Originally Posted by Dan Meyer
Any trapped air in the pressurized water system will behave like an accumulator. This often happens in water heaters. I see this as a useful thing, not a problem.
Water heaters are SUPPOSED to have a head of air in them. The instructions for both Suburban and Atwood/Dometic detail this and how to restore the air "head" which you should do every so often.
PRESSURE-TEMPERATURE RELIEF VALVE
• Do not place a valve, plug or reducing coupling on outlet part of
pressure-temperature relief valve.
A Pressure Temperature Relief Valve, dripping while the water heater is
running, DOES NOT mean it is defective. During normal expansion of
water, as it is heated in the closed water system of a recreation vehicle,
the Pressure Temperature Relief Valve will sometimes drip. The
Dometic water heater tank is designed with an internal air gap at the
top of the tank to reduce the possibility of dripping. In time, the
expanding water will absorb this air and it must be restored.
• Turn off water heater before opening pressure-temperature relief
valve to establish air space. Storage water must be cool.
TO REPLACE THE AIR GAP FOLLOW THESE STEPS:
1. Turn off main water supply (the pump or water hook up source).
2. Let water cool or let run until cool.
3. Open the hot water faucet closest to the water heater.
4. Pull handle of pressure temperature relief valve straight out and
allow water to flow until it stops.
5. Allow pressure temperature relief valve to snap shut; close faucet;
turn on water supply.
6. Turn on water heater and test.
• At least once a year manually operate pressure-temperature relief
valve (FIG 10).
When pressure-temperature relief valve discharges again, repeat above
procedure. For a permanent solution, we recommend one of the following:
• Install a pressure relief valve in cold water inlet line to water heater
and attach a drain line from valve to outside of coach. Set to relieve
at 100-125 PSI.
• Install a diaphragm-type expansion tank in cold water inlet line.
Tank should be sized to allow for expansion of approximately 15 oz.
of water and pre-charged to a pressure equal to water supply
pressure. These devices can be obtained from a plumbing
contractor or service center.