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# Expansion tank not taking the extra pressure.

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• Member Posts: 421
Maybe I am describing it incorrectly. Let me see what I can find.
• Member Posts: 421
edited December 2021
So far I found this, which backs up the math from ABOVE:

Thermal expansion
Heating water causes the volume of water to expand. Since water does not compress, the pressure inside water heater builds up.
Normally the excess pressure is not a problem because it pushes out into the cold water line and back to city water supply (or water well tank) where the pressure is equalized by the water supply.

Closed system
Problems occur with water heater when the pressure cannot travel out the cold water line.
Check valves and incoming pressure reducing valves will block the pressure, and create a closed system where dangerous pressure builds up inside the water lines.
A check valve is a one-way valve that lets water flow one direction, but not the other direction. For example, a recirculation system and other plumbing installations sometimes have a one-way check valve. Sometimes city water meters come with a check valve.
The check valve stops the water from expanding back out into the cold water line, and creates a dangerous build up of pressure especially when tank is heating water. Evidence of a closed system is a TP valve that releases water whenever tank is heating, or a bulging water heater, or even a ruptured tank if it has become rusted over time.

The solution to a closed system is to install a correctly-sized expansion tank anywhere on the incoming cold water line between tank and check valve ... the result: pressure from heated water expands into the expansion tank.

'The water in a water heating system expands when it is heated and has a greater volume. Since water does not compress like air, the expansion causes high pressure inside the water heater and plumbing system.

Thermal expansion, and the resulting high water pressure are a major consideration when designing water heaters and plumbing systems.
For example, residential water heating tanks are rated at 300 psi (when new), with 150 psi maximum safe operation.
The TP valve releases water pressure when pressure exceeds 150 psi .... but 150 psi poses a serious risk to older tanks that are rusted, and for plumbing pipes and fixtures in general. High pressure causes plumbing failures and early demise of heater.
Maximum incoming water pressure is 80 psi, with 50 psi recommended.
If pressure is above recommended, then a pressure reducing valve is installed, along with an expansion tank. The pressure-reducing valve and correctly-sized expansion tank are installed on incoming cold water line

Normal water heater operation will push the pressure back through the incoming cold water line, and out to the water main, or push pressure back into the water well tank.
Water expands approximately .00023% per degree F temperature rise.
In a normal system (not closed): ''Water expands approximately 2% in volume for a 100°F temperature rise (from 40°F to 140°F) ....40 gallon water heater, water being heated to its thermostat setting will end up expanding to about 40.53 gallons.'' This amount of thermal expansion, equal to about 1/2 gallon of water, can be easily absorbed back into the incoming cold water line.

In a closed system, the water expansion dynamic is much different.
If the incoming cold water line has a check valve at water meter, a pressure reducing valve (PRV), or sometimes a water softener can cause the problem, the pressure from thermal expansion will get backed up. The expanding water cannot be pushed out into the water main, and the pressure builds up because water doesn't compress.
For example, in a closed system where water is confined to a storage tank, a temperature rise of just 10°F (increasing from 75° to 85°F), increases pressure from 50 psi to 250 psi. This amount of pressure will quickly exceed the TP rating, causing water to be released in a series of short bursts, except when water is being used.' Troubleshoot TP valve
"...water will not boil under pressure until the temperature reaches approximately 297° F. The energy potential in the superheated water is called latent heat energy and will flash to steam when exposed to normal atmospheric pressure. This flash to steam (steam explosion) has the explosive potential of over two million foot-pounds of energy."
• Member Posts: 11,896
Chances are after removing the check valve there is enough cold/hot water usage to drop the pressure enough for the expanded water.
• Member Posts: 421
edited December 2021
Thanks Ed. What if I simply relocated that check valve closer to the regulator where the city supply enters the house? That way the extra pressure would pushing back against the entire volume of cold water in the pipes of the whole house, instead of just the short run surrounding the IWH. Would that make any difference?
If this is worth a try, then should the check valve go before or after the regulator, or no difference?
I do have a nice spot after the regulator.
• Member Posts: 1,792
I understand that the lower house pressure cannot push back against the higher incoming street pressure. Why is it that I see homes all the time that are on public water supply with a water heater installed without a potable water expansion tank and it doesn't seem to affect anything? I saw several this week that only had a pressure reducing valve after the water meter and none had any problems. Where is the expansion going?
• Member Posts: 2,802
SuperTech said:

I understand that the lower house pressure cannot push back against the higher incoming street pressure. Why is it that I see homes all the time that are on public water supply with a water heater installed without a potable water expansion tank and it doesn't seem to affect anything? I saw several this week that only had a pressure reducing valve after the water meter and none had any problems. Where is the expansion going?

Because there's a large enough Volume of water in the house, hot and cold to accept the small expansion.
A small drip anywhere also relives pressure.
• Member Posts: 18,680
SuperTech said:

I understand that the lower house pressure cannot push back against the higher incoming street pressure. Why is it that I see homes all the time that are on public water supply with a water heater installed without a potable water expansion tank and it doesn't seem to affect anything? I saw several this week that only had a pressure reducing valve after the water meter and none had any problems. Where is the expansion going?

