New Expansion tank on hot water boiler - but still getting water overflow

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Simpson148

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I've recently installed a new expansion tank on my hot water boiler. The tank is not full of water and releases air when i press the schrader valve.
However, the bucket below my TP relief valve continues to fill with water...I need to empty it once a week or I get a wet basement floor.
I have attached a diagram of the system. System pressure looks like 20 and the tank is 11 or 12.
Please let me know if there is anything I can try to do to fix the issue or if I need to call a plumber.
Thanks, Peter
 

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Anderson

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Excellent diagram. Did you ever figure this out? I am having a similar problem in my system. All seems to be working fine, but nonetheless the pressure in the system builds to 25+ after an hour or so, tripping the boiler relief valve.
 

Anderson

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I don't want to hijack this thread, but in case it's a similar issue, I'll note that in my case, it's the valve on the boiler loop. The system pressure and the expansion tank are both set to 12 (two-story house). The pressure reducing valve on the auto feed line is also set to 12, as close as I can figure.
 

John Gayewski

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There are really only three possibilities for over pressure. The expansion tank is incorrectly set, broken, or incorrectly sized. The fill valve (prv) is broken or dirty and delivering too much pressure (this isn't likley as the pressure would most likley rise much higher. Or finally there is a hole in the heat exchanger of the domestic hot water tank/coil (this also should make the pressure rise much higher).

In most cases there is problem with the expansion tank either it's been valved off or blocked off from the system and can't accept the expanded water or its too small. The expansion tank should be set with while disconnected to the system and the system should be cold.
 

Anderson

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John, thank you for your help. I will chase down each of those issues. One thing I've noticed is that as the boiler pressure reaches into the 20s, the top of the expansion tank begins to cool. My assumption there is that it stops being able to take on additional water, as if it's reached its limit. Does that help narrow things down at all? If it helps at all, the expansion tank is on the outflow side, meaning the water running past it is around 180F.
 

Reach4

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Try closing the water feed valve.

When the system is cold, knock on the thermal expansion tank. It should sound empty.

How big is the thermal expansion tank?
 

John Gayewski

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John, thank you for your help. I will chase down each of those issues. One thing I've noticed is that as the boiler pressure reaches into the 20s, the top of the expansion tank begins to cool. My assumption there is that it stops being able to take on additional water, as if it's reached its limit. Does that help narrow things down at all? If it helps at all, the expansion tank is on the outflow side, meaning the water running past it is around 180F.
The drawing posted above does show a potential issue with the orientation of the fill valve. The fill valve should be piped in on the same line as the expansion tank. One could imagine a scenario where the circulator starts and drops the pressure at the fill valve causing the fill valve to think it needs to add. If that happens and it add too much water the system could be overfilled. The expansion tank is the point in the system where the pressure does not change and this is the best place for both the circulator and the fill valve. I don't this this is at issue here as the fill valve is located behind the point of no pressure change, but I haven't tested a system with this type of setup and can't say whether or not this scenario would play out negatively.
 

Anderson

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Try closing the water feed valve.

When the system is cold, knock on the thermal expansion tank. It should sound empty.

How big is the thermal expansion tank?
I will try that. I have an Amtrol Ex-15 and a Noritz NRCB199DV. I have wondered whether the tank is undersized for the capacity of the boiler.
 

Anderson

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The drawing posted above does show a potential issue with the orientation of the fill valve. The fill valve should be piped in on the same line as the expansion tank. One could imagine a scenario where the circulator starts and drops the pressure at the fill valve causing the fill valve to think it needs to add. If that happens and it add too much water the system could be overfilled. The expansion tank is the point in the system where the pressure does not change and this is the best place for both the circulator and the fill valve. I don't this this is at issue here as the fill valve is located behind the point of no pressure change, but I haven't tested a system with this type of setup and can't say whether or not this scenario would play out negatively.
Good point. In my case, the feed valve is in a line between the backflow valve and the expansion tank, so it would seem correctly located.
 

Fitter30

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Leave pump run boiler off till water temp falls below 100° the turn pump off. 1 lb of pressure = 2.31'. 10 lbs 23.1'. Bottom of the boiler to top of the highest heat emitter whatever is that height add another 3 lbs. That's all the pressure u need. Now with a 50lb tire gauge check ex tank air pressure it should match the boiler pressure. Use a hand tire pump to adjust pressure or be very careful with a compressor. Turn pump and boiler back on.
I will try that. I have an Amtrol Ex-15 and a Noritz NRCB199DV. I have wondered whether the tank is undersized for the capacity of the boiler.
expansion tanks are sizes by system water volume and water temp.
 

Anderson

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Here is a calculator for sizing. I don't know if it is good, but it seems worth a try.
Leave pump run boiler off till water temp falls below 100° the turn pump off. 1 lb of pressure = 2.31'. 10 lbs 23.1'. Bottom of the boiler to top of the highest heat emitter whatever is that height add another 3 lbs. That's all the pressure u need. Now with a 50lb tire gauge check ex tank air pressure it should match the boiler pressure. Use a hand tire pump to adjust pressure or be very careful with a compressor. Turn pump and boiler back on.

expansion tanks are sizes by system water volume and water temp.
Thank you. That was helpful. I walked through that sizing test, and I think it was overestimating the amount of water in my system (or I should have entered a smaller BTU/hr total), so I calculated the volume of my cast iron radiator system and piping (roughly 42 gallons). Based on that, I need an Amtrol EX-30 (4.4 gallons), not an EX-15 (2.2 gallons). It's odd, though, because the system has worked fine for the past two years (it's three years old now). And I don't think it's a leak, because the pressure doesn't increase when the system is cold. But based on my calculations, I do need a larger tank.

Fitter30, I will run that test now. And to clarify, I should be matching the tank pressure to the boiler pressure when the water is cold, correct?
 
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