A hot box is a heated outdoor enclosure that keeps a backflow assembly from freezing. You need one where your city requires it or where freezing weather could reach the assembly. Flagstaff's code requires one. In the Phoenix low desert, freezing is rare, and Phoenix's code asks for freeze protection only where freezing can occur.
What is an RPZ hot box?
A hot box is a heated enclosure with a heat source inside it. It sits over the assembly, above ground, and keeps the assembly from freezing when the weather outside gets cold. Flagstaff's code, quoted below, is where the name "hot box" shows up in the sources.
The ASSE (American Society of Sanitary Engineering) publishes the standard for these enclosures. ASSE 1060 covers outdoor enclosures for fluid conveying components, and the group's news release says the standard covers protection from freezing, vandalism and tampering. The release lists backflow prevention assemblies, water meters, control valves and pumps among the parts these enclosures protect. It also says the 2017 edition added a requirement that heaters be listed for wet or damp locations.
Compliance testing under the standard, per the same release, covers items like:
- Air vents and structural integrity
- Testing and maintenance access
- Hinged access panel restraints
- Drainage performance
- Freeze protection capabilities
- Security and locking mechanisms
That list tells you a lot about what a good box has to do. It has to keep the assembly warm, but it also has to let a tester in, let water drain out, and keep out anyone who shouldn't be there.
### Heated box vs. insulated cover vs. plain cage
Not every enclosure heats. Here is how the sources use the words.
| Type | What it does | What the sources say |
|---|---|---|
| Heated enclosure (hot box) | Adds a heat source so the assembly stays above freezing | Febco's manual says the assembly must be protected from freezing using a heated enclosure, insulation using heat tape, or other suitable means |
| Insulated enclosure | Slows heat loss | Watts says that where freezing can occur, install above ground in an insulated enclosure |
| Protective enclosure for security | Discourages vandals | Febco's manual lists a protective enclosure as one way to discourage vandals, and says to leave room for maintenance and testing |
Phoenix's 2024 plumbing code amendment, Section 608.15.2, keeps the standard broad. It says the devices "shall not be located in areas subject to freezing except where they can be removed by means of unions or are protected from freezing by heat, insulation or both." A heated box is one way to comply.
When is a hot box needed?
Three things decide it: where the assembly is, what your city says, and what the maker's instructions say.
### What do the manufacturers say?
The makers use conditional wording. They don't say every RPZ needs a box. They say an outdoor RPZ has to be protected if it can freeze.
- Watts (Series 009): in an area where freezing conditions do not occur, the assembly can be installed outside. Where freezing can occur, it should be installed above ground in an insulated enclosure.
- Zurn Wilkins (975XL2): the assembly may be installed outdoors only if it is protected against freezing conditions. Exposure to freezing conditions, Zurn says, will result in improper function or damage to the device. The location must be kept above 32 degrees Fahrenheit. Zurn also says indoor installation is preferred in areas that are subject to freezing conditions.
- Febco (MasterSeries 860 and 880V): the valve assemblies may be damaged if the internal water freezes. The manual lists heated enclosure, insulation using heat tape, or other suitable means.
All three also say to check local codes first. Febco's manual adds that local codes may require changes to its instructions, and if they conflict, local codes should be followed.
### What does your city say?
Here the answer depends on the place. Two Arizona examples follow, both from the cities' own published rules.
Phoenix. The 2024 amendments to the plumbing code, described above, say devices can't sit where they freeze unless they are removable or protected by heat, insulation or both. The same section says outdoor enclosures for backflow devices shall comply with ASSE 1060. In other words, if you build an outdoor enclosure in Phoenix, the code points you to that standard. The sources reviewed for this page show no Phoenix rule that requires a hot box on every outdoor assembly.
Flagstaff. Flagstaff is a different story. Its city code, Chapter 7-03, says the approved assembly shall be installed above ground, as close as practicable to the service connection before the first branch line, and "in a hot box with electricity for heat." It adds that the heated enclosure must be ASSE 1060 certified or similar. The electrical work must follow city-approved building code and City Engineer standard drawing 19-02-025. The city's backflow page describes it this way: the preferred method is outside, close to the meter, with a heated enclosure for freeze protection. It also says a variance can be made for indoor installation, subject to city approval before construction.
