water heater leak from safety valve

[pdmcclan]

I have two 40 gallon water heaters installed side by side.  They are both less than 3 years old.  From time to time they both leak a small amount of water out of their safety valve.  Valve doen't appear to be popped/opened when I see the water on the floor. This is infrequent but seems to happen when I have the heaters turned up and there is a lot of hot water being used (when the grandkids are visiting).  Any ideas what is going on?  I even replaced one of the safety valves but it did the same thing a few days later.  Not a lot of water, less than 1/2 a cup from each heater.  Any ideas/suggestions would be appreciated by this very novice repairman.

Thanks.

[domain]

I do not have much experience with water heaters but understand how they work. Since a new valve did not cure the problem, I would suspect your thermostat may be reading incorrectly. This may cause the water to get hotter than it thinks it is, then the safety valve is doing what it needs to do by relieving water pressure. 

[ScottyMo1]

Do you have a thermal expansion tank installed?

[tgoods]

I agree with Scotty, install an expansion tank on each of your water heaters, should take care of the problem.

[jimbo6679]

The most common cause of weeping from the TP valve is excessive pressure.   The valve would "pop" full open at 150 PSI, but they are prone to weep beginning around 110 PSI.
Two things can cause pressure problem:
(1) The pressure regulator on the house main is defective.   Commonly, pressure would be normal or low when any faucet is in use, then creep up past the set point when the water is off.   House pressure will ultimately see full street pressure.    You can get a $10 gauge at the hardware store with a telltale hand, to test this.
(2)   Thermal expansion.   Water expands when heated...not much, but enough!   Some codes now REQUIRE an expansion o be installed, and all water heater manufacturers recommend it.

Since you have 2 WH, and the issue is especially noticeable when heating a lot of water.....I suspect you need the expansion tank.    Again, the little gauge will help you evaluate .

[JWWebster]

Try turning the thermostat to lowest position and see if it stops the weeping is what I would do. PS you will probably save on water bill.