By David Rubin (RBDA Chair)

In the 22 years that we’ve been living in Bonny Doon, we left home 4 times to avoid breathing smoke, most recently in August 2020, the day before the CZU fire reached our neighborhood on Thayer Rd. We appreciate the efforts of our neighbors and other firefighters who kept an eye on our house and neighborhood while we were gone. I expect we will evacuate in future fires and won’t be home to help with firefighting, so this article is my attempt help the community by passing along what I’ve learned while developing a fire sprinkler system to protect our home in our expected absence.

Before focusing on the sprinkler system, we did many fire-hardening projects: removing gutters and replacing them with drip-edge flashing; covering 200 vents with finer mesh screen; replacing 700 square ft of decking adjacent to the house with concrete patio; covering the bottom 4 ft of posts with stucco; replacing the bottom foot of siding with cement board at ground level and where second-story walls meet the roof; and trimming trees and shrubs near the house. 

Summary of sprinkler system

After our fire-hardening work, we installed an exterior sprinkler system that we can turn on and off remotely. It relies on multiple power sources (PG&E, generator, and back-up batteries), multiple communication systems (Comcast cable and connection to the AT&T cell tower in Davenport), a video camera to watch the scene at home, and underground piping from the well-water source to sprinkler risers. If our 500-gallon propane tank is half full (as low as we like it to get), the system can deliver ~45,000 gallons of water to the house and adjacent 50 ft of ground surrounding the house, over 125 hours of operation (either continuous or spread over a longer time). The article below details how the sprinkler system works—in perhaps too much detail for most readers—but it is provided to aid people who might want similar systems. Some of the electrical parts are surprisingly inexpensive; for example, the 2-channnel wi-fi relay that controls the generator and sprinklers remotely costs $17.

Details

The impact sprinklers (14 total) are plumbed in 3 lines of piping. Two lines cover the perimeter of the house, and one covers the roof. The 3 lines run sequentially, controlled by two inexpensive components. The first component is a timer relay that opens and closes a single irrigation valve. The second component is a mechanical indexing irrigation valve that moves to the next of the three circuits of sprinklers each time the water turns off and on. The timer is set to turn on water for 3 minutes and off for 1 minute off. A complete cycle of the 3 lines takes 12 minutes and delivers ~70 gallons of water to the house and surroundings. This is a tiny amount of water to compared to what a fire hose can deliver, but the water supply at this flow rate from the well is virtually unlimited, drenching the house and surroundings with ~360 gallons per hour (8600 gallons per day).

Water from the well normally pumps at a rate of 10 gpm, but to maintain pressure in sprinklers and to avoid drawing down the water level in the well, the impact sprinklers were selected so that each line of 4 or 5 of them discharges only 6 gpm. To prevent loss of water pressure in the fire sprinkler pipes, water to all other pipes is shut off when the fire sprinklers are activated (using a normally open irrigation valve).

Backup power is provided with a 14 kw Champion generator. One reason I chose this generator is because the “modern” communication with the transfer switch can be bypassed, so that the generator to be wired with older-technology “two-wire control”. This allows the generator to be turned on and off with a single switch (but still maintaining the automatic transfer switch). Using this “two-wire control” the generator is wired to operate 3 ways: (a) manual switch turns it on, (b) timer runs a specified schedule, and (c) generator and sprinkler can be controlled remotely over internet. The timer saved us hundreds of dollars during a single power outage that occurred when we were out of town. The timer ran the generator for 2 hours each morning and evening to keep the fridge cold, but we saved ~60 hours of generator use (and ~100 gallons of propane) over the 3-day outage. 

Relays that turn the generator (and sprinklers) on and off are connected to a Netgear Nighthawk M1 wi-fi hotspot, with backup battery. The hotspot is connected to the internet by Comcast cable when cable is operating, but if cable internet fails due to either PG&E or Comcast outage, the hotspot reverts to a cellular connection to the AT&T tower in Davenport. I would have preferred Starlink’s satellite internet, but it failed to give us reliable connectivity due to tree cover. Connecting to the cell tower at Davenport required luck (to have even a weak signal) and experimentation (to find a location with the stronger signal, and to try different directional antennas to boost the cell signal). Without such backup internet, remote control during a fire is unlikely to be possible, and I worry that a fire might take out AT&T’s tower in Davenport.

It is necessary to know the well’s sustainable discharge and pressure (including pressure loss to the roof) to calculate how many sprinklers the pump and well can operate simultaneously. If too many impact sprinklers are put in a circuit, they won’t rotate. This can be determined by trial and error using garden hoses before doing more permanent plumbing. (Irrigation King has detailed specs listing discharge rate and sprinkler range for a given water pressure.)

Several kinds of electric power are needed: (1) generator to run the well pump (220v AC). (2) batteries to start the generator and power the wi-fi relays that enable remote control (24v DC); these are charged by the generator, using the timer to turn it on 2 hours in the morning and evening. (3) backup power for the wi-fi hotspot, so that we can control the generator and sprinklers.

Excluding the generator, the sprinkler system cost ~$4000, including labor to dig trenches for the sprinkler pipes that surround the house. The wiring was relatively straight-forward, except for trying to bypass the generator’s “smart” technology and replace it with two-wire control. Tech support at Champion gave me a list of reasons why it would be impossible to do what I wanted, but they became helpful after I replied with details of how I planned to address the issues they raised. 

This project was a lot of expense and work (perhaps 300 hours of my time) for something I hope to never use. Much of my time went was spent searching for components and experimenting with circuitry and plumbing. I’m happy to provide information to anyone interested in more details, such as part numbers or links for where to purchase components. Contact me at drubin@ucsc.edu

General outline of parts

Sprinkler heads (14 of them at ~$10 each from Irrigation King)

Electrical timer (low power consumption) to turn generator on/off at scheduled times ($95)

Electrical timer to control cycling of sprinklers ($33, Amazon)

Normally closed valve to turn the sprinklers on and off ($20)

Normally open valve to turn off all other plumbing when sprinklers are activated ($70, Amazon)

Mechanical indexing valve to sequence the sprinklers ($60)

Netgear Nighthawk M1 MR1100 hotspot to connect to internet by AT&T cell ($195, ebay; $35/month data plan AT&T) 

Wi-fi relays to turn generator and sprinklers on and off ($17, Amazon)

power