Boiler Pressures

KevinWyman

Boiler pressure high (29 psi). Primary-secondary. Basement to third floor. Radiators and fin tube radiation. Cold was at 20 psi. Bled all radiators so I think most of the air is out of the system. Recently renovated/refilled.

Expansion tank is Extrol 30 and approximately 500 feet of piping in the system (75% 3/4 inch and 25% 1 inch).



What should I check for?

Brad White

Expansion Tank

I suspect the size of the expansion tank.



Piping water volume from what you gave is about 15 gallons. The boiler volume is unknown and could be at least that if CI. Call it 30 gallons total for the system. 



Let's also say that the cold-fill pressure is 18 psig (enough for 30 feet of lift with 5 psig residual at the top) and we allow 33% pressure increase to 24 psig when hot. The fill temperature we will assume is 60F and can go as high when operating as 180F. Sound good?



At the parameters above, you would need about 5.1 gallons of acceptance tank volume. An Amtrol #30 has 4.4 total gallons but will only accept 2.5 gallons, barely half of what you need. 



Not a big deal to add a second or even a third tank in battery, on the same line, all connecting to the same system point upstream of the circulator. Check that too, where the expansion line connects relative to the circulator.

KevinWyman

Boiler Volume is Small

Thanks for the response. Boiler volume is small (less than a gallon; mod-con boiler). Separated by hydraulic separator. I will check the expansion tank sizing  and automatic pressure fill valve (cartridge style PRV) later to make sure it is all working correctly.

Brad White

Tank Volume is Small Too

Key is to watch the pressure change as the boiler heats and cools. With all things held constantly (ie: no leaking into the system by the fill valve), the pressure and temperature should correspond at any point.



In other words, if the pressure is 21 psig when the boiler is 120 and the pressure rises to say, 26 when it is at 150, then it should return to 21 psig when the boiler returns to 120. Any higher after cooling, I would look for a fill valve leak-by.



EDIT: Unless you are using a buffer tank as a hydraulic separator, we should make a small adjustment for whatever that is, maybe a gallon so no big deal.

Also, what kind of radiators/emitters do you have? If baseboard fin-tube, is that covered in your piping? Or is it old standing cast iron (The King of Radiators!). Have to ask.

KevinWyman

System Volume

The hydraulic separator is a Taco Hydro-Sep (1 1/4"). The baseboard radiation was included in the 500 feet of piping. It is about 25% of the total emitter count and the remaining 75% is 85% sunrad radiators and 15% cast iron radiators.

Brad White

But you still

need another tank, maybe two. They are inexpensive enough, I would install two more for a total of three and park them out of the way if you like. Just keep them connected (or make sure they are connected) to the suction side of the circulator.