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Before the first hard freeze

How to Drain a Backflow Preventer for Winter (PVB, RPZ, DC & AVB)

The backflow preventer is the most expensive thing on an irrigation system that a freeze can destroy, and it is the most exposed: brass, above ground, and full of water. It is also the step people get wrong, because the four devices found on residential systems are winterized differently and the instructions that come with one do not apply to the next. Identify yours first, then follow its own sequence. One rule is common to all of them: nothing gets left sealed shut with water inside it.

What to do

  1. Step 1

    Identify which device you actually have

    Look at the assembly where the irrigation line leaves the house. A single body on a riser with a domed bonnet and two test cocks is a pressure vacuum breaker (PVB). The same shape with a relief valve hanging underneath that occasionally drips is a reduced pressure zone assembly (RPZ). Two checks with four test cocks and no relief valve, often below grade, is a double check (DC). A small body downstream of each valve with no test cocks at all is an atmospheric vacuum breaker (AVB).

  2. Step 2

    Shut off upstream before you open anything

    Close the dedicated irrigation shut-off inside the house or in the meter box first. Opening test cocks on a live line just sprays you and leaves the device full the moment you close them again.

  3. Step 3

    Set the isolation valves half open — the 45° winter position

    On any device that has them, the two ball valves go to roughly 45°, not closed. A ball valve turned fully off traps water inside the ball itself, and that trapped water is a classic split. Half open leaves a path for it to expand into.

    This is the single most common winterization mistake on a backflow device. Fully closed is not 'more shut off' — it is a sealed container.

  4. Step 4

    Open every test cock and leave them open

    Test cocks are the little screwdriver-slotted plugs on the side of the body — two on a PVB, four on an RPZ or a double check. Crack all of them to roughly 45° and leave them there until spring. They are what lets the internal chambers drain and stay drained, and because they are small ball valves in their own right, part-open protects the cock body itself the same way it protects the isolation valves.

  5. Step 5

    On a PVB, open the bonnet as well

    The domed cap on top of a pressure vacuum breaker covers a chamber that the ball valves and test cocks do not drain. Unscrew it and lift out (or at least loosen) the poppet assembly, and leave it that way for winter.

  6. Step 6

    In a hard-freeze climate, remove it or insulate it — in that order of preference

    If the device is on union fittings, unthreading the whole assembly and storing it in the garage is the safest winterization available and takes ten minutes. Where that is not practical, an insulated cover or wrap slows the freeze — but insulation on a device that still holds water only buys time, so drain it first regardless.

    Never leave an insulating wrap as a substitute for draining. It changes how long the device survives a hard freeze, not whether it does.

What “45°” looks like from above

The ball inside the valve is drilled through, and the handle tells you which way that hole is pointing. Half open keeps one end of it connected to the pipe; fully closed turns it across the flow and seals it at both ends with whatever was inside still in there.

  • Ball valve fully openThe handle lies along the pipe and the bore through the ball lines up with it, so water flows straight through.

    Open — the summer position

    Handle along the pipe, bore lined up with it. This is how the valve spends the irrigation season.

  • Ball valve at 45 degrees, the winter positionThe handle sits diagonally, halfway between open and closed. The bore is still partly lined up with the pipe, so any water inside the ball can escape instead of being sealed in.

    45° — the winter position

    Half open is not a compromise between the other two — it is the only position that leaves the inside of the ball connected to somewhere the water can go.

  • Ball valve fully closed, which traps water for winterThe handle sits across the pipe and the bore turns with it, so the water inside the ball is sealed at both ends with nowhere to expand when it freezes. This is what splits the valve body.

    Closed — what splits the valve

    Closed looks tidier and is the most common winterization mistake on a backflow device. The bore is now a sealed container, and water needs about 9% more room as it freezes than the brass around it can give.

Device by device

PVB

Pressure Vacuum BreakerAbove ground
Pressure vacuum breaker (PVB)A single body above ground on two risers, with a domed bonnet on top, two test cocks, and an isolation ball valve on each riser.

A single brass body on a riser about a foot above the highest sprinkler head, with a domed bonnet on top, two ball valves below it, and two small test cocks on the side. The most common device on residential irrigation in freeze country.

Drain it like this

  1. Shut the irrigation main off upstream of the device.
  2. Turn both isolation ball valves to roughly 45° — half open. That is not a compromise position, it is the winter position: it leaves the valve body itself open so water trapped inside the ball has somewhere to go when it expands.
  3. Open both test cocks to roughly 45° with a flat screwdriver and leave them there all winter. Test cocks are small ball valves themselves, so the same logic that puts the isolation valves at 45° applies to them — cracked part-open drains the cock body, where hard against either stop can leave water sealed inside it.
  4. Unscrew the bonnet on top and lift the poppet assembly out, or at least loosen it — the chamber under that dome holds water that the ball valves cannot drain.
  5. In a hard-freeze climate, unthread the whole device and store it indoors if the unions allow it. It is the cheapest insurance on the system.

