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Expansion Tank Troubleshooting

Member Posts: 25
I have an SX-30V expansion tank on my cast iron hydronic boiler. When heating from cold to approx 140 degrees, my pressure increases from 12-25#. If it heats above 140 degrees, pressure builds close to 30#, although the PRV has not tripped. I originally suspected the diaphragm type expansion tank may be undersized for the large volume converted gravity system. It is not water logged and pressure matches system pressure.

I estimate there is around 200-250 gallons of water in the boiler, cast iron radiators and piping and the online sizing tools I've found seem to imply that the SX30V may be sufficient.

I now suspect that the expansion tank has lost charge - it looks to be 20+ years old. If it has lost charge, would this explain the rising pressure?

I plan to wait until closer to the end of heating season to address but would like to plan in case the PRV starts tripping in the near future. I don't have an isolation valve so I will have to drain the system. I plan to add a Webstone 41672 service valve and test the existing tank or maybe just preemptively install a new tank.

Thanks in advance for any guidance!

• Member Posts: 546
edited January 22
I'm not a pro, just a homeowner/mechanical engineer who does maintenance on our 4-unit condo building with 2 hot water boilers.

Our boilers are also cold start, so they also start around 70 degrees F and finish at around 140 degrees F. In this temperature range. Within that range, the water expands about 1.5%. So if you have 250 gallons, you can expect a volume increase of roughly 4 gallons due to thermal expansion.

The SX-30V has a tank volume of 14 gallons. So when you start with an empty SX-30V pressurized to 12 psi (absolute pressure = 12 psi + 15 psi atmospheric = 27 psi) and add 4 gallons of expanded water, you've now reduced that original pressurized air volume from 14 gallons to 10 gallons. And since at constant temperature, pressure x volume = constant, we can calculate the new pressure as follows:

New absolute pressure = 14/10 * (12+15) = 38 psi.

But this is absolute pressure, which we then convert back to gauge pressure by subtracting atmospheric pressure of 15 psi: 38 psi-15 psi = 23 psi gauge.

Note this is quite close to your observed 25 psi gauge when the system is hot. The above numbers assume the compressed air in the tank stays at the same temperature, but in reality the air warms some from compression and the warm water on the other side of the bladder. So add a few psi from that warming, and you get 25 psi.

So your estimate of 250 gallons of water is probably close to correct, and your SX-30V appears to be operating as designed. But since you're getting close to the PRV pressure, you may want a bigger tank.

• Member Posts: 25
@jesmed1 Wow thank you! I did some primitive calculations that made me think that it was working as designed but your calculations are much more concise and useful!

I checked out the serial # code and it appears the SX30V is from 1985! If it's been working like this for almost 40 years, I think I'll leave it alone until something breaks. Amtrols sizing tool doesn't allow for many inputs but Wessels has one that's more sophisticated. It looks like the SX40V is a better fit for my system.
• Member Posts: 546
edited January 22
kfCT1 said:

@jesmed1 Wow thank you! I did some primitive calculations that made me think that it was working as designed but your calculations are much more concise and useful!

I checked out the serial # code and it appears the SX30V is from 1985! If it's been working like this for almost 40 years, I think I'll leave it alone until something breaks. Amtrols sizing tool doesn't allow for many inputs but Wessels has one that's more sophisticated. It looks like the SX40V is a better fit for my system.

Yes, if we run the numbers with the 20 gallon SX-40V, we get

20/16 * (12+15)= 34 psi absolute with 4 gallons water expansion
34 psi absolute - 15 psi atmospheric = 19 psi gauge when system is hot.

So the larger tank buys you 23-19=4 psi additional margin.
• Member Posts: 15,295
@kfCT1

Bladder tanks come charged with air from the factory at 12-15 psi. The bladder in the tank is not 100% airtight and air can pass through the bladder. The rate that the air can pass through (by osmosis) is very small. However, if your tank was installed in 1985 it is very likely that it has lost some air.

If you have a way to isolate the tank and get the water pressure off the bladder you could then check the air pressure and recharge it if needed. But it's probably not a good idea to fool with a 40-year-old tank.

When you do change the tank install a ball valve between the tank and the heating system. Then install a tee between the ball valve and the expansion tank. On the branch of the new tee install a drain valve. This allows you to isolate the tank and drain the water pressure off the tank so you can accurately check the air pressure.

