Hot water heating system pressure

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DIYorBust

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Why is my boiler pressure getting so high?

I have a 2 pipe hot water system with baseboard convectors. We recently drained the system to install some additional radiators. I like to keep the pressure at around 25 psi, in this building the top floor is about 40ish feet above the boiler. However a little extra pressure seems to prevent the top floor radiators from becoming airbound. Pipes are sweat copper and boiler is a navien NHB not a combi. There is an indirect water heater zone.

So we refill the system yesterday and the boiler reports 30 psi. I call my installer and he says it's probably a defective pressure sensor in the boiler, and to check pressure with a gauge. I have a brand new gauge, and I check the pressure at the purge valve - 17psi. Just when I'm thinking all is fine, the T&P relief valve on the boiler piping opens, it's a 30psi unit, and the boiler is reading 33 psi. Now I'm suspecting my gauge is not right, new gauge ordered(do i need to calibrate the gauge?).

So I do the following. I drain the system down, pump the expansion tank to 25 psi. There's no evidence of damage to the tank, it's holding air as far as I can see. Now I refill, purge, and bleed the system as best I can. I adjust the PRV for approximately 22 psi(as read on the boiler) with the system cold. It's a taco cast iron prv with a max setting of 25psi. Next I turn on the heat and wait for the boiler to read 180 degrees. The pressure is around 25 and holds for a few hours, I am happy. I come back this morning, the pressure is 32. Do I need to replace the expansion tank? The PRV? Also the system ran fine for about 4 years with this configuration.

What on earth is going on?

Thanks,

DIY
 

Reach4

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Could be the thermal expansion tank has failed. It could be the auto-refill let too much water in.
 

Sylvan

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1- The automatic feeder maybe set too high

2- The expansion tank too small

3- If you have a tankless coil there maybe a pin hole in the coil

4- 40 feet cold fill will equal 17.32 pounds (static pressure) so normally we add 4 PSI as a fudge factor to help displace air in the system


40 X .433 = 17.32 (PSI) 17.32 x 2.31 = 40 ft as the water is heated it expands so I would lower the PSI to 20 PSI (COLD) and then see how high the pressure goes

Boiler.jpg
 
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Bannerman

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In addition to water expansion due to heating, I suspect air that continues to remain trapped within the system is causing the pressure to further rise when the air expands when heated.

Air within the system will become compressed into a smaller bubble as pressure is increased. This is why increasing the system pressure is resolving the airlock situation which occured at lower pressure.

Prior to bleeding a hydronic system, I prefer to first lower the system pressure to cause remaining air to expand as much as possible, so the maximum amount of air maybe eliminated from each radiator's union or purge valve. Once the system pressure is restored for normal operation, any remaining air will then compress into an even smaller bubble which will have less effect on pressure when expanded while heated.

If the vertical rise between the boiler and the highest elevation in the hydronic system is actually 40', then the system will require at minimum, ~17 psi to ensure it will remain filled to the highest elevation point. By lowering the system pressure to 20 psi before purging each radiator, there will likely be sufficient reserve pressure to ensure the pressure will not become lowered below 17 psi while purging from multiple locations.

A suitable operating pressure for your system is likely 21 psi which will provide greater pressure safety margin below the pressure relief valve relief pressure. Once a greater quantity of air is purged from the system, you will be less likely to experience air lock even while operating the system at a lower pressure setting, and the lower quantity of remaining air will reduce the amount of expansion that is causing the current pressure variance.

A method to verify the actual system pressure is to use an accurate tire pressure gauge on the expansion tank's Schrader valve fitting while the system is pressurized to operating pressure. Compare the tire guage reading against a suspicious pressure gauge to determine any pressure difference indicated, taking into account any difference in elevation between the two locations.
 
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jadnashua

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Your expansion tank should have a precharge equal to your normal system pressure, so if you put enough water in for 21psi, the ET should have a precharge of 21psi. Note, after you've set the precharge, checking the air pressure should give you the same in air pressure as you'd get with a water pressure gauge hooked up elsewhere, less any differences based on elevation differences.
 

DIYorBust

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Thanks Sylvan, Reach4, Bannerman, and Jadnashua for your detailed suggestions. The new pressure gauge arrived today so I'll be doing some experiments. Bannerman's explanation about air expanding in the loop does like it could explain why I'm finding this counterintuitive. I raise the pressure to overcome the lock, but when heated, the compressed air expands and increases the pressure. Since we're changing the radiators, we're draining and refilling the system frequently and introducing air. I do believe the top radiator is about 40 feet above the boiler, it's on the 4th floor with the boiler in the cellar. There's no easy way to measure precisely. It's a baseboard, but we will probably change it to a radiator that could add feet or so. The top radiators have autobleeders, so I'd think most of the system would bleed out when cold, but I do usually need to purge or drain water water through the bleeder some of the other radiators to fill. Thanks again, I will try this and report back.
 

Reach4

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To eliminate one potential cause of over-pressure, you can shut the valve that feeds the autofeeder. My comment is not from experience but from reading prior posts.
 

John Gayewski

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The pin is on the top.
To be sure there is no water in it the pin should be on the bottom.

Overpressure is either, expansion tank, or fill valve. Air is a no. The pressure would drop when there's air.

What style of water heater do you have? That's also a place to look if your water is heated from your boiler.
 

Sylvan

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Expansion tanks can be installed in any direction. Whether oriented uprightly, horizontally, or even upside down, the expansion tank will function properly without any adverse effects. When putting an expansion in an abnormal position (specifically sideways or horizontally), make sure to support the tank.
 

John Gayewski

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Water will sink to the furthest point down. If your pin is at the bottom you can poke it and clearly see that your bladder is leaking. With the pin on top the tank has to fill completely full to show a leak. This is why the manufacturer has the preferred position as such. They can be installed in any position, and no one said they can't. The position to clearly show a small leak is Schrader valve at the BOTTOM.
 

DIYorBust

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Here's the picture! I did make an effort to reduce the air in the system and it seems to have helped, I haven't tripped the tp valve but I'm running at 30 right now.
 

Sylvan

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If you can isolate the expansion tank from the system and get a pressure reading you may have to pump more in around 15 PSI as operating at 30 PSI will certainly cause the relief valve to discharge
 

DIYorBust

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Well I measured and pumped it with the entire system drained down. They didn't put an isolation valve on the tank, I'll put one in next time I'm doing work in there. But I'm wondering if I need a bigger expansion tan.

The current tank appears to be in good shape. there's air in the top, and it sounds right when tapped. It holds a charge, and it's charged to 21 right now. If I fill the system to 19psi when cold it does reach 30 psi by the time the water reaches about 160-170 degrees. But if I drop the pressure much below that, I find the top floor radiators will take in air and wind up cold. I feel like we shouldn't have a pressure change that large.
 

Reach4

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The current tank appears to be in good shape. there's air in the top, and it sounds right when tapped. It holds a charge, and it's charged to 21 right now. If I fill the system to 19psi when cold it does reach 30 psi by the time the water reaches about 160-170 degrees. But if I drop the pressure much below that, I find the top floor radiators will take in air and wind up cold. I feel like we shouldn't have a pressure change that large.
Does the pressure go back down when the water goes cold? It would if the only problem was a too-small expansion tank. But one of the other problems would not let the pressure fall back to its former cold temperature.
 
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