Thermal Expansion tank for water heater - is it necessary?

I was just reading an article about the "importance" of a thermal expansion tank to extend the lifetime of water heaters, T&P release valves, hot water fixtures, etc.
On the other hand, we have never had one in our house or where I grew up and I haven't "noticed" any problems.
Specifically, we have a 40 gallon gas-fired water heater on city water with 3/4" inlet and outlet to the water heater and copper plumbing throughout the house. We have 4 bathrooms and a kitchen. The city water pressure comes in at about 85 PSI.
How important is it to add a thermal expansion tank or is this one of those "if it ain't broke, don't fix it" kind of things?
Alternatively, is there enough "give" either forward through the hot water plumbing or backward through the cold water plumbing (and ultimately maybe even back into the city mains) to allow sufficient room for expansion?
If a thermal expansion tank is recommended, any suggestions on brands, models and sizes?
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My understanding & experience is that an expansion tank is only needed when your house is isolated from city supply by some sort of backflow prevention device.
With such a device installed, cold water coming into your system to replace hot water consumed & being heated by the water heater can generate enough pressure to pop your water heater T&P valve.
The only signifant "give" in your systems is back flow into the city main. If you have a back flow prevention device then you need a relief valve (set higher than citty pressure plus hammer spikes & lower than water heater Y&P valve) or an expansion tank.
If oyu have an open system (backk flow into main) forget the expansion tank....not gonna do much.
cheers Bob
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85 PSI is pretty high, you may want to invest in a pressure reducing valve.

Depends on your city water system. Do they have an anti-backflow valve in the water meter? If yes, then you must have an expansion tank otherwise the T&P valve on the water heater will get a lot of use. Water expands when it's heated, it has to go somewhere.
If there's no anti-backflow valve in the system, don't bother with an expansion tank.
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I am not aware that there is an anti-backflow valve though I imagine it could be built into the meter (which is one of those electronic ones that they then broadcast wirelessly). Is there any easy way to check by looking or would I need to call the city and/or try to find and look up a model number.

Makes sense...
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blueman wrote:

With the exception that 85 psi is already on the high side. Do you know what it is at say 4 AM when city wide usage is light? Ours creeps up to 120+ psi around then. I installed a regulator and a thermal expansion tank. Both are inexpensive and easy to install. The other think you notice is the expansion tank buffers little surges nicely.
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If your T&P valve on the water heater hasn't sprayed scalding-hot water all over by now, then you don't have an anti-backflow valve (or DO have one, -and- an expansion tank). You would know by now.
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Well the water heater is 5 years old and not a drop has come out of the T&P valve and we definitely don't have an expansion tank... so I guess e don't have an anti-backflow valve then. Thanks
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Buy this gauge http://www.watts.com/pro/_productsFull.asp?pid=647&ref=1 at Home Depot for about $12.
Put it on the drain of your water heater and open the drain valve. If the tattletale needle indicates over 90lbs the next day you have a closed plumbing system and need a thermal expansion tank.
More info on thermal expansion here... http://www.watts.com/pro/divisions/watersafety_flowcontrol/learnabout/learnabout_thermexpansion.asp?catId=64
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screwed into a garden hose fitting right off my main cold supply so shouldn't be too hard to move it to the hot supply.
Though based on my understanding, it should be sufficient to keep the gauge on the cold supply since the piping between the cold inlet (after the valve) and the water heater is an open system (without intervening valves) with relatively large caliber pipe (1" and 3/4") so the pressure on the hot water tank should be pretty close to the pressure on the T&P valve. In fact, the whole danger of thermal expansion is presumably that the pressure (and associated wear & tear) is transmitted to all the fixtures on your system and if the pressure can be transmitted forward from the hot water tank 50-100 feet to faucets then the same pressure should certainly be transmitted backwards 10-20 feet to the pressure gauge on my main water line.
So in summary, I am assuming that if the pressure on my main line doesn't spike up much beyond the baseline then I probably don't need a thermal expansion tank.
Does this make sense or am I missing something...
Also, now that I think of it, even if I did have a backflow preventer without an expansion tank, there probably is a fair bit of reserve "capacitance" in the overall system given that the house is relatively large (bathrooms on the 3 floors plus basement) and also there is an attached in-ground sprinkler system with probably couple of hundred feet of 1" poly distribution piping (even with all the valves closed) -- also the poly piping is more flexible/expansible than copper. Of course during the winter with the irrigation system shut off there is less reserve capacitance since you only have the rigid copper piping inside the house.

http://www.watts.com/pro/divisions/watersafety_flowcontrol/learnabout/learnabout_thermexpansion.asp?catIdd
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blueman wrote:

don't bother then. in fact, unless you're one of those people who flush your water heater tank every year, and/or you have a high end water heater with a real "boiler drain" or ball valve for a drain valve, that might cause more problems than it's worth.
I have replaced all the water heater drain valves at my house (there's three...) with dielectric nipples, 3/4" threaded ball valves, and a 3/4" MIP to garden hose thread adapter (with a brass cap on the end for safety) due to having had problems with every blame one of them when I attempted to do my first annual flush after moving in.
nate
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You can leave that gauge on the cold side as long as that bib has service pressure on it. Take a bath or shower before you go to bed. The water heater will fire and as long as you don't open a faucet or flush the toilet and relieve the pressure you'll get a surprise on that gauge in the morning (or just stand there and watch it till the WH shuts off.
If you have a closed plumbing system you'll see pressures up around 130-140psi on the tattletale needle.
If that is the case you need a thermal expansion tank.
BTW, that excessive pressure of thermal expansion is at every pipe, fitting, appliance, and valve in your system and most pluming fixtures are rated to only 80psi... just a thought.
There is NOT sufficient "reserve capacitance" in any plumbing system to absorb water expansion @ 140psi. Well, there's some "reserve capacitance"... it's leaking faucets, running toilets, blown pipes, leaking fittings, and a T&P valve. Water is not progressively compressible as air is. Double the pressure and water will go somewhere... and most of the somewheres waste money or cost lots of money to fix.
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Re Re: Thermal Expansion tank for water heater - is it necessary?:

Correct.
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I filter all messages from google groups.

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Bob M. wrote:

I would say that it would indicate a definite maybe or not...
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blueman wrote:

it's required only if you have a backflow preventer or check valve on your water supply. A pressure reducing valve usually acts as a BFP as well, so that counts.
nate
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Allegheny county in pittsburgh PA added pressure tank as mandatory with all new water heater installs
They are also trying to require anti backflow valves be installed espically at home resale time
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Which is one reason why I am not crazy in getting a pressure reducing valve even though our static pressure is 85-90 psi.
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Many jurisdictions are putting in backflow prevention at the meter. If yours has such you will need an expansion tank. We didn't have one for a few years, and it was definitely hard on fixtures.
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