate portrayal.
  4. Preaction sprinkler system – This system consists of sprinklers connected to a dry system of pipes. The system will only operate when heat from a fire opens a sprinkler head and also when an electronic device senses a fire and opens the waterflow control valve.  This type of system is usually found where a false activation of the sprinkler system would cause a catastrophic loss, maybe to some irreplaceable contents.

The sprinkler is the device that actually discharges water.  Sprinklers can be installed in any of several configurations, but each configuration requires a different sprinkler design, so they’re not interchangeable.  An upright sprinkler (designed to be installed upward from a branch line) can’t be swapped out for a pendant sprinkler (designed to be installed downward from the branch line) or a sidewall sprinkler (mounted on a vertical wall).  You’re also likely to come across concealed sprinklers which have a removable decorative cover plate that releases at the proper heat level.

The deflector is a small piece of metal on the sprinkler.  The water discharge hits the deflector, creating the discharge pattern.  The design of the deflector is different for the various types of sprinklers in order to create the right type of water spray pattern.

These sprinklers illustrate the differences.  The deflector is shaped differently in these three sprinklers, one pendant (hanging down), one upright, and one sidewall.  These are definitely not interchangeable.

NFPA® 13 defines six temperature categories for sprinklers from Ordinary to Ultra High. “Ordinary temperature-rated sprinklers” have a temperature rating between 135°F and 170°F, and this is what you should expect to see in one- and two-family dwellings.  “Intermediate temperature-rated sprinklers” are rated between 175°F and 225°F.  These are also allowed in most locations, with the exact type required depending on the maximum ambient ceiling temperature.

The temperature rating of the sprinkler should be stamped into it, although it can be very hard to see and I don’t recommend that you try.  They also should be color coded, with Ordinary sprinklers either uncolored or black, and Intermediate sprinklers being white.

The sprinklers shown above have no color so you’d expect them to be in the ordinary range, which they are.

NFPA® is very clear that sprinklers shall only be painted by the manufacturer.  It’s a big problem if any installer or homeowner paints a sprinkler or modifies it in any way.  That sprinkler needs to be replaced.  Also there should never be anything hanging from a sprinkler.

There are three primary heat activation mechanisms for sprinklers, including metal fusible links and chemical pellets.  But for residential sprinklers the most common is the glass bulb.  The bulb is inserted to hold back the sprinkler plug.  At high temperature the liquid in the bulb expands, breaking the bulb and releasing the plug.  Water starts to pour out.  It’s worth noting that mechanical damage can activity a sprinkler also, so be careful.

You should expect to see an orange or red bulb in an Ordinary temperature rated sprinkler (as in the pictures above), and a yellow or green bulb in an Intermediate temperature rated sprinkler.

For residential systems in one-and two-family dwellings, NFPA® 13D does not require that spare sprinklers be provided (section  But for commercial and most multi-family buildings not only are spares required, but a wrench is required, and one spare of each of the various types of sprinklers is required.  But you’ll usually see spare sprinklers and I always tell clients to be sure to have them.

spare sprinklers

It’s likely that you’ll see a waterflow switch and alarm, although it isn’t required (unless the house doesn’t have smoke alarms, in which case you’ve got a different set of problems).  A typical switch will look something like this in the picture below.  When water flows through the system it activates the switch, which in turn should sound a local bell in the same area, and maybe a horn and strobe at the front of the house.

water flow switch

water flow alarm bell

There should be a valve and drain pipe that discharges to a floor drain or sump.  You can test the waterflow alarm by opening this drain valve, but be careful — you might call the local fire department depending on how the alarm is configured.

A backflow device isn’t required by fire sprinkler codes, but your local plumbing code probably requires it so you should look for that and note if it’s missing.

Residential systems have a different design from standard sprinklers and a different discharge water pattern.  Residential systems are designed with more of a goal towards controlling the fire to allow for occupants to evacuate, rather than putting out the fire.  Residential sprinklers spray higher than other types to help prevent flashover conditions.  Flashover is when the environment in a room is changing from two layers (hot on top and cooler on the bottom) to a single layer, well-mixed, with hot gases from floor to ceiling.  This condition isn’t survivable even by a fully protected firefighter, so preventing it is key to allowing occupants to escape.

As with most things home inspectors deal with, sprinklers need to be installed according to their listing, and that pertains to their spacing as well.  Typically you should expect a residential sprinkler to cover a maximum of 144 square feet, so the maximum spacing should be 12 feet.  But some sprinklers are listed to protect larger areas, so without seeing the sprinkler’s specifications you can’t know for sure.

In a one- or two-family dwelling you won’t need sprinklers in bathrooms of 55 square feet or less, or in most smallish clothes closets, or in garages.  You also won’t need a sprinkler in an attic with or without storage, or in a crawl space, or in a concealed space that’s not intended for living purposes.  This is consistent with the philosophy of controlling the fire to let occupants escape.  But you will need a sprinkler in a closet used for HVAC equipment, water heater, or laundry appliances.

Of course no water-based fire suppression system is going to be effective without a good source of water.  NFPA® 13D requires 18 gallons per minute for any single sprinkler, or with two or more 13 gpm.  If you’re using a stored water supply or the house is on well water, the system needs to be able to operate for at least 10 minutes (or seven minutes for some smaller single story houses).  Again, this might not be enough time to control a fire, but it should give occupants enough time to escape.

If there’s any doubt that a house can supply this amount of water, then a good fire protection contractor should evaluate the system and confirm there’s enough water or make the necessary upgrades.  To run at 18 gpm for 10 minutes might require the installation of a 180 gallon storage tank.


Of course the greatest water supply possible can be rendered moot if there’s a valve that’s shut off.  NFPA® 13D requires a single valve to shut off both the domestic water system and the sprinkler system since no homeowner is likely to shut off his sprinkler system and make do without a flushing toilet.  Or if the sprinkler system has its own valve then that valve needs to be either “supervised” by a monitored alarm system, or locked open.  (It’s not clear who’s allowed to hold the key.)  And of course anything that might restrict water flow or decrease pressure needs to be installed so that it won’t interfere with the performance of the fire sprinkler system.

I usually see separate shut off valves, and I always mention to my clients the importance of a working fire sprinkler system and urge them to never shut off the sprinkler system.

It’s easy for a homeowner to overlook a system that he hopes will never operate.  But a properly installed fire sprinkler system can be the difference between life and death.