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I have a well with a 40 gal Welxtrol pressure tank. It is a closed system. I am installing a new high efficiency electric water heater (heat pump). Do I need a thermal expansion tank if I already have a pressure tank? Won’t the pressure tank absorb the added hot water pressure in the same way an expansion tank does?

And, with the above, if the water heater already had a pressure relief valve built in, with drain, why do we need expansion tank or thermal pressure relief valve? Won’t the pressure just come out the built in valve and drain accordingly?

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  • Welcome to Home Improvement. If you'll take the tour, you'll note that we're a little different here than most general discussion forums - we work on a "one question per question" basis. Please edit your question to remove the "separate question", then post a whole new one asking that. Nobody will look down on you for doing so (as they may at other boards) - your question might actually get closed as "needs more focus" when you ask more than one. I'm sure someone who knows more about wells & plumbing will be along shortly to get you a good answer.
    – FreeMan
    Mar 3 at 13:38
  • It's all the same question, just worded poorly apparently. I've revised.
    – devnuts
    Mar 3 at 13:57

3 Answers 3

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To clarify a little (the other answers are correct):

Any expansion tank on the cold water side, whether separate just for the water heater or making use of the pressure tank for the well, is to handle normal expansion due to heating of the water. This is a relatively new code requirement (2006), so for many people (in my case, 2022) this is a new thing with the next water heater replacement cycle.

However, the TPR is to handle the crazy, unexpected, dangerous situation of something going seriously wrong in the water heater. An extreme example would be the gas burner or electric elements going on but never turning off (fused relay/contactor, bad thermostat, whatever). A typical small pressure tank would help with the pressure (because more heat = more pressure) but would not react directly to the temperature. That potentially boiling water needs to be released in a semi-controlled manner via the TPR rather than become a super-heated water bomb in your basement.

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    Speaking of super-heated water bombs, that's one of the best Mythbusters episodes ever. It gave me new respect for water heaters and the TPR.
    – KMJ
    Mar 3 at 20:01
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So long as you don't have a check valve between the water heater and the pressure tank for the well system, you don't need a hot water specific expansion tank, as the well tank does that job.

A well system should have a pressure (only) relief valve at the input from the well, but whoever is trying to upsell you an expansion tank you don't need is presumably also trying to upsell you an additional pressure temperature relief you don't need, in overpriced sharkbite format.

Ignore that idiot sales-droid.

The P/T relief valve on the tank itself (and the specifications of the tank you link to do clarify that it is, as expected and standard, a Temperature Pressure Relief Valve, not just a pressure relief valve) is what you need at the water heater. One not on the water heater won't even work right with respect to over-temperature.

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    This is DIY, just researching. Yes, check valve is between well pump and pressure tank, to prevent backflow or pressure to the pump, and has open cold line between pressure tank and hot water heater. So it would seem the pressure from the hot water heater would push to the pressure tank without issue, and thermal expansion is not necessary.
    – devnuts
    Mar 3 at 14:08
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    The pre-installed valve on the new WH will be a temperature pressure relief unless you are in an alternate universe, so that covers that aspect. Good to go.
    – Ecnerwal
    Mar 3 at 14:10
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    I'm often too subtle on my comments and answers, just because of my respect for others. @Ecnerwal you are of course correct that if there is a check valve 1) between the pump and tank, not a problem, but 2) If between the tank and the house a TPR and expansion tank are needed, like I said before, presumable there is an existing WH that isn't experiencing problems. But I do agree with manassehkatz-Moving 2 Codidact comment is that a TPR valve is pretty darn important in case the controls of the WH fail and leave it running, seriously over-heating the water. I'm just too subtle at times.
    – George Anderson
    Mar 3 at 17:52
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Depends upon your water system setup. Seems as you have a well, pump and pressure tank. If you have a check valve in there to prevent backflow, then YES, you'll need a thermal expansion tank. Presumably you already have a water heater that works OK without one, so you may not need one for the new WH. I'm not sure what code requires, perhaps others could weigh in who have more expertise than me. I'm almost sure that code requires a TPR valve. In your case if w/o a checkvalve, probably not needed for pressure, but if something goes wrong, the temp part of the TPR needs to be there "just in case". TPRs are not expensive, just do it.

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  • No, it depends where the check valve is, not that there is one at all, George. If it's between the well and the tank, it does not impact the connection between the WH and the tank. And the question states that there is a TPR on the tank already (as is standard for all new WH).
    – Ecnerwal
    Mar 3 at 14:05
  • Not sure if posting a link is ok, but it's this one: homedepot.com/p/… As with any hot water heater, it has a built in pressure relief valve and drain no the side. I just don't think an expansion tank is necessary in my set up, since pressure would be handled by pressure tank, between hot water heater and well pump.
    – devnuts
    Mar 3 at 14:09
  • "Glass-Lined Tank, Overheat Protection, Temperature Pressure Relief Valve, Vacation Mode, WiFi" so, yes, it has a built-on TPR.
    – Ecnerwal
    Mar 3 at 14:12

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