Expansion tank pressure increase

[Tim Morrison]

I had an expansion tank installed and I read here that I should see no or very little change in the water pressure when the water heater runs. I am seeing a 10 pounds increase and maybe more. What is the most acceptable pressure rise with an expansion tank? Is this tank not adjusted properly? Thanks!

[Scott Patterson]

An expansion tank by itself could not raise the pressure, if anything it might lower it. Do you have a pressure reducing valve on the supply line?

[Lon Henderson]

I had an expansion tank installed and I read here that I should see no or very little change in the water pressure when the water heater runs. I am seeing a 10 pounds increase and maybe more. What is the most acceptable pressure rise with an expansion tank? Is this tank not adjusted properly? Thanks!

Sounds like your expansion tank is performing correctly. The psig will increase as warm water expands into the tank and compresses the bladder. If you didn't see any increase in psig then, that would indicate that your plumbing system is not closed.

Over time, the expansion tank will loose air because just like a tire, air will diffuse through the bladder.

[Tim Morrison]

Thanks for your replies. Yes, there is a pressure regulating valve on the line creating a closed system.

[Jim Luttrall]

Sounds like either the expansion tank is too small or the pressure regulator is bleeding pressure through.

An easy diagnostic is to simply bleed the water pressure down to normal static pressure with the water heater off and wait long enough for pressure to build without opening any taps. Read the pressure every 5 minutes or so until you confirm either steady pressure or rising pressure. A rise in pressure indicates a defective regulator.

If the pressure rises without the water heater running then it is the pressure regulator.
If the pressure rises ONLY when heating a tank full of water from cold to hot then it is a defective or too small expansion tank.

[Lon Henderson]

Sounds like either the expansion tank is too small or the pressure regulator is bleeding pressure through.

An easy diagnostic is to simply bleed the water pressure down to normal static pressure with the water heater off and wait long enough for pressure to build without opening any taps. Read the pressure every 5 minutes or so until you confirm either steady pressure or rising pressure. A rise in pressure indicates a defective regulator.

If the pressure rises without the water heater running then it is the pressure regulator.
If the pressure rises ONLY when heating a tank full of water from cold to hot then it is a defective or too small expansion tank.
I disagree with everything you said. So, if I'm wrong then I need some major re-edumacation.

If the pressure regulator is bleeding pressure, then the purpose of a closed system is being defeated and the expansion tank may not show a psig increase. The purpose of homes having a closed system is to prevent the water in a residence's system from being backing back into or drawn (sucked) back into the city's system.

If the pressure rises markedly in the expansion tank without the water heater in heating mode, then that might be some defect in the pressure regulator but unlikely in the expansion tank. However, the psig better rise when the water heater is in heating mode. Otherwise that pressure increase from warming water expanding has found some other place to push water such as back through the pressure regulator.

In normal water usage, like taking a shower, hot water is leaving the water heater, the average water temp in the water heater is dropping as cold water is added, and once the demand is over (the shower is finished) the psig at the expansion tank should be slightly lower. Now the water heater is heating the introduced cold water which is expanding into the closed system as it heats and the expansion tank should start show an increase in psig until the water heater shuts off.

Without shutting off the main water supply and running a faucet until it dribbles; you cannot measure the actual air pressure in the expansion tank.

And BTW, the expansion tank should eliminate the chance of air hammer.

[Scott Patterson]

My SWAG is that the PRV is going bad and that is causing the fluctuation of the water pressure. I just do not see how the expansion tank could cause the pressure to rise 10psi, it just does not pass the logic smell test…. :)

[Jim Luttrall]

I disagree with everything you said. So, if I'm wrong then I need some major re-edumacation.

If the pressure regulator is bleeding pressure, then the purpose of a closed system is being defeated and the expansion tank may not show a psig increase. The purpose of homes having a closed system is to prevent the water in a residence's system from being backing back into or drawn (sucked) back into the city's system.

If the pressure rises markedly in the expansion tank without the water heater in heating mode, then that might be some defect in the pressure regulator but unlikely in the expansion tank. However, the psig better rise when the water heater is in heating mode. Otherwise that pressure increase from warming water expanding has found some other place to push water such as back through the pressure regulator.

