In 2006, the American Society of Sanitary Engineering updated their 1060 standard to state that Class-1 enclosures "Shall be heated to maintain an interior temperature of 40° with an outside temperature as low as -30°. Heater source shall be listed by an independent product safety listing organization as "suitable for wet and damp environments." You may be surprised to find that the majority of manufacturers that advertise ASSE 1060 compliance do not provide a heat source, especially one that works with installations that satisfies that requirement.
How do we know?
Now, a decade later, only three enclosure heaters are on the market that meet the requirements of this standard. The state of North Carolina recently needed to retrofit 55 double check assemblies that were in underground vaults to aboveground RPZ backflow preventers in enclosures. They knew they needed freeze protection and so put together a research team to do an in-depth review of all the available enclosure manufacturers with a heater that might satisfy the Class-1 requirements. North Carolina state eventually chose Safe-T-Cover's slab-mounted heater for their project. They informed us of their findings when they put in their order.
The options
First up is WaterSafe®. They refer to their heater unit as a radiant heat solution, but there is not readily available in depth information about the design and its capabilities. The next option is cable-wrap or tape. However, it only provides heat generation of only 5 watts per foot, so it has limited value for pipe sizes larger than 2 inches. It remains a popular method for smaller solutions, and we do sell a Chromalox heater cable. Finally, there is the Safe-T-Cover® patented sealed floor heater. This solution provides heat to both the internal equipment as well as the riser pipes beneath the slab. This unduplicated, rugged 
design has proven reliable for backflow preventer protection in severe environments for years. In fact, side-by-side tests conducted by John Chai for the Department of Mechanical Engineering at Tennessee Technological University in 1997, the floor heater allows for as much as 32% fewer thermostat on-off cycles on cold-weather days.
What's Not Approved?
The majority of fiberglass enclosure and aluminum shelter manufacturers use the wall heater method. Because there must be room for a door, wall heaters must be mounted to one side. They must also be mounted a minimum of 12" off the slab because they are not 
sealed and therefore unsuitable for wet and damp conditions. Since heated air rises, these heaters' warmth channels straight out of the top of the enclosure long before uniform warmth is achieved. In addition to being noncompliant with the ASSE standard, they are also simply unable to deliver the freeze protection provided by a slab mounted heater.
Use an Approved Solution
If your town ever gets frost or snow, you want to make certain your backflow enclosure is ASSE 1060 Class 1 approved and contains a heater that satisfies both requirements. Anywhere that gets below 32°, should be protected in this fashion so the water supply is never interrupted. Now you can make an informed decision about your heater just like the state of North Carolina did.