The mistake to avoid: Leaving the ball valves fully closed. It looks tidier and it is the single most common way a PVB body splits — water sealed inside the valve has nowhere to expand.

RPZ

Reduced Pressure Zone assemblyAbove ground
Reduced pressure zone assembly (RPZ)A larger above-ground body with four test cocks and a relief valve hanging underneath that periodically discharges water to the ground.

The big one: two check valves in line with a relief valve hanging underneath that occasionally dumps water on the ground. Required where the system carries fertilizer injection or a higher hazard rating, and usually the most expensive device on the property.

Drain it like this

  1. Shut off the irrigation main upstream.
  2. Set both isolation valves to about 45°, the same half-open winter position a PVB uses.
  3. Open all four test cocks to roughly 45° and leave them there — they are small ball valves too, and a fully-open one can still hold water in its body.
  4. Let the relief valve drain fully — it will weep for a while, which is normal and is the chamber emptying.
  5. Insulate it, or remove and store it indoors where the unions allow. An RPZ is the most expensive thing on the system to replace and it sits entirely above ground.

The mistake to avoid: Wrapping it in insulation without draining it first. Insulation slows a freeze down; it does not prevent one, and a full assembly will still split in a sustained hard freeze.

DC

Double Check valve assemblyUsually below grade
Double check valve assembly (DC)Two check valves in one body with four test cocks and no relief valve underneath, usually installed below grade inside a valve box.

Two check valves in a single body with four test cocks and no relief valve underneath. Often installed below grade in a valve box, which is what makes it the least freeze-prone of the four.

Drain it like this

  1. Shut off the irrigation main upstream.
  2. Set both isolation valves to about 45°.
  3. Open all four test cocks to roughly 45° to let the two chambers drain, and leave them there — like the isolation valves, part-open is the winter position for a ball valve.
  4. If it is in a below-grade box, that box is doing most of the work — but still drain it. A box is not heated, it is only slower to freeze.

The mistake to avoid: Assuming a below-grade device is safe by definition. It is safer, not safe: a box under a cleared driveway with no snow cover freezes surprisingly deep.

AVB

Atmospheric Vacuum BreakerAbove ground
Atmospheric vacuum breaker (AVB)A small single body on a riser downstream of a zone valve, with no test cocks and no isolation valves — it drains through the zone piping instead.

The small, simple one — a single brass or plastic body downstream of each valve, with no test cocks and no isolation valves. Usually one per zone rather than one for the system.

Drain it like this

  1. Shut off the irrigation main upstream.
  2. There is nothing to open on the device itself — an AVB has no test cocks. It drains through the zone piping instead, so it empties when the zone does.
  3. Make sure the zone downstream of it is actually blown out or drained. If the zone still holds water, so does the AVB.

The mistake to avoid: Looking for test cocks that do not exist and concluding the device has been dealt with. An AVB is only as drained as the zone it sits on.

When is your first freeze?

The steps are the same everywhere — the deadline isn't. Enter your ZIP for the NOAA freeze normals at the station nearest you — free, no signup required.

Free · no signup · NOAA 1991–2020 normals

Frequently asked questions

Do I really have to leave the ball valves at 45°?
Yes, and it is the step most people skip. A closed ball valve holds water inside the ball where nothing can drain it, and that water splits the valve body when it freezes. Half open is the winter position.
Should I take the backflow preventer off for winter?
If it is on unions and you live somewhere with sustained hard freezes, yes — it is the single most reliable way to protect the most expensive part of the system, and it takes minutes. Cap or bag the open pipe so nothing nests in it over winter.
Can I blow air through the backflow preventer?
No. Connect the compressor at the blow-out port downstream of the device, never through it. Pushing air backwards through the internals is a good way to damage the poppet and seats on the component you are trying to protect.
Mine is below ground in a box — is it safe?
Safer, not safe. A valve box is unheated and it is only slower to freeze than open air — a box under a cleared, snow-free driveway can freeze surprisingly deep. Drain a below-grade double check the same way you would an above-ground one.

Don't rely on remembering

YardLedger watches the forecast for your address and tells you when a hard freeze is actually coming — then walks the blow-out or startup checklist personalised to the zones, backflow device and shut-offs you've mapped, and keeps the record so next year is easier.

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