Expansion tank effectiveness

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SteveW

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I recently added a ball valve upstream of my expansion tank, so I could isolate it when it eventually fails (without turning off the house supply). I also have a boiler valve upstream of the ball valve, and have a maximum-reading pressure gauge screwed onto the threads of the boiler valve so I can monitor house pressure.

Just for fun, I did a little experiment. One night, I ran about 5 gallons of hot water before going to bed, and the next morning, the max house pressure overnight was 60 psi, the normal reading I always get.

Another night, I shut off the ball valve to the expansion tank (so it was "off line") and again ran about 5 gallons of hot water before going to bed. That night, house pressure reached 100 psi!

I can only imagine how high the pressure might have been if I had filled up a bathtub with mostly hot water in the evening, without an expansion tank.
 

Mikebarone

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I'm not to familiar with boilers, but you might check to see if the variance of the water pressure isn't from the water source i.e. water company. You might need a whole house pressure reducing valve installed.
You might check the pressure blow-off valve on the hot water tank too, and make sure it is working!

bell_6001_637.jpg
 

SteveW

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Thanks for writing, Mike!

I realize now I wasn't terribly clear in my post about a couple things --

I actually don't have a boiler -- I added an expansion tank to my gas water heater since I do have a pressure reducing valve and have heard some funny noises coming from the pipes in the middle of the night.

The other tip-off to the problem was the temperature/pressure valve on the water heater dripping off and on.

I learned from this site that a pressure reducing valve makes for a "closed system" when the water in the water heater heats up, and having nowhere to go, causes pressure to go up dramatically in all the pipes (and fixtures) in the house.

Interesting side note -- when I went to a local plumbing supply house, the owner told me I didn't need an expansion tank -- I needed a TPR valve with a higher opening pressure! (Would not have been code, or safe, as near as I can tell... but he says he does it all the time...)
 

Brownizs

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Actually the expansion tank is better then a TPR. It sounds like the Utility company that owns the water lines for where you live, has Backflow prevention valves. What this means is that due to the fact that the water line system in a house is supposed to take the expansion of the pressure from the water system, if there is no place for it to go, then the Water heater TPR is supposed to relieve it.

Basically what this means, is on most systems without the Backflow prevention valve on your incoming water line, the pressure would flow back out to the water main when any valves are closed (ie faucets), due to water is not flowing).

Think of it, like a ball that you are pumping air in from a compressor. The pressure is static at 60psi, but the ball will still inflate until it is not able to anymore. You would actually be able to measure a higher PSI on the ball, even though the incoming PSI was at 60psi.
 

Mikebarone

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I am not a professional plumber, just a handyman that has dabbled in almost everything. I always thought that an expansion tank was to eliminate water hammer. Although, I could see where it could take up some of the excessive pressure in the lines, for a limited time.
I think that browinizs, (Junior Member) hit it on the head, that there is a check valve in your water company's line, going to your house somewhere. I personally would not modify the TPR valve that was designed to work with your hot water tank. Besides, that wouldn't do anything but add additional pressure going into the house.
You may be able to install a pressure reducing valve coming out of your hot water tank line, that goes to the house. That would limit the PSI going into the house from the hot water tank. This is kind of way out there, but you could also install another expansion tank, in line with the one you have now, just in case the first one failed. This would also give you a little more volume for any excessive pressure your hot water tank might accumulate. If your were really concerned about monitoring the TPR valve, you could put a water alarm in a bucket located at the discharge point. This would let you know when the expansion tanks couldn't handle the excessive pressure.
And to go way out there...if you had an electric pilot light on your hot water tank, you could install a limit switch, (like they have on well systems) that would, (via an Electric gas valve), shut off the gas going to your hot water tank, when the pressure in the water line got over a certain pressure. I would check with a professional plumber, or the manufacture of you water heater, before doing this one, to make sure it would work.


Good luck, Mike
 
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hj

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tank

1. an expansion tank is not a very effective hammer preventer. That is not its intended function although it can happen under certain specific circumstances.