The pressure from the expansion is greater than the pressure in the public water supply, and the water does back out into the mains a bit. Many pressure reducing valves do not, in fact, act as check valves, and in a system with a pressure reducing valve, but no check valve, the house pressure will rise to the street pressure and then backflow.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 298
edited December 2021
SuperTech said:

Why is it that I see homes all the time that are on public water supply with a water heater installed without a potable water expansion tank and it doesn't seem to affect anything? I saw several this week that only had a pressure reducing valve after the water meter and none had any problems. Where is the expansion going?

Two Reasons:
• They never take the entire volume of their water heater from 50F to 140F.
• They are using domestic water (cold or hot) while the water is heating and expanding.
Not a completely foolproof system though. Take the scenario of a gas supply interruption in winter. Uncommon, but it could happen. You have been away over the weekend, and come home to find a cold house with cold water coming out the hot tap. The area around the water heater smells like gas. You open the windows, cool the house more, close the windows, light the water heater pilot, crank up the house thermostat, and immediately go out to get warm and have dinner.

If your lucky and have good pipes and a working pressure relief valve on the water heater you will have 1/2 a gallon of water on the floor next to the water heater. Alternatively, if one spot in your domestic water system lets loose before the PRV pukes, you will have a deluge until you shut off the water.
• Member Posts: 298
In Mike's house he said the city pressure is 110psi. If there is no check valve in the water meter, his house pressure could rise above 110 and push the half gallon back into the municipal supply.
• Member Posts: 18,680
WMno57 said:

In Mike's house he said the city pressure is 110psi. If there is no check valve in the water meter, his house pressure could rise above 110 and push the half gallon back into the municipal supply.

Exactly. Fortunately, most US plumbing fittings can withstand that much pressure.... though they really shouldn't be asked to.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 421
SuperTech said:

I understand that the lower house pressure cannot push back against the higher incoming street pressure. Why is it that I see homes all the time that are on public water supply with a water heater installed without a potable water expansion tank and it doesn't seem to affect anything? I saw several this week that only had a pressure reducing valve after the water meter and none had any problems. Where is the expansion going?

Exactly my point. You stated it better than me.
• Member Posts: 11,896
Most all PRVs are check valves.

Water cannot be compressed.

In the many houses with PRVs and no expansion tanks a little water usage "uses up" the "expanded water"

It's as simple as that. Why make it something it is not?
• Member Posts: 1,792
I understand it now. It seems like a situation where just because a water heater can be installed without a potable water expansion tank in a home on a public water supply without problems it doesn't mean that it's the right thing to do. This is why backflow prevention is important.

It does seem like I come across a lot more leaking T&P relief valves due to lack of expansion tank or tank failure in the winter when the water supply to the home is colder.
• Member Posts: 421
I removed the SharkBite check valve last night.
And the pressure still increases, the same as before, except now all of the pipes (cold now included) are subject to the pressure increase all the way back to the regulator at the street supply. And yes, the regulator is acting just like a check valve.
Maybe I will reinstall the check valve back where I had it.
• Member Posts: 11,896
Expansion tank issues can drive you nuts.

Water can't be compressed.

We installed a brand new job about 10 years ago. It was chilled water (glycol) I don't remember the size of the chillers (maybe 2 30 ton chillers) but we had installed a 500 gallon buffer tank because it was a process load to prevent short cycling. It had a bladder type expansion tank that sat on the floor. The pipe came out of the top of the buffer tank (where the air would collect) over and dropped down to the expansion tank with a valve on the pipe that was open to the tank. The system piping was all 6" so it was a decent sized system.

About a month after start up (i did not do the start up) the customer called and said they had to constantly drain and add water/glycol to the system as the pressure would be all over the place sometimes high sometimes low.

I went over and the chillers were shut down and the water was room temperature. The system pressure was high. On chilled water you only get a 10 degree TD when operating, you only have to worry about expansion when the chillers are down the water warms up and the expansion tanks are small compared to heating systems.

Because they had glycol in the system there was no make up system connected.

So I was thinking either the EX tank was too small or maybe lost it's air.

I drained literally a coffee cup full of water from the system and the pressure dropped 10 psi...unreal.

So I knew it was an expansion tank problem.

I valved off the tank took the union apart........no glycol??

I checked the air pressure on the bladder 12psi no problem. I reconnected the tank and opened the valve didn't here any water rush in.

I climbed up on top of the 500 gallon buffer tank and found another ball valve in the ex tank line that had never been opened.......you couldn't see it from the floor.

• Member Posts: 18,680
And then you went out to the truck and pounded on the steering wheel for a while?
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 421
I did reinstall the check valve back where I had it last night.
• Member Posts: 4
I think your problem is the tankless water being connected after the indirect. Normally the tankless coil is disconnected. Second installation instructions for most expansion tanks want the inlet facing up Wirenut
• Member Posts: 421
Well I have messed with this all I am gonna. I'm done.
Everything is working the way it should, I just didn't expect to see that much of a pressure increase.
Stupid me for having gauges to monitor the system.
The extra pressure is not a problem and the expansion of the heated water is going into the tank.
Even if the tankless coil was not there, what comes after that is closed faucets, so there wouldn't be a difference. I could test your theory easily because I already have a bypass valve installed should the tankless coil fail in the future.
I guess I just expected the pressure to remain at 50 psi as it did before with just the tankless.