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The myths:
Water heaters require an air space at the top of the tank for the water to expand.
The P/T valve leaks when heating water because there's no air cushion in the water heater.
The P/T valve on my water heater must be bad because it's leaking.
The manual for my water heater says the P/T valve leaking while heating water is normal.

Those people that are weekend campers, or the occasional seasonal camper that drains their tanks after the short week long trip, or when moving from site to site may never see these symptoms. Those that full-time will likely be much more in tune with what it is I have written here. Regardless, this information can easily apply to everyone and I provide it here for the benefit of everyone that reads it.

The facts:
Taken from an old post of mine about water heater "air-cushion" and leaking Pressure/Temperature (P/T) valves, here is a discussion I hope may help to clear the air about the mystery of the leaky P/T valve.

Some people think that an air-cushion inside the water heater will prevent the P/T valve and fixtures within their rigs from leaking. While this is true to an extent, during normal use, the air within the water heater diminishes over time. When this happens and the water expands from being heated, it has to go somewhere. In a CLOSED WATER SYSTEM, such as normally found in standard RV plumbing, the places the water can go are; out a faucet, toilet, shower, out the P/T valve or, into an accumulator tank!. Another possible location could be through a cracked or otherwise damaged plumbing component (fitting or line) caused from excessive system pressure. This would be the worst case and if left unchecked could cause major damage to your rig.

Other people seem to think that when the pressure valve starts leaking it's because the valve is defective. While this can be quite true in some instances, usually the valves themselves are not the problem at all. In the event your P/T valve leaks or dribbles all the time, whether the water heater is actually heating water or not, then you probably have a defective P/T valve and should be replaced. On the other hand, if you notice that the only time the P/T valve leaks is when the water heater is actually heating water, then you do not have a defective valve. In fact, the valve is doing exactly what it is supposed to do and that is to relieve excessive pressure from within the system. In a properly functioning water system, these valves should last many many years without ever leaking. When was the last time you replaced one on your home water heater? Likely, never and nor should you have to on an RV water heater. These systems work exactly the same way. The only real difference between them is the overall size of the system is much smaller in an RV than it is in your home system.

If you have a closed water system at home, there will be an accumulator or expansion tank installed somewhere in the plumbing, usually at the cold water pipe that feeds the water heater.

The theory of water hydraulics:
As stated above, heated water expands. If there is an air cushion within the water heater, the P/T valve won't leak because the expanding water will compress the air inside the tank. Once the air space has diminished, as it always does with normal use, this expansion can cause the P/T valve to leak or seep along with other potentially serious problems. Expanding water can cause metal pipes to expand and eventually crack open causing a flood in your rig. Plastic pipes can be stressed beyond their limits and break open also. Leaking faucets are another common symptom caused by heated water. In reality, the 'air-cushion' really is required, BUT, the location of it residing within the water heater itself is a poor one because of the fact it does indeed diminish over time requiring periodic replenishing.

The pressure/temperature relief valve is supposed to protect the water heater AND plumbing from these potentially serious problems. When you see it leaking, it's doing it's job just as it's supposed to. This means the air inside the tank has diminished completely and there's nowhere for the expanding water to go, except out the P/T valve or other location as mentioned above. If you want to be one of those persons that goes out and pulls the drain plug or anode rod from your water heater to drain it and reestablish the 'air-cushion' every few weeks, by all means go ahead and do it. Personally, I have other things to do and little minor things like this are just bothersome. It's simply not necessary and there is a simple, better solution.

The solution:
An accumulator tank (also know as an expansion tank) provides a permanent air cushion for the expansion of heated water. The addition of an accumulator simply moves the air-cushion from within the water heater, to a remote location outside the water heater which is a much better place for it to be. It solves the problem of having to deal with reestablishing the air-cushion in the water heater on a periodic basis. It will give you more hot water! Because the air-cushion inside the water heater is replaced by the accumulator tank at a remote location, your water heater can be completely filled, thus giving you the entire capacity of water it was actually designed for! An accumulator tank is the proper cure for P/T valves that leak due to water expansion from heating. It will also help with water pump oscillation (constant cycling) and help to quiet down an otherwise noisy water pump. There are no downsides to installing one of these devices to your current system. It truly is a worthy upgrade.

In conclusion:
Very simply put, a water system that leaks anywhere is an inferior system to one that has no leaks at all. Would you put up with the P/T valve on your home water heater leaking? If your kitchen or bathroom sink leaked every time the water heater came on, would you be happy with that? Of course not, you'd probably call a plumber to fix it or, those that are handy about the house would fix it themselves. There's no valid reason why your water system in your rig should be treated any differently than your water system at home! If your water system at home doesn't leak, then the water system in your rig shouldn't leak either.

Fix the REAL problem, not the symptom. Get an accumulator tank, install it, then CAMP ON!

Yo-