2. The utility does not have a check valve in his system. The TPR valve is the equivalent of a check valve and causes the closed system.

3. Why stop at two expansion tanks? Three or more will give even more protection against failure.

4. Capacity is not a factor. Water being essentially non-compressible will create a large pressure increase from a very small amount of increase in volume.
 

Mikebarone

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The expansion tanks do limit water hammer. I don't see how the TPR valve serves as any kind of a check valve for the water lines. It does prevent the water that has blown out from coming back into the water tank I guess you could call that a check situation. If there is not a check valve somewhere in the lines, why doesn't the excessive pressure just balance out to the incoming water line from the water company? There are many applications that duplications are performed to increase dependability¦ i.e. a gluelam beams. I do not see any solutions in your response that may help out Steve W.
 

Bob NH

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From what I read of Steve's remarks he has everything in control. Some of the other remarks are confusing.

1. A TPR valve should operate only if there is a system failure of some kind. It is NEVER to be used as an operating control.
2. A TPR valve is not a check valve; it is a relief valve. It actuates if the temperature exceeds the set value (usually 210F), OR if the pressure exceeds the set value (usually 150 PSI). Your system should never reach those values.
3. An expansion tank should be used where a system is closed, as when an incoming Pressure Reducing Valve (PRV) with a check valve is used to reduce incoming pressure. An expansion tank is not required with a well system that has a hyropneumatic tank.
4. The expansion tank should be precharged to the nominal system pressure (the PRV setting) so that under normal conditions it is full of air. That provides the maximum expansion capacity for when the cold water in the water heater expands when it is heated.
5. If you don't have an expansion tank, or the tank is too small, you can protect the system with a small relief valve that is piped to a drain. A 1/2" 75 psi relief valve such as is often used on submersible pump systems, installed in a cold water line, will protect the system against overpressure. This is not a recommendation to use a relief valve in lieu of an expansion tank.
 

SteveW

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Bob -- thanks for clarifying -- your remarks are right on the money.

In my situation, the PRV (pressure-reducing valve) acts as a check valve and made mine a "closed" system (and I think this is also what hj was referring to).

The expansion tank (5 gal. capacity) seems to be doing its job well and preventing system pressure from exceeding the PRV setting when the water heater runs and no water is being used.

Just wanted to share my success story for anyone considering installing an expansion tank. I was surprised just how well it stabilizes the pressure.
 

Mikebarone

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My bad, (Like the kids say today). I thought you were having problems with the expansion tank installed. I guess that's what I get for not reading your first letter more carefully, (now that I've gone back and read it again ¦Ha, ha).
Thanks for the input on the results of the tank, I'll remember this one.

Mike
 

SteveW

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Not a problem! Thanks for taking the time to respond -- I think this was a useful discussion, and probably will be helpful to future listers trying to get a handle on this issue --

I learned some things, too -- like hj's comment about small volume changes causing large pressure changes, and Bob's comment about the option of a separate pressure relief valve instead of a tank.

Steve
 
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Gary Swart

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Please note that Bob did not recommend a TPR for pressure control, that's the purpose of the expansion tank.
 

Jadnashua

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The guy that suggested installing a T&P valve that released sooner was just wrong...
 

SteveW

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Yep, amazing how much bad info is all around us. This gent had never heard of a thermal expansion tank being installed for this purpose.

Another alternative to Bob's idea of a 1/2" relief valve is Watt's Governor 90-M1 toilet ballcock with built-in pressure relief valve -- it's set at 80 psi. Again, the consensus seems to be that an expansion tank is the best choice, but for those who can't for some reason install one, this might be a reasonable option.
 

SteveW

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I did another little experiment today. Just got back from vacation, and I turned the water heaters off while I was gone. I turned them back on (2 50-gal. units) and isolated the expansion tank while the WH were warming up. I decided to open the valve to the expansion tank when house pressure hit 120 psi. Once the expansion tank was back on-line, pressure came down to 65 immediately.

Makes me very glad to have an expansion tank. Seems like it would save a lot of wear-and-tear on appliances, faucets, etc.
 
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