Well pressure tank as water heater expansion tank ?

[VanHelsing]

We recently converted to city water from a well. The well water line is dead headed although the pressure tank is still in the house tee’d off the now city supply cold line leading to the two water heaters connected in series. It seems this might already function as our expansion tank rather than cutting another small tank into the line ? Thanks !

 

[Jadnashua]

As long as there's not a valve between the WH and the bladder tank, it should work.

I do wonder, though, about how much exchange there might be in the water in that tank when used where it is. A generic expansion tank will have very little water in it except when the WH is reheating, then, most of that will be expelled upon the next water use. That won't happen with the large volume of the existing tank.

Personally, I'd lose that tank, free up the space, and install an appropriately sized ET between the WH inlet shutoff and the inlet to the WH.

 

[Sponsor]

 

[hj]

If the tank has the proper "precharge" it will function EXACTLY like a smaller one.

 

[VanHelsing]

Hi guys, appreciate the help- I forgot to include that I still have the filters in place from the well system. I just realized to utilize the existing tank as an expansion tank, water will have to backflow through these filters. Probably not the best idea, eh?
Thanks again !

 

[Reach4]

I just realized to utilize the existing tank as an expansion tank, water will have to backflow through these filters. Probably not the best idea, eh?

Not so bad. Thermal expansion is really slow.

 

[Jadnashua]

Some filters might not care, some might.

The amount of expansion depends on the following factors:
- the amount of water that is being heated
- the delta of the heating (how cold is it coming in versus when fully heated)

It generally isn't all that much - in your case maybe a quart or so. It's not that hard to calculate.

If the water passes through your bladder tank, it won't get stagnant. IN a typical ET, nearly all of the water that gets pushed into it is expelled the next time you use water, so there's not much of any left, and it flushes itself each cycle. If it sits on a dead end, it will not flush itself. It should have more than enough capacity to accept the expansion if you've adjusted the precharge to your new municipal water pressure (note, that may not be constant over the day).

Personally, I wouldn't want that much water sitting mostly stagnant on a dead end. They're getting a bit more sensitive to dead ended lines these days. Functionally, yes it will work.

 

[VanHelsing]

Thank you all. Good point above about the turn over of water in a large volume dead end tank. I think I will leave as is until all ‘ home for summer’ people are gone & then put a small ET as most of you do, in the line between the filters and the heater inlet.

 

[Reach4]

Thank you all. Good point above about the turn over of water in a large volume dead end tank.

If you increase the precharge to 3 to 10 psi above the highest water pressure, the tank will be empty of water, except when it is accepting thermally expanded water. That will happen fairly often.