When the temperature and pressure (T&P) safety valve on the water heater leaks, it is usually a simple repair method; just replace the valve. These valves cost less than $15, and replacing the valves is a very basic job—just drain some water from the water heater, remove the drain pipe, and replace the valve. no big deal.

Unless it starts leaking again.

If the newly replaced T&P pressure relief valve starts to leak again, it may mean that the pressure relief valve is only working properly; it can reduce the excess pressure in the water heater. When this happens, the repair will be more complicated. Let me explain.

When the water heater heats the water in the water tank, the water expands. When this happens, the water usually eventually expands back from the cold water inlet, all the way back to the water supply system that enters the house. The house’s municipal water supply is like a huge expansion tank...no one noticed. This is shown in the figure below.

What happens if a check valve or check valve is installed on the water supply pipe of the house? The water has nowhere to go. When the water heater heats the water, the water expands, increasing the pressure in the now basically closed system. When the pressure is high enough, the T&P safety valve on the water heater will come into play and release the excess pressure by leaking water.

In Minnesota, it is rare to install a check valve on the water supply line of a house, but it is quite common to install a pressure regulator. When the pressure of the water supply entering the house is too high, it is necessary to install a pressure regulator on the water supply main to prevent damage to the plumbing components in the house. The problem that these regulators may cause is that they will act as a check valve; they will allow water to enter the house, but they will not let the water flow back. This creates the so-called "closed system".

When this happens, the T&P safety valve of the water heater may leak. This does not happen every time a pressure reducing valve is installed, but other problems may occur in the house, such as the toilet filling valve randomly refilling the toilet, or the faucet dripping for a long time.

Solution When there is a closed system on the water distribution pipeline in the home, an expansion tank needs to be installed somewhere in the pipeline system. This is a fairly simple and straightforward fix; the expansion tank will send the water somewhere when the water expands, and the T&P safety valve on the water heater will stop causing the problem.

This rule also applies to hot water heating systems; when the boiler heats water in the circulating heating system, the expansion tank allows the water to expand without leaking the pressure relief valve. If the pressure relief valve on the boiler system leaks for a long time, even after it is replaced, it may mean that the expansion tank is faulty.

Reuben Saltzman, Structure Tech Home Inspections-Email-Maple Grove Home Inspector

Reuben Saltzman is a second-generation home inspector who is passionate about his work. Naturally, this blog is about house inspections and family-related topics in the Shuangcheng Metropolitan Area. In addition to working at Structure Tech, he is also a real-life sales assessment for housing in Minneapolis, St. Paul, and several other cities.

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