Guide

How to Measure a Backflow Preventer for a Freeze Bag

5 min readBy The Backflow Depot, Rocklin CA

To measure a backflow preventer for a freeze bag, take the full installed assembly—length, width, and height including shutoffs, test cocks, and fittings—plus the pipe size stamped on the device. With those we match one of the five standard bag sizes (the next size up if you’re between), and for 3" and larger devices send photos of the tag and the assembly instead of guessing.

What to measure on the backflow preventer (and what to skip)

A freeze bag (insulated pouch) wraps the assembly. Backflow freeze bag sizing means matching bag dimensions to the device footprint—not building an ASSE 1060 enclosure with service clearance and heater space. Your backflow preventer measurements only need to cover what the bag has to wrap.

Measure:

  1. Length — along the pipe run, end to end of the assembly.
  2. Width — widest point, including handles, test cocks, unions, and any gauges.
  3. Height — from the lowest pipe/fitting to the highest component.
  4. Pipe size — ¾", 1", 2", 4", etc. (from the device tag or shutoff stamps). Pipe size is the fastest first filter: ½"–2" devices take a standard bag, 3" and larger are sized from photos.

If anything sticks out past the body, include it. If you’re between two sizes, go up one size so insulation covers everything.

Fast path: read the tag, then size

Before you pull a tape, clean the metal ID tag. Brand, model, and size on that tag are what we use to match bags and kits.

Standard backflow freeze bag sizes (Sub Zero Covers)

We are an authorized Sub Zero Covers dealer, and their insulated covers are made in the USA. Standard bags fit ½"–2" devices and come in five sizes, listed on the shop as 20x20, 24x24, 30x24, 36x24 and 42x24.

Pipe size only tells you that a standard bag applies. Your L×W×H decides which of the five. Send us the three numbers, or call, and we will match the size; if you are between two, go up one.

For 3" and larger devices there is no size to pick yourself. Submit the tag and assembly photos through the photo sizer and our staff pick the bag for you.

FAQs and brand story: Backflow freeze bags by Sub Zero Covers. Shop the five standard sizes: Backflow Freeze Bags collection and the shop freeze-bags page: shop freeze bags.

Install check (so the bag actually works)

  • Cover all water-bearing brass—body, shutoffs, and test cocks.
  • Close straps/closures snug; leave the bag on through cold snaps; pull it for annual testing.
  • If you also shut down and drain the assembly for winter, California requires a field test by a certified tester before it goes back in service (CCCPH §3.3.3). Check with your water purveyor.
  • Bags buy time by holding residual heat. They’re not a heated enclosure. For when each wins in NorCal winters, see Freeze bag vs heated enclosure.

Still unsure?

Email a clear photo of the tag and a side shot of the assembly to sales@thebackflowdepot.com, or call (916) 251-7071. We’re at 4487 Pacific Street, Unit E, Rocklin CA 95677, Mon–Fri 7am–4pm.

A worked example: a 1" residential RP

Say the plate on a residential reduced-pressure assembly reads 1". Pipe size puts it in standard-bag territory, so the question is which of the five. Now measure:

  • Length along the pipe from the outside of one shutoff to the outside of the other.
  • Width across the widest point, which on most RPs is the test cocks and shutoff handles, not the body.
  • Height from the lowest fitting to the top of the tallest component.

Send us those three numbers, or call with them, and we will tell you which of the five standard sizes covers the assembly. If it is close between two, we go up one. A bag that is slightly loose still insulates; one that is slightly small leaves brass exposed, and exposed brass is where the freeze damage starts.

Measuring mistakes that lead to the wrong bag

  • Measuring the body only. The shutoffs, test cocks and unions are part of the assembly and all of them hold water.
  • Forgetting the handles. Ball-valve handles add width. Include them, or turn them parallel to the pipe before you measure.
  • Ignoring height above grade. A bag has to reach down over the lowest fitting, so measure to the bottom of the pipe, not to the ground.
  • Rounding down. Between two sizes, go up.
  • Trusting the pipe size alone. Pipe size is the first filter, but layouts vary. The L×W×H is what decides the size.

When to use the photo sizer instead

For devices 3" and larger, or any layout with extra fittings, meters or an unusual orientation, the tape becomes a fight. Use the free photo sizer at app.thebackflowdepot.com: a square-on photo of the tag plus a front and a side view is enough for us to match a custom bag.

Frequently asked questions

Do I have to take the bag off for the annual test?

Yes. The tester needs the test cocks. Pull the bag, test, and put it back on. The bags are built for that.

What if my assembly is between two sizes?

Go up one size so the insulation covers everything. A little extra room does not hurt performance.

Can I get a bag for an odd layout?

Yes. Sub Zero Covers makes custom-sized covers, and as an authorized dealer we can order them. Send the tag photo and measurements and we will quote it.

Does the bag need to cover the shutoffs?

Yes. Cover all water-bearing brass — body, shutoffs and test cocks. That is why width and length are measured to the outside of the shutoffs.

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