In normal water usage, like taking a shower, hot water is leaving the water heater, the average water temp in the water heater is dropping as cold water is added, and once the demand is over (the shower is finished) the psig at the expansion tank should be slightly lower. Now the water heater is heating the introduced cold water which is expanding into the closed system as it heats and the expansion tank should start show an increase in psig until the water heater shuts off.

Without shutting off the main water supply and running a faucet until it dribbles; you cannot measure the actual air pressure in the expansion tank.

And BTW, the expansion tank should eliminate the chance of air hammer.


I get where you are coming from, but you are thinking of water bleeding to the city rather than to

the house. Water will flow only from high pressure (the city) to low pressure (the house).

While the purpose of a closed system may be to prevent water from flowing back into the city

system, the REASON it is a closed system (when a pressure reducing valve is present) is that the

city pressure is higher than the house. Water from a lower pressure zone (the house) cannot flow

back to higher pressure zone(the city) simply due to physics, not that it is intentional just to

create a closed system. The closed system is simply a byproduct of the physics of a pressure

reducing valve.

Now if you have a one way check valve to intentionally create a closed system you may have a

point. A failing check valve would allow water to flow back to the city and there would be no

pressure rise due to thermal expansion.


When the water cannot flow back to the city because of a check valve or a pressure reducing

valve, we have a closed system and therefore a thermal expansion device is needed to

accommodate the increased pressure due to the growth of the water. If the capacity of the

thermal expansion tank is too small, then the pressure in the system will rise once the thermal

expansion tank is overwhelmed. Agreed?


IF the thermal expansion tank can be overwhelmed by being to small, then it is also possible to

overwhelm the tank if higher pressure water from the city bypasses through a defective pressure

reducing valve and the pressure in the system will rise.


To put some numbers to the explanation, say you have a house which has a pressure reducing

valve set to 60 PISG and the city pressure is at 120 PSIG. There is no way water is ever going to

leave the house lines and go to the city lines unless something causes the house pressure to

double and even if the pressure more than doubles and water flows backwards through the

pressure reducing valve no one really cares (except maybe the city) but the homeowner would

never know.

If however in the same scenario the pressure reducing valve is defective the house pressure will

rise to whatever level the pressure reducing valve allows. So quite easily the pressure could rise to

the same as the city pressure. I see this on occasion where the pressure will be one reading

immediately after attaching the pressure gauge and a few minutes later it will rise. Open the valve

and close it and the pressure falls again to the original reading.

[Jim Luttrall]

Sounds like your expansion tank is performing correctly. The psig will increase as warm water expands into the tank and compresses the bladder. If you didn't see any increase in psig then, that would indicate that your plumbing system is not closed.

Over time, the expansion tank will loose air because just like a tire, air will diffuse through the bladder.

I disagree with this :).

The thermal expansion tank is designed to prevent excess pressure in the system. Since water does not compress easily and air does, the captive air space in the tank gives the expanding water some place to go without raising the pressure in the system. It may raise the pressure a bit but 10 PSI sounds like the tank is too small or not functioning properly.

[Jerry Peck]

Water will flow only from high pressure (the city) to low pressure (the house).

The reason for the thermal expansion tanks is that, when the water is heated, it does flow back into the city supply (the house pressure is now elevated above the street pressure).

Utilities do not like water flowing back into their systems (the water could be contaminated) and thus many utilities require check valves.

When the street pressure is high, a pressure reducing valve is installed, the thermal expanding water meets a greater resistance going back into the utility supply (the pressure reducing valve acts like a check valve and does not easily flow back into the city supply), resulting in the house pressure increasing within the now confined house piping system - which is why the thermal expansion tank is needed.

[Bob Harper]

You need to first test the Schrader valve to see if water spits out. If so, then you have diagnosed the pressure rise from the failed diaphragm. Also consider the size of the tank for the system and BTU rating. If there is a boiler feed, valve that system off. Next, either isolate the tank from the system if a valve is present or drop the system, remove the tank then check the air pressure referenced to atmospheric pressure. The tank pressure should equal the normal static pressure in your house. With the tank still removed and capped or valved off, open the water and read the pressure. Observe the static pressure with NO water flowing and the WH at standby. If the pressure creeps up, you have a bad PRV. Note- many PRVs have strainers that rarely get cleaned. If the PRV tests ok, reinstall the tank with it properly charged and let her go. If the pressure rises about 80 psi while cooking water, replace the PRV. Don't forget to open the boiler feed once done.

