Saturday, December 03, 2005

Suddenly feeling the pressure

I previously reported on my experience installing a pressure reducing valve on the main water inlet to our house. Well, it seems there's still some kind of problem. We've been noticing that one of our showers occasionally starts to drip water. I had thought that it was a bad seal in the show spigot (it's kind of a cheap fixture). Today it started dripping again and just on a hunch I went outside to check the pressure gauge that I installed on the main water line.

It read 130 PSI.

Holy shit! That's double what it should be, and 40 PSI higher than the maximum pressure that the pump is able to produce.

This is really bizarre. I have no idea how the pressure could have gotten up that high. The inlet pressure at the main is 30 PSI and the pump only pushes that up to 90. Somehow we're getting an extra 40 PSI from somewhere, and it's getting past the PRV, which is set to 65 PSI and is supposed to be good up to at least 150 PSI on the inlet.

The only thing I can think of is that we have an air bubble trapped somewhere in a hot water line. Maybe the air is heating up, expanding, and compressing the water in the pipes in the house. It seems pretty farfetched, but that the pump could suddenly produce twice the pressure that it normally (and that this would coincide with a failure of the PRV) seems pretty farfetched too.

As the King of Siam would say, is a puzzlement.

If anyone has any idea what might be going on here please let me know before my pipes burst.


torq said...

Saw this same issue discuss on Ask This Old House...

(towards bottom of page),16663,1110925-1115943,00.html

Richard installed a Thermal relief tank in the hot water line, near the hot water heater.

News to me as well, but with a pressure reducing valve (or one-way valve) on the main water line, the expansion from the water tank actually has no where to go. Normally the piping to the street (and city network) actually acts to absorb the extra pressure. But with a pressure reduction valve, which acts as a one-way valve as well, you need to install on of the thermal expansion tanks.

Linked per the Ask TOH page:

...more details here, as well as install instructions.

The guy on the show thought he had a bad Temperature & Pressure Relief value on the hot water tank, as it kept leaking (even after replacing it twice). Kept having to empty buckets under the discharge pipe, a couple times a day.

torq said...

was actually installed on the cold water supply side (not the hot), next to the water heater.

See the link manufacturer page, and pdf with charts and graphs. Last page of pdf has the quick sizing guide - after getting lost in alot of data.

Ron said...

Thanks for the pointers, torq.

NFLRN said...

torq is right. What happens is that as your hot water heater heats the water it expands a little. Since water is essentially noncompressable this little bit of expansion results in large pressure increases. You apparently don't have a tank (like a home with a well) for this expansion to be absorbed. Since you have a backflow preventer between you and the water main, your pressure will shoot up.

To fix it you need an expansion tank placed somewhere in your house. Just an added bit of trivia that same thing can happen if you place a check valve on the cold water side of your water heater.

I typed all this in case somebody like comes along and doesn't click through to the articles that torq linked.

Good Luck

Raf M said...

Its been two years since you posted on this topic. With all this posts, it surely must have gone well. Check out these most cited water heaters in 2007