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bus driver
water heater bypassing

the gas water heater, which is 11 yrs. old, is constantly bypassing from the overtemp/overpress valve. I have replaced the valve, drained and refilled the tank and replaced the thermocouple.It's not much,just a trickle. If you put press on the valve stem/lever it stops, but as soon as you release it starts again. Water temp is set at the A setting ( Hot,A,B, Very hot) The burner is not constantly on, and there is plenty of hot water. No leaks noted. Is it time to "bite the bullet" and get a new one,or is there a fix ?

Re: water heater bypassing

Do you know the water pressure coming into your house?

Are you on city water or a private well?

Re: water heater bypassing

Just a few thoughts (most likely culprits) to start with......

Since you've already replaced the T&P safety valve, odds that the new one is faulty are kinda slim. It *can* happen though.

If the water isn't scalding hot, then the odds that the temp is set too high is virtually nill.

If you're on city water....is there a pressure reducing valve in place? If so, it may be malfunctioning; in need of repair or replacement.

If you're on city water and they recently installed a check-valve to prevent back-flow from your house into the city line, then you may no choice but to install a small in-line expansion tank to absorb the increase in pressure from the expanding water (when the heater fires up).

bus driver
Re: water heater bypassing

OK It's city water,there is a press reducing valve that's set at 50 psi.The tank was drained when I replaced the overpress & overtemp valve. Water temp is approx 130. Is it possible that there is air trapped in the tank not submerging the valve sensor ?

Re: water heater bypassing

If you flip the T&P valve open and only water comes out......no, not likely there's any problematic air trapped around the T&P.

If any air comes out, leave the valve open until only water comes forth. Now it should be completely submerged.

You could also try reducing the temp setting a bit. The thermostats (aquastat) in gas heaters have a tendency to develop a "flat spot" after a while....which means they may "hang". Seems they usually hang-up on a call for heat, but yours may be hanging when it should turn off the gas/burner. I'd say try turning it down approx. 10 degrees F or so and see what happens. If that resolves the problem, leave it there for about two weeks or so. Then turn it back up to your desired setting and see if the problem returns. It's a cheap and easy trial to see if the thermostat is at faltering. (I have to do this to our gas WH about once every 4 years or so. Two weeks at a different setting seems to "remove" the flat-spot and we're good to go for a long time.)

If the above doesn't resolve the problem, you may have a totally shot thermostat........ or a pressure reducing valve in need of cleaning/repair/replacement. To ascertain the function of the pressure-reducing valve, you'd have to install a pressure gauge somewhere downstream to acquire a reliable reading...if there isn't one in place now. If the thermostat is shot altogether, then it might be time to think about getting a whole new WH rather than investing that much time & money in an older machine.

The other possiblility is that you did indeed manage to drag home and install a faulty new T&P valve. It happens. Not that often, but it does once in a great while.

Evidently you have no reason to believe that the city recently installed a back-flow-preventer valve on your supply. ???

There's also a chance that you have a tiny piece of debris lodged in the seat of the T&P valve. Flipping it open and letting water flush out may remove that bit of debris. But....if the debris has been trapped in the seat for weeks on end......it may not be easily dislodged now.....or may have created a crease in the seat which will cause the leak to continue even if the debris gets flushed out.

Edit: Upon further thought (and the light of a new morning )...........it seems you should be able to narrow down the hunt a bit pretty easily.

Turn off the WH and run say, 5 gallons or more of water from a hot water tap/faucet somewhere. This should substantially lower the temp of the water in the holding tank. If the drip continues, then the problem isn't being caused by too high of a water temperature. The problem is then likely related to either a faulty T&P valve, a T&P with a piece of debris lodged in the seat......or from too much water pressure.

A pressure gauge should show you if there's a problem with the pressure-reducing valve.

Re: water heater bypassing

The pressure reducing valve is a backflow preventer. You need to install an expansion tank. Your relief valve is merely doing the job that it is supposed to be doing...relieving excess pressure.

Re: water heater bypassing


I was thinking that his pressure-reducing valve may be malfunctioning and his standing pressure is actually above 80...thereby causing the dribble from the T&P.

Perhaps presumptuous on my part, but I got the impression that this is dribbling is a new occurance on an old existing installation.

If he has air-cushions (anti-hammers) in the plumbing........those might have been absorbing the expansion in the past when the PR valve was functioning properly and pressure was 50-60ish. At 80 they might not have enough left over capacity to do that job anymore.

Re: water heater bypassing

Dave is right.

You need to install a thermal expansion tank. When water heats it expands and this is creating pressure in your system.

They did a segment on this very problem on Ask This Old House. The homeowner’s T&P valve had a constant drip. When Rich attached a pressure gauge to the outlet he read 170 psi. After they put the thermal expansion tank in the pressure dropped way down.

Re: water heater bypassing
goldhiller wrote:


I was thinking that his pressure-reducing valve may be malfunctioning and his standing pressure is actually above 80...thereby causing the dribble from the T&P...

That's exactly what happened with my system. The Pressure regulator for the whole house was malfunctioning, and letting the pressure creep up. The T/P valve on the water heater - set for 120 psi - was blowing intermittently, due to the faulty regulator letting the pressure build up. I actually measured the pressure build-up with a gauge. It would climb slowly after each water use. One other symptom, btw, was that the initial opening of a faucet - hot or cold - anywhere in the house, would get an initial 'burst' of pressure, then drop to normal.

Replacing the regulator solved the problem. Guy at the plumbing supply house said it's a very common problem as regulators age. He said average life of one is ten to twelve years (mine was fifteen). BTW, the new regulator cost $68.00. I replaced it myself in about fifteen minutes, and it solved the problem.

Just out of curiosity, I took the old regulator apart. It was full of lime-like build-up from the minerals in our water.

Re: water heater bypassing

Let me repeat...A pressure reducing valve IS a backflow preventer.

The problem is not a faulty valve...the problem is that there is a valve.

Add the expansion tank.

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