That depends. A house that I owned up until 6 years ago was on top of a
hill. When the water supply failed (e.g. when there was a fire on my
circuit and the pumpers went whole hog) the water from the tank got sucked
back into the mains and the heating elements on my heater burned out ...
happened several times ... I was forced to install a check valve.
As the others have asked, why? Siphoning the water tank dry sounds pretty
unlikely if all taps are closed.
But anyhow, your water meter may already have one as I believe most newer
meters are required to have them (
There are (at least) 2 possible reasons for a check valve on the inlet
of a hot water heater.
1 - Some municipalities require a check valve to prevent the flow of
water from the house back to main.
2 - A check valve is used when a recurculating system is used to
prevent long waits for hot water at fixtures far from the heater.
If you do use a check valve, you should also use an expansion tank in
between the valve and the water heater. If you don't, you run the risk
of having water leak from your pressure relief valve when the water
heats up and has no place to go due to the check valve.
- I am tempted to say that is impossible. Even if a siphon could be
- established, if no taps are open the water in the heater couldn't get
I'd like to agree, but this thread seems to indicate that its possible:
That is strange. They require "backflow preventers" (2 check valves)
on every water service around here. You might be able to contaminate
your own water lines but your water will never get back to the street.
If it is required by code, it will be required at the house supply
inlet, usually where the service enters the house (unless it is part of
the meter). It won't specify that it is neededon the water heater.
Someone else pointed this out but I will add it again. If a check
valve is placed on the heater inlet, then an expansion tank should be
The solution is to install a syphon breaker valve. Run the supply line up above
the top of the tank and place the syphon breaker there. Tee off just below the
syphon breaker and run pipe to the water heater inlet.
If there is any syphon on the water line, the valve will open and let air in,
breaking the syphon. Because the valve is above the tank, the water in the tank
is trapped. This is code where I am.
If you use a check valve, it has to be a two way check valve with a relief port
to allow expansion to escape back into the water line when the water is heated.