Field note
A new 2-inch DCVA on a vertical riser, with an expansion tank
A 2-inch double check valve assembly (DCVA) was installed on the vertical riser after the water meter. A thermal expansion tank was added, because the check valves turn the building's plumbing into a closed system. The tank absorbs the expansion instead.
Before and after
The service enters beside the door, passes through the meter and rises to the ceiling. The assembly went on the vertical section, which keeps the test cocks at eye level and the floor clear. The expansion tank hangs off the horizontal run below it on the building side.
Why the expansion tank
Before the assembly, water that expanded as the heater warmed it could push back toward the main. Two check valves in series stop that, which is the purpose of the device. They also make the building a closed system. With nowhere to go, the expanding water raises the pressure until a relief valve opens. A thermal expansion tank gives that water somewhere to go, so the relief valve stays shut and the pressure stays steady.
This detail is easy to miss on a backflow installation. In our experience it shows up later as a dripping relief valve on the water heater.
Permit and first test
A new assembly needs a building permit (in Toronto, a stand-alone plumbing permit). We test it at installation and file the report with the municipality. The Toronto page sets out both steps for the City. The other program guides cover Peel, Halton, York and Durham.
Questions
Do I need an expansion tank when a backflow preventer is installed?
Usually, yes, if the building has a water heater or boiler on the domestic side. The check valves stop expanding water from pushing back into the main, so pressure rises as the water heats until a relief valve opens. An expansion tank absorbs that expansion instead.
Can a DCVA be installed vertically?
Many models are listed for vertical installation when the flow direction matches the listing. The manufacturer's listing and the municipality's program decide.