When changing a failed tank use extreme caution. If the bladder has failed, the tank will be full of water and very heavy. No broken toes. Drill a hole in the tank to let the water out before removal.
• Member Posts: 25
Thanks, @jesmed1. Will go with the larger tank when the time come.

@EBEBRATT-Ed Thanks for the tips. Luckily it's floor mounted so it will make the swap much easier. The webstone 41672 valve looks like a great product for serviceability and allows for the addition of an air vent. It has an integral T and drain valve. The tank definitely is not waterlogged and I was shocked to see the 1985 date on there. I typically hear of these only lasting for around 10 years.
• Member Posts: 15,295
@kfCT1

The Extrol tank in my house was there when I bought the place in 1986 so I don't remember how old it was when I bought the place, but it lasted until 2019. The newer ones don't seem to last as long for some reason.
• Member Posts: 25
@EBEBRATT-Ed unfortunately true for most things made today. The expansion tank is original to the 1985 installed Utica 250k BTU cast iron boiler. Both serviceable after 40 years even though the boiler condenses pretty hard. Hoping to get at least a few more years out of it.
• Member Posts: 983
When I replaced my expansion tank, I made the incorrect assumption that the new tank would keep the pressure at 12-15 psi at all times. WRONG. The tank gives the expanded water a place to go, but does nothing (or nearly nothing) about the pressure increase. When you heat water in a closed system, the pressure is definitely going to increase.
• Member Posts: 25
@MikeAmann that's ok I expect some increase. I expect 12-15 cold and 20 when heated. Right now I'm pushing 25 PSI heated and almost 30 PSI with extended runtimes when trying to increase my t stat set point.
• Member Posts: 1,228
kfCT1 said:

When heating from cold to approx 140 degrees, my pressure increases from 12-25#. If it heats above 140 degrees, pressure builds close to 30#

Your tank has failed and is becoming waterlogged. Tapping on it is not going to tell you if it is still good. The pressure rise you describe indicates a failed tank.
I DIY.
• Member Posts: 25
@WMno57 I'm confident it's not but thank you for your opinion
• Member Posts: 4,677
kfCT1 said:

@WMno57 I'm confident it's not but thank you for your opinion

Then, pull the tank and check it. Then you'll know for sure.
• Member Posts: 1,228
OK, then pump some more air into it. You can do that now. You don't have to wait for spring.
I DIY.
• Member Posts: 1,228
edited January 23
As Pecmsg said, the correct method is to pull it. If you don't have an isolation valve, then you can incrementally add small amounts of air, without pulling it.
I DIY.
• Member Posts: 25
I'd rather not pump air into a 39 year old diaphragm tank. System is operating fine as is. If it starts tripping the relief valve, I'll swap it out. At this point I feel confident that the pressure rise is due to an undersized expansion tank.
• Member Posts: 4,677
When did it become "Undersized"?
• Member Posts: 2,119
If it was ok for that long then it's very likely the adequate size! What could have changed in nearly 40 years? It likely has lost its air charge, especially if it hasn't been recharged annually like it should have.  I'd just replace it.  Install a Amtrol Pro series tank and pressurize it properly before you install it.
• Member Posts: 25
Refer to Jesmed1's comments above. I have owned this home for a couple of years. As far as I can tell the system has operated this way since it was installed in 1985.
• Member Posts: 1,228
I applaud you for being cautious around pressure vessels. In this case, there is little to no danger or risk. Both danger to your personal safety and risk of possible damage to the tank.
I don't know if that is a bladder tank, or a diaphragm tank. It doesn't matter. Either the air has leaked out the Schrader valve, or the bladder/diaphragm is compromised and air has leaked into your hydronic system. If the bladder/diaphragm is compromised, what you now have is a Compression tank with bits of rubber floating around. It will still absorb volume changes from heating, as long as it has some air in it.
Ask yourself; Could this have been undersized and operating with these pressure changes for 39 years?
I DIY.
• Member Posts: 25
Thanks all for the insight. As I said previously, I plan to replace the tank in the near future and add service valve while doing so. Based on system volume calculations, the SX30V is likely undersized and I will move up to the SX40V model.