Other cities may have their own wording, and this page does not cover them. If you're outside Phoenix and Flagstaff, your water provider's cross-connection office is the place to ask.
How real is the freeze risk in Phoenix and in higher-elevation Arizona?
Freeze risk is low in the Phoenix low desert and real in Arizona's high country. The National Weather Service records below show the gap.
### Does Phoenix really need freeze protection?
Honestly, not often. The National Weather Service Phoenix office's 2024 year-in-review lists the number of days with a low at or below 32 degrees. The normal (1991 to 2020) is 0.6 days a year. In 2024 the count was 0, and the lowest temperature was 35. That is one official station's record, not every neighborhood's. Still, it shows how rare a freeze is there.
For a Phoenix-area business, that means:
- A hot box is generally not the fix for a Valley assembly.
- The maker's instructions still apply. They talk about freezing conditions, and if you have a rare cold snap, an unprotected assembly is the risk.
- An enclosure may still make sense for security. That's a separate question from freezing. For theft, see what to do when a backflow preventer is stolen or damaged. For general freeze risk to pipes on the low desert, see can pipes freeze in Phoenix.
### What about higher-elevation Arizona?
Here the picture flips. The National Weather Service Flagstaff office says the average January low in Flagstaff is 15.4 degrees Fahrenheit, and that temperatures can dip into the single digits and even below zero. Its page also says most high terrain areas in northern Arizona can see near and well-below freezing temperatures throughout the entire winter. Flagstaff's record low, it notes, was minus 30 in January 1937.
Flagstaff's own page ties its heated enclosure to freeze protection. If you own or manage a property in the high country, freezing is a normal winter event, and the assembly needs real protection: heat, not just a cover.
This page reviewed only Flagstaff's code in detail. Other high-country towns may have their own rules. Don't assume a rule from one city carries over to another.
What else should a hot box get right?
A box that keeps the assembly warm can still create problems. The sources point to a few:
- 1Access for testing. Febco's manual says to install any protective cover so that adequate clearance is available for maintenance and testing. Zurn's installation drawing shows the protective enclosure as optional, which fits its wording that the assembly must be protected against freezing one way or another.
- 2Drainage and a clear relief port. Flagstaff's code says all test cocks and relief ports must be accessible for testing and for release of water from the relief port during a discharge event. An enclosure has to leave the relief water a way out. For the drain side, see why an RPZ valve needs an air gap and how it is piped.
- 3No fiberglass wrap. Flagstaff's code says fiberglass insulation can't be wrapped or placed around an assembly for freeze protection, because it allows condensation and later damage. It also says there must be adequate air flow around the assembly.
- 4Electrical work done to code. A hot box has a heater, so it needs power. Flagstaff points to city-approved building code for the electrical work.
For seasonal shutdown, Febco's manual also describes a drain-down procedure for its assemblies, including a shutoff valve upstream of the freeze-protected area. That work touches a backflow assembly, so follow the maker's instructions and your water provider's rules. Arizona's rule requires a test after installation, relocation or repair, and testing has to be done by a certified tester.
For the wider placement rules, see where an RPZ backflow assembly can be installed. If the assembly spits water, why an RPZ backflow preventer spits water explains what that means. For annual testing rules, see how often a commercial backflow preventer needs testing in Arizona.
When should you call a plumber?
Call for help if:
- You're not sure whether your city requires a heated enclosure.
- Your assembly sits outdoors in a place that freezes, with no protection at all.
- The enclosure blocks the test cocks or the relief drain.
- A cover or box was damaged or stolen and the assembly is exposed.
Testing the assembly must be done by a certified tester. For plumbing help around it, such as drain lines, supply piping and fixes to keep water from going where it shouldn't, HQ Plumbing & Air is licensed and insured, and gives free estimates and upfront pricing. We're available 24/7 for plumbing emergencies.