Originally Posted by lewster
Not exactly sure why, but it may be an RVIA design and certification requirement. AFAIK, a check valve on the hot side (I have seen these on just about every water heater installed by the OEMs) will keep the cooler water that resides in the piping from back-filling the tank.
Now it makes sense.
A check valve on the inlet
side will keep hot water from back-filling into the supply cold water piping. A check valve on the outlet
side will keep cold water from back-filling the water heater.
However, a check valve on the inlet side does not provide a second line of defense against overpressure in the water heater (the first being the T&P valve on the water heater itself).
As the water in the heater tank expands, a check valve on the outlet
will allow the excess pressure to escape out of the pressure relief valve in the inlet water piping before it pops the T&P valve.
On the other hand, if the check valve is on the inlet
side, the pressure will only have one way out, the T&P valve. The T&P valve is set at a much higher pressure than the regular relief on the inlet piping and may overpressure the piping before it pops.
This assumes you don't have an accumulator to absorb the excess pressure.