Advice Wanted - Pressure Differential between boilers

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tmy23

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First thanks for all help I receive. Let me describe my situation. This is a residential hydronic. Large, 3 floor victorian, cast iron radiators, 11' ceilings so probably 35-38'' rise from boilers in basement to top of the 3rd floor radiators. Also some long runs (+/- 60') in basement from boilers to vertical turn to upper floors. System is 2 WM HEII's for heat and WM DHW tank controlled by Tekmar controller, outside temperature monitor, etc. All original equipment is about 18 years old.

System was pushing water through boiler relief valve on one of the two boilers. Tried to test the relief valve and it was partially frozen, sticking open and also not re-seating properly. Replaced both boiler relief valves with OEM 30 PSI valves. Still pushing water out of one boiler. Suspected autofill valve. Took autofill valve out, and thought to put in a pressure gauge on the system side of the autofill between the autofill and a pre-existing shut-off valve. Reinstalled the autofill and with the shut off closed, the autofill would not hold pressure and regardless of setting would creep up to about 28PSI. Took the autofill out, disassembled, it was cruddy...cleaned, reinstalled, retested the autofill over a few nights and it held pressure. By completely uninstalling the autofill, I lost the original setting and am trying to readjust system pressure. With the shutoff on the system side of the autofill closed, I have it set to hold 12 PSI, which seems low with my vertical runs, you'll see where I am heading.

With the entire system shut down (circulators, boilers, etc), all the shutoffs open, the expansion tank and the new pressure gauge near the autofill valve both show 12PSI. One boiler gauge shows about 18PSI, the one that still leaks shows about 23PSI. When I start the system and it really heats up i.e. 180+, the boiler with the "resting" 23 PSI vents through the relief valve, and I noticed some seepage around the air vent on the top of the boiler also. The boiler pressure gauge goes up to near 40 psi. The other boiler doesn't vent, and stops around 28 PSI. At this stage the pressure gauge near the autofill is showing about 22 PSI and the expansion tank shows about 22 PSI also.

So for some reason, (Finally my question) , although both boilers are on the primary loop, one boiler over pressures and the other doesn't. I switched the new pressure relief valves, thinking maybe I got a bad valve, but the same result, the same boiler over pressures.

So the only thing I can think of at this point is to (1) turn down the autofill, but I'm concerned I won't get heat to the third floor, (2) check the expansion tank, what pressure should that be set at? or (3) put a 35 PSI or 40 PSI valve on the boiler?

thanks for all advice.
 

Jadnashua

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IF your autofill is out of the picture, and the pressure rises when the boilers are running, your expansion tank is probably shot. Check it. It should be precharged to your normal system pressure. This must be done either before you install, or if in place, with all water pressure removed and a drain valve open, otherwise, you'll just be reading the water pressure. The gauges on the boilers may not be accurate - when operating, you can use a good tire pressure gauge on the expansion tank to check it...it will read the same as the water pressure unless your precharge was excessive. If the system was regularly getting high, that puts more stress on the expansion tank, and it is probably bad.

You want enough pressure in the system to prevent boiling or zero pressure issues. Take the rise from the boiler outlet in feet to the high point and divide by 2.31. That would be your minimum pressure - I'd probably add a couple of pounds to that. If it is more than half of the pressure relief valve, you may want to switch it up IF the system is designed for it to give you a little working room. In a typical 2-story house, 12-14# is enough, and a 30# relief valve works. You may want one higher.
 

tmy23

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Jim,

thanks for the help. My next step was going to be removing the expansion tank and checking it out.

Do you have any thoughts why one boiler is holding different pressure than the other when they are "at rest" with no circulators running? Could mineral scale be doing that??? I guess I am thinking ahead to finding out that the expansion tank is fine, and then having to decide to increase the relief valve PSI without knowing root cause. I hate bandaiding things without knowing the actual cause.

I do have one final question...I'm assuming I set the pressure in the expansion tank to the "ambient" pressure of the system when cool (in this case, with 37' height) I'm assuming around 18 psi. You know, it occurs to me that if the expansion tank (which was replaced about 4 years ago, was left at the factory setting of 12 PSI, it would fill with water as soon as the system is pressurized, even before heating begins, and there would be no room for thermal expansion...
 

Jadnashua

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Your expansion tank won't fill with water if the precharge is low (but it has to be intact) - as water comes in on one side, it compresses the air, and the bladder will only stop when the pressure on both sides is equal...IOW, the air pressure will rise because the volume is smaller when the water hits the other side. It's just that under ideal conditions, it won't have to move too far (assuming the precharge is correct), which puts less stress on it and makes it last longer. This is why I mentioned it may be toast...excessive stress by your pressure going up from the leaking autofill valve. Since you now have that resolved, replacing the ET will probably make things work.

As I said, boiler pressure gauges are not always the most accurate things...the two could be the same. If the ET is sized and working properly, when the boilers turn on they will keep the overall system pressure almost constant. Only when they are bad (leaking) or undersized for the volume of water you have would it rise a lot.
 
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