Hot water expansion problem

[kk]

I have a System 2000 boiler which has a seprate hot water storage tank which is heated by a heat exchanger. Everything worked fine for over a year after it was installed, but lately it has started over-pressuring, which causes the T&P valve to open. I put a gauge on the tank to verify what I already knew - yes, it's really over pressuring. The telltale sign was the brief high pressure occasionally when turning on a faucet.

In looking at the system, there is no expansion tank on the hot water side, and the only thing inline is the PRV at the water meter, which is a Watts 25AUB. This PRV is supposed to have an internal bypass if the internal pressure exceeds the street pressure, so it would seem it should prevent the 150-160psi I'm seeing.

Finally my questions:

1. Could the street pressure really be greater than 150psi? If it was, the bypass would not do anything and my system would overpressure. I called the water company but they only had a really old reading of 100psi for my neighborhood. Not sure how good that number is.

2. Could the bypass in the PRV be stuck? It appears from the drawings it's a spring loaded ball of some sort. I think the PRV was installed about 11-12 years ago.

I hate to put in an expansion tank if the problem is really somewhere else, and I hate to replace the PRV if I really need an expansion tank.

Any input would be appreciated!

-kk

 

[hj]

Prv

Unless you know the city pressure there is no way to tell if the bypass is working. If the pressure stays at a constant low psi when it is not heating, then the PRV itself is functional. You cannot go wrong by installing an expansion tank because that will minimize the pressure reaching the level of the city supply whatever that is.

 

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[Jadnashua]

Just to be clear, the T&P that is dumping is on the storage tank (indirect water heater)? If it was on the boiler, then the boiler would usually shut down.

The by-pass won't do much for you if the street pressure has spiked high as well. An expansion tank is probably a good idea, regardless.

Depending on how high the street pressure is, you may need two PRV's in series.

 

[kk]

Just to be clear, the T&P that is dumping is on the storage tank (indirect water heater)?

Yes.

Water pressure when this isn't going on is 60psi, so it seems the PRV is working.

Perhaps this problem was on the edge of happening all along and the city water pressure increased just enough to push things over the edge. I was eyeing the drain port on my main shutoff valve (on the city side of the meter) wondering if I could come up with a way to measure the pressure there without taking anything apart.

-kk

 

[kk]

Wanted to follow up on this - I put an Amtrol ST-5 expansion tank on the cold feed to the hot water tank. Problem solved. It doesn't take a very big expansion tank to make a very big difference in pressure in the system.

Thanks for the advice.

-kk