Air pressure for Thermal Expansion Tank above Water Heater

[cparke]

The expansion tank is the point in the system where the pressure does not change ...

I just had to install a new thermal expansion tank myself since the old one failed on Friday afternoon and you can't get a plumber until Monday unless you want to pay an arm and a leg for emergency services.

Anyway, the above quote from an old thread has me a bit concerned. To figure the air pressure, I temporarily installed the water pressure meter on the pipe where the old expansion tank was. I notice that if you run the hot water, the pressure drops, as one would expect. So is this normal, or maybe I am taking the above quote on the old thread that water pressure should never change somehow out of context?

Also, the value of the water pressure has concerned me. Most people are saying it is normally 50-60 psi. However, I measured 125 psi at the garden hose and 121 psi at the expansion tank connection. Both readings taken when I was as certain as possible that nobody was running the water. These values are double what others are saying. It's a brand new water pressure meter and came with the manufacturer's accuracy test results (measured within 1 psi at higher pressures this). Is this high water pressure possible in some locations, or am I maybe somehow taking the reading wrong?

Finally, how close does the air pressure really have to be exactly equal to the water pressure? I imagine the air pressure in the expansion tank normally changes somewhat with the weather (high vs. low pressure storm systems)? I think I got it pretty close to 125 psi, at least no more off than +5 psi, but is a value that is slightly off like this enough to cause a problem or cause the expansion tank to fail prematurely? Can I check the current air pressure or add/release air in the expansion tank (without removing it) by shutting off the cold water and letting the sinks run till they stop?

 

[Reach4]

You need a pressure-reducing valve (PRV).

I guess testing your gauge against another house could be worth trying, since a PRV is not a trivial install.

Yes, some areas do have such high water pressures. You can ask your water department what should be expected at your house.

 

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[Master Plumber Mark]

you need to kick down your home pressure to about 65 psi....
then set your thermal expansion tank at around 80 psi

 

[Fitter30]

Residential plumbing faucets, clothes washers and dishwasher are designed for lbs max. Yes they can handle more. Ex tanks should be within a couple of lbs of standing water pressure.
If you have 125 lbs water pressure surprise that the hw heater relief wasn't dripping sometimes or more. Their set at 150lbs.

 

[cparke]

You need a pressure-reducing valve (PRV).

I guess testing your gauge against another house could be worth trying, since a PRV is not a trivial install.

Yes, some areas do have such high water pressures. You can ask your water department what should be expected at your house.

Haven't really noticed any problems with the water pressure in the house, and the water does seem to run normally. Maybe worth getting another water pressure gauge in case this number is totally bogus?

 

[Master Plumber Mark]

Residential plumbing faucets, clothes washers and dishwasher are designed for lbs max. Yes they can handle more. Ex tanks should be within a couple of lbs of standing water pressure.
If you have 125 lbs water pressure surprise that the hw heater relief wasn't dripping sometimes or more. Their set at 150lbs.

This guys pressure is waaaaaay too high in his home

The general idea is to throttle down the pressure to around 60- 70psi so when you flush your toilets
they dont sound like a F15 Tomcat jet taking off from an aircraft carrier...

the high water pressure is not good for any plumbing fixtures and any materials made of plastic
that could snap--crack under stress from the strain ---- like ballcocks, washing machine mixing valves,
washing machine supply lines, toilet supplies , dishwasher control valves., ect....

...

 

[cparke]

You need a pressure-reducing valve (PRV).

I guess testing your gauge against another house could be worth trying, since a PRV is not a trivial install.

Yes, some areas do have such high water pressures. You can ask your water department what should be expected at your house.

It's a condominium with multiple buildings and 4 attached homes in each building, if that makes a difference. The main water line enters my unit but serves all four units in the building. I can check the water pressure at the other end, but I suspect it won't be much different. I got a new, different water pressure gauge, same reading, so probably not a defective reading. I am going to ask the complex supervisor what he thinks.

 

[Master Plumber Mark]

It's a condominium with multiple buildings and 4 attached homes in each building, if that makes a difference. The main water line enters my unit but serves all four units in the building. I can check the water pressure at the other end, but I suspect it won't be much different. I got a new, different water pressure gauge, same reading, so probably not a defective reading. I am going to ask the complex supervisor what he thinks.

With you being in a condo project with a common water supply for a bunch of units about the only thing you can do would be to install a prv valve going directly into the cold line going into your water heater if you do not have a separate cold line with a shut off to your unit... they will not gear down the whole complex

 

[John Gayewski]

you need to kick down your home pressure to about 65 psi....
then set your thermal expansion tank at around 80 psi

Why would he set the expansion tank at 80? 80 is the max for any domestic expansion tank. That's about like saying when you drive you should mash the gas pedal to the floor any time you need to accelerate.

 

[John Gayewski]

The expansion tank should be set at whatever pressure you set your prv. It should olny take on water when the water is heated and expands. The expanding watter will raise the pressure and spill into the tank. Properly sized, the tank will keep the pressure in the system below 80 and preferably near the set point of your prv.

 

[Master Plumber Mark]

The expansion tank should be set at whatever pressure you set your prv. It should olny take on water when the water is heated and expands. The expanding watter will raise the pressure and spill into the tank. Properly sized, the tank will keep the pressure in the system below 80 and preferably near the set point of your prv.

This subject is debatable and always has been,,, Water pressure in the area can rise and lower depending on the amount of use
in the area and the incomming water pressure can vary at what time of day you test it..... Set it wherever you want to ,, but I normally like to set them more about 15- 20lbs above wherever you set the prv valve at

 

[cparke]

UPDATE: I actually had a Pressure Reducing Valve (PRV) and all the units do, I just never knew it was there. All buildings in the complex get fire hydrant street curb pressure and each unit needs to step it down with a PRV. My PRV failed, and I guess that's why the previous expansion tank burst when the pressure suddenly jumped from 60psi to 120psi. Once the tank began to leak afterwards, it basically served as a sort of pressure reducing valve, so we did not notice pressure problems elsewhere in the house. I guess you could say the expansion tank took the beating and sacrificed itself to save my other faucets, etc.

I changed my expansion tank, and plumber changed my PRV and set pressure at 60. So the expansion tank air pressure should now be set to 60psi or how much higher?

 

[Jeff H Young]

better to follow the simple instructions they are all on line if you lost them . i dont think 70 would hurt but why do what they dont tell you ? exp tanks do go bad i dont think it sacraficed if pressure goes to 150 it spittles out the tand p no biggie but continued daily going go to 150 or 135 is hard on everything we dont want pressure that high but stuff shouldnt just blow out either sounds like you got it handled good luck