Expansion tank for hot water heater. What is it?


Old 01-24-01, 08:46 PM
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My gas hot water heater, (not my girlfriends broken one), has always leaked out of the pressure relief valve. Within 6 months of the new heater being installed, I replaced the pressure relief valve. A couple hours later, still leaking.
Not a real leak, just some water passing thru the valve, ya know. Enought to keep a 4 gallon bucket of water filled all the time without overflowing, before evaporating down.

My old water heater did the same thing. So 2 heaters, 3 valves, always a leak.

I just found this article which I will post here that seems to answer my problem. You should know that my house has a pressure reducing valve on the main line.

The relief valve (on water heater) will operate if either the water temperature is to high or if the pressure gets too high. If the inlet to the water heater is fed directly from the water main with no pressure relief valves or check valves between them then when the water heater heats the water and it expands, the extra volume of water simply flows back toward the water main. If however there is a blockage such as a check valve or pressure reducing valve with a defective bypass then the increase in water volume has no where to go and the pressure with increase dramatically. Placing an expansion tank in the line at the inlet will give the increased volume of water someplace to go and prevent operation of the relief valve.
I've never seen this so called, expansion tank. Where do you buy them, Home Depot or plumbing supply? Are they small 1 gallon units that fit easily into tight quarters? Should I get one?

P.S. My pressure reducing valve on the main line may be back blocked as suggested above, but it still works and keeps the pressure down to 40psi. The valve is 15 years old and I do not plan to replace it. UNLESS you believe it will fix my problem alot quicker than an expansion tank.

[Edited by Mark Chapman on 01-24-01 at 11:53]
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Old 01-24-01, 10:22 PM
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These can be bought at Home Depot or a Plumbing Supply House.

Install it on the cold side of the water heater, the trick to installing these are as follows.

If copper , sweat on a 3/4" copper tee, then the stub up, then the 3/4" female adaptor to accept the expansion tank.

If dealing with screwed type of pipe, galvinized, then use screwed pipe for the installation.

I show theses pictured not cause I don't think you know what the parts look like, but for others that may not know

Now I will sweat the female adaptor on the the stub up before screwing it onto the tank.

Now a few steps need taken before you you can even start installing the tank on the water line.

You need to find out what the static pressure is at, this is the 1st step, using a pressure gauge, turn the outside bibb on for about 10 sec, this will release residual pressure built up on the system, now connect guage to the outside hose bibb, turn the bibb on, and read the gauge, this will now read the static pressure.

This is the pressure you need to pressurize the bladder to, + or - 2 lbs You will see the spot on the top of the tank to attach the airpump to.

Now your set to install the tank

Hope I was of help on these tanks, and installing one.
Old 01-25-01, 07:18 AM
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Wow! That was a GREAT explanation! I now see that it acts as a really BIG shock pipe. The picture was most helpful.
I also like the pic of the gauge. I have stood in the plumbing aisle before, hand fitting parts together to make me one of those, but have yet to find the right parts.

Thanks PB2000 !
Old 01-25-01, 11:13 AM
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Just returned from Home Depot, plumbing dept. I saw something on display called a "pre-charged diaphram pressure tank", 8.6 gal for $69, and it looks like your picture. They go all the way up to 112 gallons! Is this the expansion tank?

If so, it will never fit in my water heater closet. What are the larger ones used for? I think I have seen these units at farm supply stores, in the well water section.
Old 01-25-01, 07:45 PM
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Go to a plumbing supplier house, they have em smaller then that, I think the last one I put in was a 2 gal or 2.5 gal one.
Old 01-25-01, 08:11 PM
Join Date: Dec 1999
Location: Northeastern NC On The Albemarle Sound
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The tank that you were looking at is for a water well system that is air-charged to maintain water pressure.
What Ron showed you is much smaller, and is to alleviate water pressure in your water heater tank.
Look similar in a photo, except size. Two opposite functions (alleviate OR maintain water pressure).
Look for an EXPANSION tank and the fittings just as Ron advised.
Good Luck!
Old 01-25-01, 08:29 PM
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Tanks OldGuy! All's well that end's well!

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