Expansion Tank Necessary?

I've read a lot on here about PRVs and expansion tanks. I live in a new home in the Tarrant County area of Texas built in 2006 and we have two electric water heaters and a recirculation pump. From what I can tell we do not have an expansion tank. Are they necessary?

The last time a plumber was at our house they told me my pressure was about 70.

Just curious. Thanks!

If you have a "closed" system, meaning water enters the structure but cannot return back to the main,

yes.

Electric water heaters are somewhat less susceptible than gas to building up pressure and needing an expansion tank. The reason is that they heat more slowly than gas. The principal is still the same....heat water and it expands a little. A PRV may block that expansion and thus cause a rise in pressure in the house. Even if the PRV has a bypass, it will still allow the house side to rise equal to the street side.

You need an expansion tank only if there is something closing your water system off from the supply. Two things do that: a PRV and a check valve. When you use hot water, it gets replaced with denser cold. Then, it gets heated and expands. If the valves are all closed, it actually pushes back out into the supply to the street. If it is blocked by a check valve (there's one in a PRV), then the only thing that can happen is escape at the weakest point. Since water doesn't compress, it has to go somewhere. Common places are toilet valves, breaking washing machine hoses, plastic pipe to filters or icemakers, or, if all of those are strong enough, the safety valve at the water heater called a T&P valve (temperature and pressure).

A leaking T&P valve can be a symptom of needing an expansion tank, but it could also be an old valve, or an overheated WH tank.

Note, some utilities put check valves in as it makes the distribution system safer since you can't push water back into the main that might have been polluted by something the homeowner did.

If you are under the IPC you need an expansion tank reqardless.

tanj

quote;
If you are under the IPC you need an expansion tank reqardless.

Correction, you do not necessarily "NEED" one, but they may REQUIRE one, which can be two entirely different things.

HJ nailed it. If it is required, end of story, but it is only needed if you have a closed system which is created when you have a pressure regulator valve or a check valve the prevents expansion for being absorbed by the city main. As pointed out, electric heaters create less problems with expansion because the heat slower than gas, however I would install an expansion tank if the system was closed even if there was no problem with the T/P tripping just to be on the safe side. Others might disagree with that philosophy. 70 psi is on the high side of desirable pressure, but is still within reason as long as that is a more or less constant reading. To really know that, you need your own gauge and check the pressure at different times of the day.

Gary Swart said:

As pointed out, electric heaters create less problems with expansion because the heat slower than gas,

Click to expand...

Unless I'm missing something, delta T is the same, regardless of the type of heater, so it doesn't matter gas or electric. What matters is how big the tank is, how much you've used, and the amount you need the water heated. Now, a gas WH MIGHT have a quicker recovery, but in the end, it's how much that water was heated that matters, not how fast you get there.

Re:

Thank you very much for the helpful responses. I did some research. We do not have a PRV. And definitely not an expansion tank. I called the water company and they said when they set meters they also do not include a PRV. So we do NOT have a closed system. I went to Ace Hardware and picked up a pressure gauge and tested the outside hose bibs and we are actually getting about 55 psi around the house, so not as high as I thought. When I checked the side of the house by our master bath, the initial reading was higher but then it lowered to about 55 after few seconds. Not sure why that was but it was the first time I've ever checked anything like that.

Our Water Heaters were drained and flushed in the fall and when that was done our T&Ps were also checked.

I want to be sure on our pressure, so our plumbing company is coming out Monday morning to check our pressure with a more sophisticated pressure gauge.

While the utility company rarely installs a PRV, they MAY install a checkvalve...they aren't the same thing (even though a PRV may have a checkvalve as part of its internal design).

If your gauge has a second hand that shows max pressure, leave it connected overnight. Daytime is one thing, overnight is another. It would be especially interesting to watch it before and after a large hot water use. A true open system would not show any effects from the WH reheating water, a closed system would, and it could be dramatic.

hj said:

quote;
If you are under the IPC you need an expansion tank reqardless.

Correction, you do not necessarily "NEED" one, but they may REQUIRE one, which can be two entirely different things.

Click to expand...

Yes indeed.

A lot of plumbers think that if there is a well system, the well tank will take care of the expansion and truth told, it probably will. However the IPC don't say that nowhere, nohow. It probably should.

an addition

quote: Beware of Tankless heaters and those that would fill your head with lies and useless information about them.


You may have to add expansion tanks to that warning,