David Starr wrote:
I've got a 4 year old 40 gallon gas water heater. Every now & then,
the T&P valve opens and 3 or 4 gallons drain out. The temp isn't set
too high - 1 on a 1-3 scale - so I don't think it's getting too hot.
It seems to happen when I'm using a fair amount of hot water for a
long period of time.. I have a home darkroom & sometimes need water
at 75 degrees for a couple hours; this is when I notice it happening.

Would it be a good idea to replace the T&P valve, or what? Do these
valves deteriorate over time? The valve appears to be doing what it's
supposed to, but I can't figure out why it's happening. Our previous
water heater never did this.




- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Dave Starr, Senior Shop Rat Emeritus: 14,647 days in a GM plant.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -


I'll hazard a guess that you don't have an expansion tank in your system
and there is some kind of a check valve in the home's water supply line
which prevents backflow into the city supply line.

If that's the case, then from your description, it would seem that when
you draw hot water for an extended period of time you're probably taking
nearly all the "hot" water from the tank and it's filled with "cold"
water. When that cold water gets heated up it expands and if there's no
place for the larger volume of water to go, the pressure rises and the
pressure relieving function of the T&P valve lets some water out so the
plumbing system doesn't burst at its weakest point.

If you're not familiar with how expansion tanks work, think of a thick
rubber balloon connected to the piping. When the water expands, the
balloon gets a little larger and makes room for the water.

HTH,

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public
schools"