My county requires a permit to replace an electric water heater. They also require installation of an expansion tank. I looked up the purpose of an expansion tank and learned municiple sourced water can vary in pressure at end points, possibly exploding the water heater. But I have a well. Can someone explain why well owners are required to spend this extra 1K+ please? Also, where can I access data on exploding water heaters and losss of life?

  • Do you have anything fitted that blocks the expansion path from your HWH's cold port to the existing pressure tank in your well system? Do the local water utilities have any requirements for backflow prevention in water service lines, for that mater? Dec 25, 2020 at 17:14
  • 1. I have no idea, this is the first time I've received info cation of a pressure tank in my well. Had the well pump replaced last summer (1988-2020) by aucensed professional well pump fella, so I'm sure he would have mentioned if anything else needed attention. 2. I have heard of backflow, though I have no idea if I have that either. Fabulous question for me to write down and ask the county about requirements. I've been here 17 years and only 2 years ago did I finally get a main shutoff installed and functional...I now see I should've warned y'all about my back-woods lifestyle. Dec 25, 2020 at 23:07
  • Did you confuse an "expansion tank" ($40) with a $1000 pressurization system that assures good service water pressure despite low well water pressure? The requirement is for the $40 one, and the $1000 one does not meet that requirement. Dec 25, 2020 at 23:26
  • A 40 gal Rheem + expansion tank delivered and installed for almost $1,400. The install service HD offers is likely appreciated immensely by some, and an expensive "only option" for others. Having no history in such matters, I assume permit cost were incorporated into total. Additional info: my well is unusually productive, can't recall the gpm but no low pressure issues. Regarding that $1400, yeah, I should have requested an itemized list to learn more. Frankly, I felt like crying and probably did a tad prior to hanging up. I was shocked. That total is probably a minimum 60% profit. Dec 26, 2020 at 0:35

2 Answers 2


An expansion tank is $40. Did you get an installation quote of $1000 just for the expansion tank?

Expansion tank allows for expansion of water when heated. Cold water occupies less volume than hot water. When your water tank is pressurized with cold water and then that water is heated it now exceeds the pressure for your water supply system. Depending on your setup, if you have a backflow valve that prevents water from going back into the source (well or city) then pressure relief valve on your hotwater tank will notice the pressure being too high and discharge. The valve is really only supposed to discharge in unexpected scenarios - it shouldn't be discharging all the time. The more the valve is used the more likely it is to break. If it breaks then there is potential for your hot water tank to become over pressurized.

  • I was about to purchase a 40 gallon electric WH from HD for $395ish. The fella just happened to mention additional items and that's how I learned about my county code changes. His quote for parts and install was almost $1,400. Your explanatory is appreciated, I'll reread it until I get it. You mentioned it's a method of avoiding overheating., Funny, that's why (I think) it kept tripping off and needing me to push that reset button. When I took a peak at the pan with a flashlight, I saw the lower foot split along a seam and rusty stuff and immediately cut all power. Dec 25, 2020 at 23:14
  • 1
    No, that means a better, contractor-grade water heater that won't cause go-backs is $600, a better expansion tank is $60, and there's $700 labor on a site-unseen installation of unknown scope. That's what that means. Dec 25, 2020 at 23:31
  • Yup, site-unseen and when I mentioned WH is in my closet....ugh. Dec 26, 2020 at 0:36

Here's the deal.

When you heat water, it expands. If it can't expand freely, it increases pressure.

When you're on city water, that's no big deal. The extra volume can just push back out the water main to the street - maybe an inch or two of water movement, not a big deal. So there is no increase in pressure.

Imagine if we stuck a check valve on the inlet, so water CAN'T flow backwards to the street. Some cities actually want this. Now what? Now, water can't expand freely. All the valves are closed, and the check valve prevents backflow into the city system. Since water can't expand freely, it increases pressure.

Well systems effectively have a check valve at the bottom of the pump. That's to keep the column of water in the well pipe from falling back down!

So with all valves closed and the check valve on the supply, the system is "solid" with water - a state called "hydraulic lock".

Water does not compress. So, if volume increases, pressure must also increase, to infinity if needed, until something gives or breaks. That is why a water heater on a well is a big deal. If normal pressure is 34 PSI, this can drive pressure up to 200 PSI or more until the weakest valve or pipe joint fails.

These high pressure instances can also be simply from slamming a valve shut fast. Water has momentum and when it's flowing toward the valve and you shut it, it "piles up" at the valve causing a pressure spike. This is also what causes pipe knock or rattle.

The solution is to have an air volume in the system. Air does compress. So if volume increases (a little), the air compresses (a little) and pressure increases some tiny amount (like 34 PSI to 34.1 PSI).

The expansion tank is simply an engineered vessel with an air space and water space. And a rubber seal between them, to stop the air from effervescing into the water and being lost.

We're talking a $40 item easily added. It only needs one pipe connection.

  • The pipe rattle information is fascinating. I priced an appropriate expansion tank at same store and they averaged $50. Regarding only one connection, my home was a first project for the builder. (Lots of curiousities here). I was told it's best to also install a "cut off" or was it a "stop valve" at either end just to make potential future maintenance or minor repairs easier. Dec 26, 2020 at 0:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.