Pressure Problems on Gas Fired Hot Water System

[bewing]

New to the world of radiant heat. I have a big house (5000sqft) built in the 1920s. It has a 10 year old Weil-McClain CG-8 boiler on it which also has a 30 gallon Bell & Gossett Hydro-Flo air expansion tank (not a diaphragm type). The boiler is in the basement as is the expansion tank (closed 2 pipe system) and my house has 21 cast iron radiators covering 3 floors and the basement (3 on 3rd floor, 7 on 2nd, 9 on 1st, and 2 in basement). Circulating pump is a Bell & Gossett SLC-30 model. Here's the problem. My system pressure keeps going over 30 when doing a cold startup and popping the PRV. The PRV and the fill valve (set at 12 psi) have been replaced in the last week. When cold, my pressure is around 12-15, but on cold start it cranks too high. Would draining the expansion tank solve this? Is the tank big enough for the house? For a house this size, what should the normal operating pressure be when hot? Lastly, is it better to keep a fairly constant temperature on a 24 hour basis, or use a programmable thermostat to raise/lower temps as you come/go/sleep/etc?. Not sure which route is the more efficient way to heat given the size of the house.

FYI, I can isolate the supply/return lines on the boiler and the cold feed line. I can't isolate the expansion tank directly. It has a weird 3 way valve that is either directing flow to the system or to the drain. I isolated the boiler and fill line, then drained the tank (takes a long time as I can't break the vacuum). I estimate I got 20 gallons out of it cold. How much water should be in the tank when cold. Sorry for all the details. Just trying to give as much info as I can the first time around. Appreciate the help.

Brooks

[Ed Imeduc]

Do you have water in the bleeders on the 3rd floor with 12 to 15lb psi?????Should be about 18 to 20 some times.
With the boiler warm Id drain the expansion tank all the way That new auto full might not get water up to the top floor . Most come set for 15psi. If you dont have water there you what to kick up the pressure on it a little.
Most of the time 160o on180o off works fine the tstat should turn on and off the pump. A lot is in how it is wired up. A home that large Id set the tstat and forget it or put zones in it if you can with not to much work.

ED ;)

[bewing]

I'm at work now but will check the pressure when I get home. Talk more on the thermostat piece. My thermostat is turning the whole system on/off. Burners, pump, etc. Temperature setting drops to low 60s when we are at work and I know the water temp drops to around 80 and pressure down to 12-15. My PRV runs into trouble around 145-150 degrees on the cold restart. I'm puzzled by the 160-180 on/off bit. Perhaps my system isn't optimally installed......we've only lived here 3 years and again I've never in my 40 years lived in a house with anything other than forced air heat.