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.




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




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

"David Starr" wrote in message
...
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.




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Dave Starr, Senior Shop Rat Emeritus: 14,647 days in a GM plant.
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They can be defective just like everything else that is made. Water use has
nothing to do with it. The valve should not open unless the temperature
exceeds the safe boundary.

On a 4 year old heater you should be able to still get it out and install a
new one.


Colbyt

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.

The T & P will open for either of 2 reasons:
Hi Temp
Hi Pressure

Unless the water is scalding (you say it's not),
very likely hi pressure is to blame.
And that is often due to "thermal expansion"
which occurs when the water is heated.

Good info he
http://www.stateind.com/expansion/expansion.htm

Or...do an Advanced Search he
http://groups.google.com/
It's been discussed on this ng many times.


Jim

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 was not going to reply since Jim and Jeff covered the answer very
well, but I have to congratulate you for giving such a accurate and complete
description including observations few people would notice. You made it
easy to identify the problem. Expansion tank it is. (defective or none
existent)

--
Joseph Meehan

26 + 6 = 1 It's Irish Math

On Sat, 05 Mar 2005 00:13:31 GMT, "Joseph Meehan"
wrote:

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 was not going to reply since Jim and Jeff covered the answer very
well, but I have to congratulate you for giving such a accurate and complete
description including observations few people would notice. You made it
easy to identify the problem. Expansion tank it is. (defective or none
existent)


Thanks to all for the help. I'll have to look into an expansion tank.


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



Thanks to all for the help. I'll have to look into an expansion tank.


If space is tight, there are some "tall skinny" expansion tanks which
became available in the past few years. They can slip into the space
between a round tank shell and the walls of a corner it's siting in.

To put some dimension on the expansion of heated water, raising the
temperature from 50F to 180F will increase its volume by about 3%, which
equates to over a gallon of extra volume for the amount of water in your
40 gallon tank.

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

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