Hot Water Heater - Pressure Relief Valve Keeps Blowing

[Kevin Autrey]

Hi -

I've got an A.O. Smith HWH that repeatedly triggers the pressure
relief valve to open. The valve is rated at 210 degrees (F) and 150
PSI.

The plumber has replaced the valve a couple of times, so I don't think
that it's a faulty valve component that's at work here.

When it happens, it's usually when an outside tap is turned off -
although it has also happened when an inside tap is turned off.

Could there be something related to rapid changes in water pressure
that might cause this problem?

Can anybody suggest solutions?

Thanks,
Kevin

[Speedy Jim]

I suspect that your street water pressure is very high (over 100 psi).
The sudden closing of a tap creates a shock wave of very high pressure
which is enough to trip the T & P open.

You can get a pressure gauge to attach to a hose tap at a hardware/
sprinkler supply house if you want to measure the presure.

If it *is* high, you really should have a pressure reducing valve
*and* an expansion tank. The reducing valve cuts down the pressure to
all fixtures within the house (the outside taps could byapss it).
The expansion tank is fitted to the hot water heater to absorb
pressure created by expanding water when it's heated.

You *might* get away with just the expansion tank on the heater.

This may all sound like over-kill, but it's what is really needed
to solve problems like this (and is required in many localities).

Check the pressure first to decide where to go.

Jim

[Jay]

}I've got an A.O. Smith HWH that repeatedly triggers the pressure
}relief valve to open. The valve is rated at 210 degrees (F) and 150
}PSI.
}The plumber has replaced the valve a couple of times, so I don't think
}that it's a faulty valve component that's at work here.
}When it happens, it's usually when an outside tap is turned off -
}although it has also happened when an inside tap is turned off.
}Could there be something related to rapid changes in water pressure
}that might cause this problem?
}Can anybody suggest solutions?

}Kevin

What is the water pressure on the supply line coming into the house?
Plumber should be able to tell you. You may need a pressure regulator.
Normal city water pressure is in the 50 - 60 psi range.

[Mark Cravatts]

I had the exact same problem.
This is when I replaced my original unit
with a larger 50 gallon A.O. Smith unit.

This is what happened in my case:

I have my house pressure set to 70psi, a little
on the high side, but we enjoy strong showers.
First make sure that the house water pressure
is within range. Typically they are set to
50psi. If after turning off all of the faucets
with the hot water heater turned off, the pressure
keeps creeping higher and higher until the relief valve
pops, I would suspect that the seal in your house regulator
needs to be replaced. If this is the case, it may be easier
just to replace the entire unit ($35-$50). Our street
pressure is about 150psi, which is enought to pop the
pressure relief valve all on its own. My street
regulator was creeping up slowly, so I replaced it.
But even after I fixed this, the pressure relief
valve popped again!

With all of the faucets turned off in the house, I
opened the utility sink faucet and let about 20 gallons
of hot water out. I then stuck a pressure gauge on the
water line. My initial reading was 70psi. Of course
at this time, the hot water heater needed to turn
on in order to reheat the 20 gallons of cold water
that replaced the 20 gallons of hot water I just
let down the drain.

We would expect the water pressure to increase
as the 20 gallons of cold water is heated.
Increase it did! It increased enough (over 100PSI)
until the pressure relief valve did its job again.

Why didn't this happen to my older unit? Maybe
the tank was thinner and was able to expand
slightly to accommodate the heated water. Maybe
that dripping faucet that I recently fixed helped
keep the pressure at bay. Maybe having a larger
capacity water heater has something to do with it.

Nevertheless, hot water expands, and the slack
has to be taken up somewhere.

Solutions:

1. Let a faucet drip ever so slightly. In my opinion,
this is a lame way of fixing the problem, but could
be used as a tempory solution.

2. Install an expansion tank. Amtrol makes these and
they are readily available in any good plumbing supply
house.

The expansion tank is T'd into the cold water supply.
This type of expansion tank uses a bladder inside of
it, and is quite small compared to the older type of
expansion tanks. They come in different sizes, and
are sized to the capacity of your hot water heater.
I believe that is cost me somewhere between $40-$60.

If it makes you feel any better, I was told the following:
At least here in NJ, many local building codes now require
these tanks to be installed in new construction.

-Mark

[Dan Bolt]

Sounds like you have a check valve on the inlet side. Perhaps built into a
pressure reducing valve. I think you need to install an expansion tank in
the line to limit pressure increase on heating. These are readily available
in home centers.

Dan

Kevin Autrey <exu...@exu.ericsson.se> wrote in message
news:7td1qp$j7f$1...@abc.exu.ericsson.se...
> Hi -

>
> I've got an A.O. Smith HWH that repeatedly triggers the pressure
> relief valve to open. The valve is rated at 210 degrees (F) and 150
> PSI.
>
> The plumber has replaced the valve a couple of times, so I don't think
> that it's a faulty valve component that's at work here.
>
> When it happens, it's usually when an outside tap is turned off -
> although it has also happened when an inside tap is turned off.
>
> Could there be something related to rapid changes in water pressure
> that might cause this problem?
>
> Can anybody suggest solutions?
>

> Thanks,
> Kevin