Bigger expansion tank?

[tcianci]

Ok, a little history first... 32 yo HB smith cast iron wet base boiler, tankless coil, oil fired. Earlier in the week, the relief valve started to vent, so I put a bucket under it. The pressure on the system was high, maybe around 40 psi so I'm figuring possibly a bad expansion tank or fill regulator. I relieved the pressure on the system and shut off the feed. The pressure still built when the boiler got hot, so I think I was looking at a new expansion tank. Tonight I purchased a new tank, fill regulator and relief valve. The relief valve was mounted in a horizontal position and I think that was the reason that it was still dripping even when the excess pressure was gone...I have seen sediment build up on the back side of the seal in this orientation and the valve doesnt seem to seat well when operated. Also the expansion tank was "T"eed into the line coming from the boiler to the relief valve and the tank had a service valve mounted in the connection to the tank but the tank was mounted wet side down. I know that it will function fine in that position but it will be messier to change since all the water will want to run out of it when it is changed. So, since I had hit the Schrader valve on the tank a couple of times in the process of checking things, I charged the tank (off the boiler) to 12 lbs and checked it periodically over the course of an hour. It lost about 5 lbs over an hour so I pronounced it dead and Installed a new expansion tank in the tapping on the bottom of the Spiro-vent. I also installed a new temp and pressure gage in the tapping on the front of the boiler. I did this because I was never quite sure of the accuracy of the original one since the boiler always ran at just under 30 lbs ever since it was new. I filled the boiler and fired it off. I came up to temperature but I think the settings on the aquastat aren't quite accurate either since the boiler always seems to get hotter than where the high limit is set. If I understand the function of the aquastat settings, the boiler should cycle between the high limit MINUS the differential when in the heating mode and it should cycle between the low limit PLUS the differential when calling for domestic hot water. Please, someone correct me if I'm wrong about that. Well anyway, the boiler still runs up to just under 30 lbs when it's hot. I have come to the conclusion that I need more expansion tank capacity...what do you guys know?

 

[BadgerBoilerMN]

Expansion tanks are sized for system volume at design temperature.

If you have a tankless coil, it could be leaking and pushing city/well pressure to the hydronic side. In any case, get rid of it. A tankless coil in a hot boiler is the least efficient way to may DHW.

You should have an indirect water heater. Set the system pressure (cold and static) to 12 psi to match your tank and turn down the operating limit in the coldest weather until the thermosat gets behind and then up again until the room temp. agrees with the stat.