Water heater T&P valve question

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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David Starr wrote:

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
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Jeffry Wisnia

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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
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David Starr wrote:

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 here: http://www.stateind.com/expansion/expansion.htm
Or...do an Advanced Search here: http://groups.google.com / It's been discussed on this ng many times.
Jim
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David Starr wrote:

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)
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Joseph Meehan

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

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

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
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