presure surge/expansion tank question

ALH_4

Did you check the pressure on the air side of the expansion tank before you installed it? Sometimes they are significantly off. Could either be too high or too low. It can only be checked with the system side at atmospheric pressure.

Sometimes the diaphragm "sticks" to the inside of the tank and has to be popped loose with higher pressure. Although 30 psi certainly should do that.

Is there an automatic boiler feed valve? If so, is it functioning correctly? It could be overfilling the system. What is your cold fill pressure?

Sarah_3

pressure surge/expansion tank question

The situation: hydronic boiler with weeping relief valve, pressure up in the low 30's. Seems like an obvious diagnosis: change the expansion tank.

Changed tank to same size (old tank diaphragm was indeed broken, tank was totally full), but problem continues. The pressure surges up into the high 20's and the relief starts weeping. I attached a photo of the piping on that part of the boiler. Although there are some less than kosher things about the installation (not done by me), ie, circs on return, no spiro vent, etc, the boiler was working before the original expansion tank failed. I shut off the fast fill to be sure it wasn't over filling, so that isn't it. The air vent you see is new. New exp tank same size as original, a #30 for a smallish one fam. house with 2 zones, one of which is rads, one baseboard. circulator works, zone valves work.

Am I missing something totally obvious? It is the end of a long week...Please help! Thanks, y'all.

Sarah

Norm Harvey

Does the boiler have a tankless? The tankless coil may yhave a small leak, leaking street pressure domestic water into the low pressure boiler.

Install Isolating valves on your tankless if they are not already there, reduce the boiler pressure to be about 15 psi with the boiler off on its high limit, then wait a day and observe any pressure changes.



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Sarah_3

pressure off?

interesting idea about the exp. tank pressure being off- can I check it by disconnecting it and hooking up a gauge to the lower side of the tank and taking a reading? Then, what is on the other side of it, ie, nothing, would be at atmospheric pressure.

otherwise, what does it mean to have the boiler at atmospheric pressure--open to the atmosphere? or reading 14.7psi?

the boiler feed valve has been shut off; it isn't the problem. there's no tankless either. cold fill pressure is a nice 12 psi. pressure only surges as the water heats up.

Norm Harvey

Lets narrow down the problem,...

the relief valve was weeping because of a rupturerd exp tank,.. so you changed the tank and the weeping continues,..

The seat of the relief valve may have some crud on it preventing it from closing properly.

Id take the pressure off the system and change the relief valve (making sure it is in a vertical position), and at the same time check your exp tank pressure.
As the boiler heats to its high limit drain water as necessary to maintain between 12psi at its low limit setting and 15-20psi at its high limit setting.

That way when the boiler is as cool as its going to be (at low limit) the fill valve wont add any water, and your max pressure at its high limit shouldnt be in danger of opening the relieve valve.

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