How to test a thermal expansion tank?

I have an oil fired hot water system in my house (about 10 years old) and everything up to this point has been fine. My water is supplied by a well.
Recently, I've notice big "spikes" in my water pressure. Sometimes when I turn on the water the water seems to come blasting out for the first few seconds. In addition, the last few days I've notice some water coming out ot the pressure relief valve on the hot water tank. These things seem to happen most after using the a lot of hot water, for example when someone takes a bath.
My system has a thermal expansion tank, but I'm beginning to suspect that it might be bad. Does anyone know how to test these tanks. Also, is it something that I can replace myself (I'm fairly handy). It looks like it simply screws into a fitting in the water line?
Thanks in advance.
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Seems you've lumped heated tap water with water used in a closed loop to heat the house. Split them in your thinking, and take another look at the systems.
One possible cause of high pressure of domestic hot water: use of lots of heated water, which is replaced by (obviously) cold water. This is now heated and expands. Being trapped in a closed volume between check-valve and faucets, the pressure rises. Also, the water in the non-house-heating part of the furnace may be over-heated (like, from extended burner run-time heating house), or the water may have lots of dissolved gases which then outgas at elevated temps.
There are lots of DIY books that'll get you oriented.
J
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snipped-for-privacy@hotmail.com wrote:

You need *TWO* expansion tanks: one for the Hydronic loop and one for the domestic hot water tank.
Jim
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snipped-for-privacy@hotmail.com wrote:

I'm going to take your words literally Joseph and understand that what you are asking about is an oil fired hot water heater, and NOT an oil fired hydronic heating system which warms the living areas.
If I'm reading you correctly then it does sound like you have an expansion tank hooked up to the DHW system and that tank is likely "waterlogged".
The simplest older expansion tanks were just that, a tank with an "air space" above their typical water levels. They usually have something which looks like a Schraeder valve (tire valve) on top through which you can pump some air with a bike tire pump to replace what has been lost through absorption in the water.
Newer style tanks use a rubber diaphragm inside to separate the air from the water to avoid losing air through absorption. That kind of tank will hold its air space for years, until the rubber finally fails and develops a tear, then the air space will decrease slowly through absorption in the water. Those tanks usually also have that Schraeder valve on them through which you can periodically pump some air in until you get tired of doing that and decide to bite the bullet and replace the tank.
With regard to "the test" grab a bike pump and squirt some air in. If the "high pressure" problem goes away, you've got your answer.
HTH and Happy Holidays,
Jeff
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Jeffry Wisnia

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Thanks for the info. The thermal expansion tank has a schraeder valve on the bottom, so I assume it's the bladder type.
When I put a pressure gauge up to the tank it doesn't show any pressure.
I tried to just push the valve in to see if there was water leaking out, but it's dry.
So I plugged in a bicycle pump and tried to pump it up to 40 psi. But as soon as I disconnect it and recheck the pressure it seems like it hasn't "taken" any pressure.
Do you have to turn off the water and take the tank off to recharge it, or can you charge it while it's in the system?
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On 13 Dec 2005 16:01:26 -0800, snipped-for-privacy@hotmail.com wrote:

Yes, you cant have any pressure behind the tank while charging it. Remove it. This will also give you a chance to inspect it. Bubba
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Bubba wrote:

I'll go along with that and am embarrassed I didn't think to mention it.
If the entire system and the expansion tank are all filled with water and there's no way out for it, when you try to pump in air there's no place for the air to "fit" other than the tiny bit of volume gained by further "stretching" of the plumbing and water heater tank. (Water is essentially incompressable for practical purposes.)
If you want a quick try at it without removing the tank, shut off the water supply to the water heater and open a hot water faucet. That should let you pump in some air in situ and let you see how that works for you, but I bet you're gonna have to remove that tank eventually anyway.
Jeff
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