expansion tank question

If my PRV is broken and lets my incoming presure rise to around 110psi will that cause my expansion tank to fail also as it will just fill up with water and then pressurise the whole system to 110 then open my t&p valve when the water heats?

Also I have no expansion tank and my system is closed it sits at about 48psi. when my heater cycles to heat water it rises to about 70 psi but once it stops heating it drops back to 48 again I thought once it heated and expanded it would hold at the high setting. So once the water is at temperature does it contract back. No outlets were opened I am sure. Or is pressure excaping somewere?

Sorry for all the question tring to help a freind out.

It doesn't take a lot of a leak to reduce the pressure...could be from a faucet or it might, after time, leak back out to the supply. The prv normally acts like a one-way valve, but it may not be perfect in the backwards direction (that's my guess, I'm not a pro).

I dont think it is leaking out it only takes about 10 minutes for it to go back down. I think when the water heater is done heating the expansion is decreased. But I am no expert either I hope someone who knows will chime in. Everything I read says when water is heating it expands not hot water has more volume then cool water. But not sure.

hw

When it expands it has more volume. Once it expands it stays expanded until it cools down, or you open any faucet, hot or cold, and get a momentary rush of high pressure water before it reverts to normal pressure. Then the cycle starts over again if you used hot water.

so mine goes from 48psi to about 70 then back again when it cycles does that seem normal? I have left my guage on for a week and it never went over 75 psi. I dont think I need an expansion tank. But I have a wilkins br4 reducing valve that makes my system closed. Some people say every closed system needs tank but all the plumbers I have talked to around here say no if it dont go over 80 alot dont worry about it as it will not save you in a catastrophic water heater falure. Just test your T&P valve regularly and if it does start to discharge at some point then reevaluate your needs then.

And these are not bad plumbers one is the water inspector for our town.

Because water is relatively un-compressible, the additional volume ( cause by heating) is very small, maybe an ounce or two. This is why the pressure relieves so quickly when any water flow out. \


As far as returning to normal pressure a little while after heating: there is some natural convection up into the hot outlet piping and cold inlet piping. This causes some gradual cooling of a certain amount of water and a gradual reduction of pressure. Also, all the rubber components in all of your fixtures can compress imperceptively. All these thing cause the pressure drop you observe.

So jimbo do you guys think my plumbers advice is ok.
I just dont want my water heater to explode and destroy my house.

Not having an expansion tank is not going to cause something to explode. Every time the pressure rises because hot water is used and it expands upon being heated does cause (probably minor) stress on everything, but not enough to cause something to explode. Maybe, just wear out a little sooner or show up a worm gasket or something sooner than it might otherwise. My unprofessional opinion. Most things, pipes, gaskets, etc. are at least a little springy - they'll expand a minor amount under pressure (just not much). The expansion tank is designed to take that expansion, and it relieves stress on those things not designed for all of that cylcling. Course, they wear out, too, but they are cheap.

Jim in you unprofesional advice then what my plumber said sounds ok?
Since right now I am never getting over 80 psi even with expansion. I should be ok with out a tank. But to watch for discharge from my T&P and check it for flow every 6 months. And in the future if I do get discharge from the T&P from expansion we can install a tank then? Just looking for a little reasurance that my plumber is not just being lazy.

Thanks for your help and patiance.

You're probably okay. What I'd do for peace of mind is maybe turn the hot water heater off overnight, run some water out of it to check in the morning that it is truely cold (may not be depending on the insulation), run it until it is cold. This way, you start with a tank that is going to expand as much as it possibly could like if you have a teenager that runs the shower until the water is cold. THen, don't let anyone use the water. Turn it back on and see what the pressure gets to. Then you will know the worst case situation. Since it is winter, the incoming water is probably about as dense and cold as it gets, so the water will have to be heated the maximum possible. Then, see if you can live with it. Ideally, it shouldn't get much above 80 psi and definately shouldn't trip the prv. If it does, you might want to rethink the expansion tank, but again, you'll probably be able to skip it.

Labor wise, I'm guessing, but it shouldn't take much more than 1/2 hour to cut the pipe, install the t, shutoff and screw the tank in. Parts aren't much, relatively, either. Again, my unprofessional opinion.

if it does ever trip the T&P I will for sure get the expansion tank I did something simerlar to your test I ran the shower untill it was cold then no one turned nothing on even turned off the ice maker got to like a little over 80 only for about 2 minutes then went slowly back down to like 55. I may at some point get the tank but money is tight right now. But dont want to put my family at risk ya know. I will check for discharge from the tp valve and if I get any at all I will call the plumber and get the tank.

Thanks