I need to replace my water heater tank since it’s leaking. I’d like to add an expansion tank at the same time. Since space is limited, I’d have to install it horizontally. How high do I need to install the tee adapter on the cold side so the expansion tank can spin during install? Also, it looks like I would need a shorter flexible feed line. What’s the minimum length they’re available in (12 inches?). Would teflon tape and pipe dope be adequate to seal the tee connection and the rest of flexible feed lines? I plan on using a brass tee connector. Should I use a different material? I’m not home and unable to measure the amount of room I have above the tank so I apologize if my questions sound silly. Just trying to get a head start on this. Thanks. enter image description here enter image description here

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  • In some municipalities there is no check valve between the water main/water meter and the residental plumbing, therefore no expansion tank would be needed. A pressure reducing valve might also function as a check valve, but some municipalities have low enough pressure in the final mains that no pressure regulator is needed. Does the OP know that other houses near him have expansion tanks? Does he know one is needed/recommended? Is PT (TP) relief valve dripping? Dec 28, 2023 at 22:14

2 Answers 2


The expansion tank can be anywhere that's convenient on the cold water plumbing, so long as there isn't a check valve or PRV between the expansion tank and the water heater.

It does NOT need to be anywhere near the water heater. You don't have much space at all there, from the pictures.

If using pipe dope, don't use tape - pick one, and use it. I currently prefer modern pipe dope. I used to prefer tape .vs. old-fashioned pipe dope. Don't use either on a threaded joint with a sealing washer.

Consider providing a ball valve so you can easily change the tank when it fails in a decade or two.


I wouldn't use flexible lines. I've never seen that on a water heater, only at the end of the line (washing machine, toilet, sink, etc.). The pipes are always under full pressure and a break (and flexible lines are much more prone to bursting than regular pipe) means cutting off hot water to the whole house.

The usual standard for decades in the US has been copper pipe, which means a lot of sweating (soldering) of fittings, but now there are copper press fittings and battery-powered tools to do all the work and it is amazingly fast. I was really surprised what a difference it made when I got my water heater replaced.

As I understand it, the pressure tank can be anywhere because the cold water line is under pressure. With things relating to drains there are a lot of constraints because everything has to be gravity flow. But with water under pressure, location of different components doesn't matter much.

But most important, in my mind, is to do something about the exhaust. Check the installation manual for a new water heater before you buy. And measure carefully. Many have particular rules (which may themselves be part of the plumbing code) regarding (a) vertical distance required before the first turn of the exhaust duct, (b) distance between any exhaust duct and walls, ceilings, etc. What you have right now doesn't look very safe to me.

  • Flexible line are pretty standard. They break less in earthquakes, among other things. Given the tank is strapped to the walls, that's likely applicable here.
    – Ecnerwal
    Dec 28, 2023 at 16:27
  • @Ecnerwal Not being in an earthquake-prone area (Maryland, we get earthquakes but very rarely and usually very mild) I didn't think of that. But in that case I'd go for the steel braid type rather than plain rubber. Dec 28, 2023 at 16:30
  • Appreciate the advice on exhaust routing. As far as the flex water lines, I’ve also read online that they could withstand earthquakes better than copper pipes as Ecnerwal mentioned above. Thanks.
    – J C
    Dec 29, 2023 at 12:20

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