Designing a Residential Fire Sprinkler System
Wed, Feb 20, 2019We are in a Wildland IR1 fire zone. In addition to using fire-retardant materials and setting up a 100’ defensible zone (trees and shrubs need to be cleaned or cleared within this radius), we are required to have a fire sprinkler system inside the home.
Actually, all of Kitittas County is considered an IR1 zone (requiring sprinklers). You can apply (and pay) for mitigation to have this requirement removed, but with the burnt remains of the Jolly Mountain fire visible from our lot, we doubt the fire marshal will grant such a request.
Why Have a Residential Fire Sprinkler System?
Before getting into the details, we should go over what a residential fire sprinkler system is. In short, its a system designed to give you up to 10 minutes of additional time to escape your home in the event a fire breaks out inside. I say “up to” 10 minutes, but maybe it could be more. In the end, the codes that govern these systems are targeting 10 minutes, so I would not assume you’re getting much more time.
A residential system will only help slow fires that originate inside. A fire that starts outside will quickly overwhelm the system.
A residential system will not stop a fire (at least, its not the design intent). The intention of having this system in place is to give you enough time to escape the house before it becomes fully engulfed.
These are some important considerations you should factor into your cost/benefit equation. For us, our chance of being engulfed by a forest fire is pretty high (in my opinion), in which case this system will not help.
We are required to use many fire proof or fire retardant materials. So our risk of a fire starting should be lower than typical. This is a bit naive to say though as most modern furniture and other fixtures that go into a home spread fire much quicker than they did a few decades ago.
The cost/benefit equation becomes more complicated, but for us its easy: the county requires it.
So what goes into a residential fire sprinkler system?
NFPA 13D
I could not find many specifics on the web. Most articles were discussing the pros and cons of including such systems, not on how they should be designed. The county website was not much help either. The requirements for the fire sprinkler system permit (yes, it’s another permit and it’s not cheap!) just says the plans must use a recognized national code, without giving any examples of such codes.
After some searching, what seems to be the most popular national code for residential fire sprinklers is NFPA 13D. This code is overseen by the National Fire Protection Association, and 13D is the section of their code that addresses residential systems. You can view the code for free by registering an account, and using their online-only reader, or you can purchase a copy for download.
NFPA 13D is only 67 pages, and most of those pages are prescriptive lookup tables or illustrations. It’s a fairly quick read. There are a few interesting details in the code.
In general, you design a system such that it can disperse a specific design density, or the number of gallons per minute (gpm) of water per square foot of house. The 2019 code specifies the system should disperse 0.05 gpm/sq ft. What is interesting is that the code specifically says not to produce more than this. My best guess is that in the event of a fire, the water supply may be limited (many systems are supplied by a private reservoir in the garage) and you wouldn’t want to waste it.
A large portion of the code describes how to layout sprinklers around obstacles. Beams on ceilings are considered an obstacle. A floor to ceiling cabinet is an obstacle, etc. I was able to skim most of this as the specific circumstances did not apply to us.
Types of Sprinkler Systems
There are multiple different types of sprinkler systems. You can have a wet system, where water is already in the pipes, or a dry system, where water does not feed the pipes until a fire breaks out. The benefit of a dry system, which is more complicated, is that you don’t have to worry about pipes freezing. Or you can have a wet system with anti-freeze, which has its own technical challenges, namely, you don’t want the antifreeze to ever mix with your potable water supply. There are quite a few different options.
I want a system that was cheap, reliable, and easy to maintain. The multi-purpose looped system seemed like the best option for us. In this system, the fire sprinklers and potable cold water supply would run off the same plumbing lines, and the main plumbing line would be a big loop back on itself (ensuring there were no dead-ends). This system is extremely reliable because if there is ever an issue in getting water to the sprinklers, there will be an issue with at least one of the plumbing fixtures too. In other words, there is no way to accidentally turn off the sprinklers without also turning off water to your whole house.
Due to its simplicity, its typically cheaper too. We don’t have to buy or maintain equipment to prevent backflowing since its all one system. You do have to be aware of which sprinkler heads you buy as I found many contained lead.
If we were hiring out the plumbing work, we could save a bit as a standard plumber is able to plumb the rough-ins for all the sprinkler heads. But like everything else, we are planning to do the work ourselves.
Sprinkler Placement
Surprisingly, NFPA 13D does not give any specifics as to how far apart sprinklers should be placed. For this, you turn to the manufacturers. The manufacturer will specify maximum distance between sprinkler heads, based on the gpm and pressure you can supply each head. Once you know your max distance for placement, and the dispersion that can be provided by each head, you can run the math to figure out how far apart each head must be. This gets a whole lot more complicated if you have obstacles in your path, but for the most part, our rooms are rectangular and clear of any obstacle.
Pipe Size
The final piece of the puzzle (for our needs) is determining what size pipe we need servicing these sprinklers. NFPA 13D lends assistance here. If you are on a city water supply (with a minimum capacity), you can use a prescriptive lookup table to determine your pipe size. We are on a well, so we had to use the calculated method.
The calculation is fairly easy on paper, but will take some work to track down all the parameters. Essentially, you start with your source water pressure (in psi), and subtract from that all the things that will reduce water pressure. These are all specified in the code, but include things like the length of pipe from the source (well) to your house (this is reducing pressure via friction), the size of water meter, any filters that may be installed at the house, the pressure needed by the sprinkler head, and some others. The left over pressure after these subtractions is used in a lookup table to tell you the max length of pipe for the diameter you want.
We want to use PEX. When I run our numbers, we can have a maximum run of about 100’ on 3⁄4” PEX, but a little over 300’ on 1” PEX. I made some assumptions on some of my calculations since I need to contact the water company still to get the actuals. But as long as these are close, we should be able to get away with using a 1” PEX line for the main line in the loop of our system.
Final Words
I should also mention that NFPA 13D describes how a system should be tested and maintained too. Even if you are choosing not to design a system yourself, its worth looking at this section of the code so that you can be familiar with how your system will be tested, and that its done properly. In the end, your installer should explain all this, and provide you documentation on the system (this handoff is a specific part of the code— its section 12.1 if you need to reference it).
Disclaimer: I am not a professional fire sprinkler system engineer and you should not take anything here as advice. I share my findings and opinions just a way to help you get started in your research.
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