What you'll learn:
- Which kind of backflow preventer you have, and whether it's the kind that freezes
- What actually cracks when a backflow freezes
- How to shut off and drain it before a hard freeze, including the 45-degree ball valve trick
- What to wrap it with, from towels to a fitted cover and insulated rock
- Which DFW cities require the above-ground RPZ type
Experience level: Beginner (you'll need to find your shut-off and turn a couple of handles)
- Most DFW homes have a double check in an underground box. I've never seen one of those freeze
- The ones that freeze sit above ground next to the house: pressure vacuum breakers on homes, and RPZs on commercial properties and in cities that require them
- What breaks is usually the riser pipes and the ball valves, not the whole device. On a commercial unit, a split 2-inch brass ball valve gets expensive
- Before a hard freeze: shut the water off, drain what you can, and leave the ball valves at 45 degrees
- Then wrap it. Towels work. A fitted insulated pouch with an insulated rock cover over it works better
Which Backflow Do You Have?
Every sprinkler system in Texas has a backflow preventer between the city water and your zones. It keeps lawn water from siphoning back into the drinking water. Whether yours needs freeze protection depends almost entirely on which type you have and where it sits.
Double check in an underground box. This is what most DFW homes have. It sits in a rectangular valve box in the yard, usually near the water meter or between the meter and the house. Being below ground is what protects it. I have really never seen one of these have a freeze problem. Some people stuff insulation in the box, and I don't think it's necessary unless the device is basically touching the lid, which I just don't see.
Pressure vacuum breaker (PVB). This one sits above ground, usually right against the house, with copper or brass risers coming up out of the dirt. It's the most common type on homes that do have an above-ground device, and it's the one that freezes.
Reduced pressure zone (RPZ). Also above ground, usually near the building. You see these mostly on commercial properties, and on homes in cities that require them. They have the same freeze problem as a PVB, with more brass to crack.
Anti-siphon valves. These are zone valves with a built-in backflow device, mounted above ground. I've only seen a few of them around DFW.
What Actually Breaks
When an above-ground backflow freezes, it's usually not the whole device that's ruined. Water trapped in the pipe expands as it turns to ice, and it splits whatever is weakest. In my experience that's:
- The risers, the copper or brass pipes coming up out of the ground into the device.
- The ball valves, the shut-off handles on each side. Water sits trapped inside the ball, and when it freezes it cracks the valve body.
On commercial properties it gets expensive. I've seen 2-inch brass ball valves split open on commercial units, and a 2-inch brass valve isn't a cheap part.
What to Do Before a Hard Freeze
When a hard freeze is in the forecast, do this the day before, while it's still above freezing:
- Turn off the water to the system. Close the irrigation isolation valve, the shut-off between your house water and the sprinkler system. If you're not sure where yours is, my emergency shut-off guide walks through finding it.
- Drain what you can. With the water off, open the small test cocks on the device and let the water run out. If you have a way to push a little air through the backflow itself to clear more of it, even better. This isn't a whole-system blowout, which North Texas doesn't need. It's just getting water out of the one part that sits above ground.
- Leave the ball valves at 45 degrees. Turn each ball valve handle halfway, so it's sitting at an angle instead of fully open or fully closed. A half-open ball can't trap a pocket of water inside the valve, which is exactly the water that cracks it.
- Set the controller to off so it doesn't try to run a zone into a frozen pipe.
Be realistic about step 2. The test cocks drain the device, but not the vertical risers coming up out of the ground. Febco, which makes one of the most common pressure vacuum breakers, says in its own winterization instructions the assembly can't be adequately drained through the test cocks. Their answer is a drain valve below the device, but plumbing code doesn't allow a buried stop-and-waste drain on the drinking-water side, so you won't find one on a typical DFW install. On an RPZ with a Y-strainer ahead of it, opening the strainer would let some of that water out, but the strainers are usually buried and hard to get to. Assume the risers still have water in them. That's why the wrapping below has to go all the way down to the ground.
The double check itself won't freeze, so skip the wrapping. But I'd still shut the water off: turn off the isolation valve if you can find it, and set the double check's own ball valves to 45 degrees. The reason is what's downstream of it.
Here's a job that shows why. A homeowner left the water on to the sprinkler system over a freeze. One of the zone valves had a tiny, tiny leak, not enough to notice, and it was slowly feeding a set of copper risers in a flower bed against the house. Normally the low-head drainage would have emptied those risers after the zone shut off. With the valve weeping, they stayed full of water. When it froze, they split all the way up and down. With the water shut off, that leak would have had nothing to feed it.
How to Wrap an Above-Ground Backflow
Once it's drained, cover it. Any of these works, roughly in order from quick fix to best:
- Towels. A couple of heavy towels wrapped around the device and the risers, held in place so they don't blow off. Cover the risers all the way down to the ground, not just the brass body.
- A fitted insulated pouch. They make insulated covers shaped to slide down over a backflow. They fit better than towels and don't get soaked.
- A pouch plus an insulated rock. Put the pouch on, then set a fake insulated rock cover over the top. That combination does pretty well, and it looks better in a flower bed than a pile of towels all winter.
- Heat tape. For a device that's especially exposed, you can wrap thermal heat tape around the pipe and device under the insulation and plug it in. It needs an outdoor outlet within reach.
Freeze Relief Valves
There's also a small add-on valve made for above-ground backflows, the Zurn Wilkins ZWFR freeze relief valve. It threads onto the downstream test cock of an RPZ or a pressure vacuum breaker. When the water inside drops to about 35 degrees, it opens and starts dripping, which pulls warmer water up from underground and through the device. When the water warms back to about 40 degrees, it closes.
It works the opposite way from everything above: the water stays on, and water runs through the device all night instead of sitting still in it. A few things to know before you rely on one:
- It doesn't protect everything. Water only moves up through the inlet riser and the device to where the valve is mounted. The outlet riser heading back down to the zones has no flow, so it still needs wrapping.
- It discharges water. Zurn warns that the water it lets out has to drain somewhere it won't damage the property around it.
- It's not a substitute for winterizing. Zurn's own instructions say it "is not intended to replace the standard winterization process." It's backup for a surprise freeze, not a reason to skip the wrap.
DFW Cities That Require an RPZ
An RPZ is the above-ground type, so if your property is required to have one, freeze protection isn't optional. Texas irrigation rules require an RPZ where the system carries a health hazard, most commonly a home on a septic system, a system with chemicals injected into the water, or a system tied into a second water source like a well, pond, or rainwater tank. Beyond that, it comes down to the city:
- Mesquite requires an RPZ on commercial irrigation. Residential systems in Mesquite use a double check in a box below grade.
- Heath requires an RPZ when the property is on a septic system.
- Fate treats a septic system with irrigation as a health hazard that requires an RPZ.
- Plano requires an RPZ (or an air gap) when the system has chemical injection or quick-coupler hose connections. Plano also requires an RPZ to sit above ground, and the city recommends a freeze-proof enclosure on a concrete slab.
If you're not sure what your city requires, the city's backflow or cross-connection office can tell you. Whatever the reason you have one, an RPZ sits above ground, and it needs the same draining and wrapping as any other exposed backflow.
Get It Done Before the Freeze
If you'd rather not crawl around the side of the house in the cold, or you have a commercial RPZ with big brass valves you don't want to gamble on, book a freeze prep visit. I recommend scheduling it for January. I'll shut the system down, drain what can be drained, set the valves, and get the backflow covered, and you'll know where your shut-off is for next time. Then in March, after the last hard freeze, it gets turned back on.
For the rest of the system, my North Texas winterization guide covers the controller, exposed pipe, and spring start-up.