T&P valve draining - expansion tank issues?

Question

I am having issues with my Water Heater. It's a slantfin tankless boiler (TR-30 PT). It's running on a 4 zone setup. I typically only use one of the zones though. We've been in the home for a year and a half. Towards the end of this winter I started finding a few gallons of water draining from the T&P valve every few days.

I tested the expansion tank and found it to be leaking water from gauge, leading me to believe the diaphram has ruptured and we needed a new one. I bought the same model replacement. An Amtrol #60 and gauges to test our home water pressure and PSI on tank. I attached the water meter to our outside hose line and read about 56 PSI. I then took the precharged (to 12 PSI) new tank and charged it further up to about 55 PSI before installing it. I then removed the old expansion tank to find it almost completely filled with water. Replaced it with the new tank. Turned water line back on and power to water heater. Gave it a day or so.

Now I'm still seeing water, perhaps more. So, even though the exp tank was shot, what else could be causing this?   Autofill valve?  I don't think the T&P valve is bad as it is draining water once the meter reaches the max level on the valve which is 30 PSI.  The slantfin temp range was set at 160 - 180. I lowered this to 140-160 and still same issue.

I then turned on the water from my faucet for a while, and the pressure on slantfin was hovering between 2 and 10. Got the temp on slantfin below 140, down to 120ish. It didn't seem to instantly turn back on, but then turned on the first floor of our home (ZONE 1 of 4), slantfin initiated and watched the gauge on the unit. As it heated the water from 120 to 140 (where min is set) it gradually increased pressure until right about hitting 140 the pressure then hit 30 PSI and T&P valve started draining.

Should I not have charged the new tank? I was under the understanding this needed to be same if not a few PSI below water pressure. Does this change if on a multi-zone house. If using a tankless system?

Should my T&P valve be rated to take on more pressure than 30 PSI?

Should the precharge be the same as the water supply line even if the tank will be placed after a "water pressure regulator valve" set between 12-25 PSI?

Any information would be greatly appreciated. I'm stumped...

Tester101 · Answer

The tank should be charged to the pressure at the attachment point.  If the tank is attached to a point in the system where the pressure is supposed to be 25 psi, the tank should be pre-charged to 25 psi.  If the tank is attached to a point in the system where the pressure is supposed to be 55 psi, the tank should be charged to 55 psi.

When the water pressure raises above the air pressure in the expansion tank, the water will begin to fill the tank and compress the air further.  As the water pressure decreases, the air pressure in the tank forces the water out of the tank. If the tank is charged too high, water will not be able to compress the air and fill the tank.

Note: Measuring the pressure in the tank while it's connected to a pressurized water system, will show the pressure in the water system not the pre-charged air pressure of the tank. To measure the air pressure of the expansion tank, you must isolate it from the system.

Mnc123 · Answer

Your hydronic heating system should operate between 15 and 20 psi. The expansion tank, which is pressurized by factory, should be pressurized close to the operating pressure. At no time should your system ever exceed, or come close to exceeding the pressure relief valve rating which is 30psi.

Most residential closed loop systems operate at 15-18psi. If the system is in fact exceeding 30psi then you have an issue. This can be caused by a faulty automatic fill valve, or pressure reducing valve.

Since you also replaced the expansion tank then you must have opened the closed loop system. You should make sure to purge any and all air out of the system for it to work properly. Also if the system is cold and filled to the desired pressure, when the system heats up it will increase the pressure. You will then have to relieve his excess pressure manually.

One last possible scenario is that your boiler has filled with calcium, or there is a restriction in the system. Normally if this is the case the boiler will eventually fault out on lack of water flow due to plugged up heat exchanger, or dirty inlet filter if the unit has one.

gbronner · Answer

I actually own a very similar setup.

The pressurization of a hydronic system needs to be calculated based on the height of the top radiator:

Please see this calculator.
https://www.achrnews.com/articles/97047-pressurization-of-closed-hydronic-systems

In my case, I was calculated 22 PSI for my expansion tank, and charged it to that level, but I live in a tall townhouse -- 55 PSI seems excessive.  I also used an accurate tire gauge and pressurized it before installation.

Additionally, I have had T&P pressure relief valves go bad, and it is relatively easy to replace one.

DaveM · Answer

I ended up doing this twice;  The heat exchanger coils for the HW supply in the boiler sprung a leak, and the pressure from the town CW supply was overpowering the T&P valve.

After the second failure, the plumber special ordered a better quality coil with a coating to resist hard water.

T&P valve draining - expansion tank issues?

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