It is not uncommon for houses that have supply water pressure regulators to have leakage from the relief valve on the water heater. Most water pressure regulators simply close on over-pressure and do not relieve so they act as a check valve. The solution is to add an expansion tank.
hot water tanks have pressure relief valves on them in case the thermostat fails on and also for expansion of cold water. when you use a lot of water you will find water leaking out of it as it expands.
I thought it was more an issue of closed system vs. open system. A domestic hot water heater serves an open system, right? Plus the temperature is what, 140 deg F max? A residential basebaord system would be closed and there normally is an expansion tank. A boiler for heating purposes might run at higher temperatures (180 to 200 deg F or more, commercially anyway).
Also, like someone else eluded to, the domestic hot water heater has a temperature and pressure relief valve that will discharge (leak might be a poor term) to drain when necessary.
The relief valve on a water heater is for safety in case of some malfunction and will soon fail if used constantly to relieve expansion pressure, which is seen as dripping from the valve. So I would not call it a discharge. The first attempts to fix the problem are usually to replace the "leaking valve".
What do imply when you use the word "require"? Codes may "require" differently as it relates to BFP, while systems( with leaking PRV's) may "require" due to some physical need relating to the hydrostatic pressurization as it relates to the process.
Using a relief valve rather than an expansion tank is an acceptable solution but it should not be the one on the hot water tank. Hard mineral build-up from leaking hot water and subsequent evaporation will soon ruin the valve. The valve is best located in a cold water line and the leakage flow needs to be managed. Most tank installations are not designed for regular flow from the relief valve.
Most residences have pressure reducing valves installed on the cold water supply to the home. These are generally "drop tight" and allow no creep in downstream pressure during no flow situations. They will also act as a check valve as the seat is held closed by the high pressure side. A relief on a domestic HWT is usually a Temperature/Pressure relief type which is a safety valve and should not be used to accomodate the release of water due to expansion. Most residential domestic piping systems are capable of accepting the increase in pressure during HWT heat up. If the tank relief valve is dripping, an expansion tank should be installed, or the system pressure reduced. Here in Canada, most DHWT relief valves are set to release at 125psig. If your tank is reaching that pressure and the valve is releasing, you are probably overpressuring the plumbing system overall.
