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Saturday, October 31, 2009

P&T Relief Valve Questioned

I have a two year old travel trailer. Recently, the pressure relief valve on the hot water tank began dripping regularly. The water appears to be over-temperature. What is my problem? Could it be the thermostat? Can I fix it myself?
S. Farmer, (e-mail)


Steve, though a faulty thermostat may be the cause of your symptoms, I would first look closely at the pressure/temperature relief valve itself. The P&T relief valve is an interesting device. Contrary to what some may believe, this valve will normally drip some water during each and every heating cycle. Here’s why. As any containerized liquid is heated, that liquid will expand causing not only a rise in temperature, but also a rise in pressure, (a percolator effect, if you will).

Since the RV water heater is a tank filled with water, if there were no method to absorb or control this expansion during the heating cycle, the unchecked pressures and temperature could rupture the tank resulting in serious injury. Temperatures below 210-degrees Fahrenheit are considered relatively safe. Therefore, all P&T valves on today’s water heaters are pre-set (non-adjustable) to open at 210-degrees F.

In the small confines of the RV water heater, the water is heated very quickly with a rather inordinately large LP burner flame, so keeping up with the drastic fluctuations of both temperature and pressure is no easy task. Additionally, in RV water heaters there is usually a cushion of air at the very top above the water level that acts as an accumulator and buffers the water. It also allows space for the water to expand into while being heated. This air (oxygen), is eventually absorbed into the oxygen portion of the water. At this point there is no place for the expanding water to move into since the tank is literally completely full. The P&T valve then does its job of becoming a virtual hot water faucet – it opens. Expelling hot water from the outlet of the valve allows more cold water to enter the tank (lowering the temperature) and the valve snaps shut.

Usually, draining some water from the water heater tank will reinstate this cushion of air if excessive dripping is encountered. The fact that this dripping of the relief valve seems to be more prevalent today is attributable to the design of the modern water heater. Back in the good ol’ days all water heaters were thermostatically controlled manually. Today, with the prominence of electronic ignition, manual control of the temperature of the water is taken away. On many units the thermostat is a pre-set thermal switch which electrically turns off the heating sequence when the pre-set temperature has been reached.

There will continue to be P&T valves that simply fail over time, but by and large all will drip occasionally. They must, however, drip only during the heating cycle. If indeed they drip or weep during non-heating phases and the pressure within the fresh water system is less than 150 PSI, then the relief valve may indeed be faulty. The P&T valve is easily replaced, though it may take a special tool. And it is relatively inexpensive, considering. All well-stocked RV part stores will carry them.

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