Designing a WEEDS System

What’s New?

Over the past year or so, several new water spray systems that are based on the WEEDS wildfire protection system have come online – two in California and one in Montana. A number of common questions and problems came out of these installations, and I’d like to take this opportunity to present the lessons learned.

Where are the plans for WEEDS?

The best collection of diagrams for WEEDS can be found in the presentation on the website:
Mbar_WEEDS_Comparison_web.pdf

There are no “plans” or “instructions” per se, since every installation is going to have its own challenges and requirements, and these will require having an experienced plumbing contractor who knows how to run numbers. If you are one yourself, then the technical details in this document should help you plan your system. If there’s something missing, drop me an email and I’ll try to answer your questions.

What is the biggest problem that people have had when building a WEEDS system?

The biggest issue people run into when constructing a WEEDS system is generally due to the nozzles. The nozzles I chose (Champion SF-9, also called S9F) are optimized to run at 1.5 to 2.0 gallons per minute (gpm), whereas I run our system at 1 gpm. In my documentation, I recommend 0.5 to 1.0 gpm based on the simulations that I did in conjunction with crib experiments and theoretical calculations, which are all described in my Fire Safety Journal paper. When people try to use the SF-9 spray heads under 1.0 gpm, though, performance becomes poor and they do not get a good spray pattern. They have often managed to circumvent this problem by reducing the number of heads or using a different nozzle.

What other sprinkler nozzles have people used?

I also use SH-9 heads (half-circle) in our system wherever it isn’t necessary to have a circular spray pattern, for instance where we have a double roof line. One person used Champion SQ heads, which give a quarter arc spray pattern 15 feet in diameter. These produce smaller droplets than the SF or SH nozzles, which wont stand up as well against wind. Another home WEEDS system was successfully built with Rainbird nozzles (will update when I get the exact model).

Keep in mind that these nozzles are adjustable, which is very important because you can get a significant pressure drop long your line and you need to adjust them in order to get equal flow for all of the nozzles.

One other important note: I use PVC pipe to mount my brass nozzles. Warning – do not use iron or steel piping to mount brass nozzles. This sets up electrolysis and causes corrosion and nozzle clogging.

And finally: do not use heat-triggered fire nozzles on the outside of your home. If you are experiencing enough heat to trigger them (which you probably won’t unless you have insufficient defensible space or your home is already burning), the water spray is likely to shatter your very hot windows.

Why do I have my WEEDS system divided into two branches?

Two reasons: First, I thought it would be useful to independently operate the north and south sections. It isn’t, except for letting me do a demo without getting our balcony wet. Second and more importantly, it allows for a shorter run of pipe, thereby reducing the overall pressure loss. My 1¼-inch feed line branches into two 1-inch supply lines (the cross-sectional area is about the same before and after the split). As I said before, you need to run the numbers to make sure you are going to get the pressure you need throughout your system, so hire an experienced contractor if you can’t do this yourself.

How much water does a WEEDS system really need?

According to the calculations in my Fire Safety Journal paper, nozzle flow rates of around 0.5 to 1.0 gpm would be more than enough to extinguish a “crib”, the standard stack of sticks used by fire scientists to do their measurements. Since my nozzles are 8 feet apart, this works out to around 0.06 to 0.13 gallons per linear foot of exterior per minute.

How low could you go and still prevent ember ignitions? We do not know. One home not far from here that experienced similarly severe conditions during the Cedar fire used a set of misters on its wrap-around wooden deck. With 10,000 gallons of gravity fed water, it also survived, though its flow rate was much lower than WEEDS. Some statistical data collected during the Paint fire in 1990 suggested that sprinklers (of undefined type) operated during and after the fire were effective in reducing ignitions, though the statistics are too weak to make a strong claim.

The WEEDS design is conservative in that it uses enough water to suppress small fires. Systems putting out less water might still be effective, but for our own home we prefer to have this margin of safety.

What is the most important design consideration for a WEEDS system?

The most important balancing act in designing a WEEDS system is the balancing act between system lifetime and the amount of water it lays down. In order for a water spray system to be effective, it should operate DURING and AFTER the passage of the fire front. This is tricky. In this sense, our system is under-designed, since we only get about 3½ hours of protection. Ember attack can last for hours after a fire. Also, it can be dangerous to delay evacuation unless one is certain that evacuation routes are clear, so the longer you can do this in advance the better. This is where having a larger reservoir really helps.

Another important consideration is to make sure your WEEDS system is safe itself. Pumps and generators should be in an open or protected area, and horizontal supply lines should be metal, buried, or covered.

What would I do differently if starting from scratch?

Overall, the components my contractors chose for this project have been superb and have operated reliably for the past six years. The only thing I’d change is to use a larger water tank – 10,000 gallon versus the 5,000 gallon we have. Also, I think putting the pump and above-ground pipes in a heated (non-flammable!) enclosure would make our water supply more reliable on the very rare occasions it gets much below freezing here (we use the tank for our home supply as well as WEEDS).