[Jim Luttrall]

The reason for the thermal expansion tanks is that, when the water is heated, it does flow back into the city supply (the house pressure is now elevated above the street pressure).


I agree... somewhat. The reason for check valves is to prevent water from flowing back.
The reason for thermal expansion devices is to deal with the resulting pressure rise.
Same thing... kinda but not quite.

[Lon Henderson]

I disagree with this :).

The thermal expansion tank is designed to prevent excess pressure in the system. Since water does not compress easily and air does, the captive air space in the tank gives the expanding water some place to go without raising the pressure in the system. It may raise the pressure a bit but 10 PSI sounds like the tank is too small or not functioning properly.

We don't know how large his plumbing system is, but even at a 10pisg increase, it sounds like the expansion tank is doing its intended job. The OP stated that pressure only went up when his water heater was operating. He may have a huge water heater. There is nothing in his OP that gives suspicion that his regulator is "bleeding" excess pressure into his system.

I think the more interesting discussion is what expansion tanks and valves do. A few years ago, I attended a plumbing seminar where the speaker got it his explanation about expansion valves (not tanks) completely wrong. He later told me that of the 30 or so attendees, I was the only one that pointed out that he was mistaken. Some plumbers install these devices because the city code now tells them to do it without understanding what they do, how they work, and amazingly, even why.

Around here, only recently have thermal expansion tanks or expansion valves been required and they are required for any water heater replacement regardless of home age in most of our municipalities.

And in the FWIW category, I always check the Shrader valve. Once a month, an expansion tank will spray water or not have any air in it. The failure rate is high.

[Jim Luttrall]

We don't know how large his plumbing system is, but even at a 10pisg increase, it sounds like the expansion tank is doing its intended job. The OP stated that pressure only went up when his water heater was operating. He may have a huge water heater. There is nothing in his OP that gives suspicion that his regulator is "bleeding" excess pressure into his system.

I can see where you could assume that is was only when the heater was operating, but he actually said "I am seeing a 10 pounds increase and maybe more."

On the subject of "he may have a huge water heater" goes back to sizing the expansion tank to the size of the heater.
I still stand by my original theory that either the PRV is defective or the expansion tank has a problem. Otherwise there would be no pressure rise.

[Lon Henderson]

I had an expansion tank installed and I read here that I should see no or very little change in the water pressure when the water heater runs. I am seeing a 10 pounds increase and maybe more.


I can see where you could assume that is was only when the heater was operating, but he actually said "I am seeing a 10 pounds increase and maybe more."
You left out the end of the previous sentence that clearly establishes the circumstances.


I still stand by my original theory that either the PRV is defective or the expansion tank has a problem. Otherwise there would be no pressure rise.
You should be suspicious if there is no pressure rise as the water heater operates. Water expands when heated and the dissolved air in it will expand even more. And as you stated, because water is not compressible it has to go somewhere and, assuming a closed system, that will shove the bladder in the expansion tank compressing the air inside and raising the psig. Once the water heater reaches its preset temperature and shuts down, the system pressure should stabilize and likely decrease incrementally until the stored water in the water heater cools sufficiently to reactivate the water heater. The greatest change in psig will be when the water heater has been flushed and is heating a full reservoir of water. It's really simple.

[Lon Henderson]

I have to say that this was one of the most interesting threads I have seen in days. LOL

Always great when you can combine education with entertainment, but it has been a curious thread. As for myself, I think I've beat this horse to his last step.

[Jim Luttrall]

Lon, I think we agree on much more than we disagree. Mainly the disagreement is in the semantics and degrees.
If the pressure rose... say 40 PSI and he said it was only when the heater operated then both of use would arrive at the same conclusion. Or vice versa if the pressure was rising only when the heater was not operating and rising only 10 PSI we would likely be on the same page. Take away the degrees and ambiguities and bang, we are on the same page.