anybody know about hydronic (hot water) home heating? - PeachParts Mercedes-Benz Forum

jsmith · #1 01-09-2004, 07:40 PM

trying to figure out how to drain and fill the expansion tank properly (fixed). keeps discharging excess pressure which seems to indicate that there isn't enough cushion in the tank...

BF_JC230 · #2 01-09-2004, 08:23 PM

if it is a rafter/over head tank shut off the make up water supply at the PRV - turn off the pump - if there is a valve leaving the bolier and entering the boiler close those - there should be a boiler drain midway on the expansion tank - open it - drain it until it stopps comming out of midway - close valve - open the two at teh boiler (make up water valve at the PRV) and start the pump. if expansion tank keeps flooding it sounds like your PRV is set too high - system should have 12PSI when the pump is off and the boiler is cool.

bladder type tank there is a shradder (bike tire) valve that pressure should be the same as gauge on booiler, shut pump off at amke up water - put bike gauge on bladder tank - there should be no water ever coming out of valve, if there is bladder is ruptured, if the pressure in the boiler is 12PSI check that bladder is at 12 and put air pressure in the bladder to 14PSI to push back bladder - and start system - they should equalize after some run time and bladder should center. - Jake

jsmith · #3 01-09-2004, 08:55 PM

it's a rafter/overhead - i read a procedure that says to fill it until water comes out of the air relief at the drain which i think is what you call the midway because air comes out of there. the complete procedure involves running the pumps and boiler independently which i don't know how to do so i just run it. i have the PRV to about 12psi cold but when i turn the boiler on the pressure starts climbing over 25psi so i keep relieving the pressure. it seems to get better as time passes - i don't know exactly why - maybe because the trapped air is passed into the tank? it's in early morning where the pressure fluctuation is high because the water in the system is still cold (timer) and the boiler is on for a long time. otherwise, when the water is hot and it is just in a maint cycle then everything seems ok.

BF_JC230 · #4 01-10-2004, 02:12 PM

shut off the water at the PRV - shut of the boiler - drain the tank - then close teh valve at the tank - check in the house for a bleeder on the fin tube radiation on a elbow, or is it a cast iron system where a vent could be ontop of the radiator...with the system closed up and teh pump turned off open the valve at the highest point in the house - open the water supply at the PRV - vent ti tuntil water comes out - close the vent start teh pump - shut the pump off after 10 minutes and vent again - cycle this until the air is out of system - thhis way the expansion tank will have enough air voulme and water to capturee any air left in the system and expand when the water is heated - it is normal for the water pressure to rise to 25lbs at 180 degrees you should have a 30lbs saftey on the boiler.

if the pressure keeps kreeping up - drain a little pressure of the system - keep the make up water valve closed if system does not build up pressure after running it for a day - your PRV is leaking by / shot...not even worth playing with it - go to HOmedepot or LOwes or a a shop - if you have to change PRV you should change the saftey too, youll propably have to drain the system anyway - do it all.

jsmith · #5 01-10-2004, 05:55 PM

now i'm confused. yeah it's a cast iron baseboard and i have vents at many points in the house. yes i can vent the at the top floor baseboard. there is a shutoff before the tank so i shut it off to drain the tank.

you seem to be saying not to refill the expansion tank the way i've been doing by opening the shutoff (also the PRV fastfill) and let it fill the tank until water comes out the tank vent.

are you saying that after i drain the tank that i should then:

-close the drain, open the tank shutoff
-open the vent at the highest point in the house
-open the PRV fast-fill until air does not come out the vent at the highest point in the house.
-turn off fast-fill and run system
-turn off boiler and purge air again (repeat as required)

this will certainly add more "air cushion" in the tank since it is empty instead of half-full with water the way i do it (from what i can understand).

thanks for taking the time to explain. part of the problem is that i've found I can't keep the cold pressure at 12psi and have to keep it higher at about 16psi because one of the circulators leaks at the impeller seals at 12psi. i don't think that this would cause the large swings in temp as i did not have this problem before...

thanks for all your help

BF_JC230 · #6 01-10-2004, 06:53 PM

ok- shut off teh water makeup - if the prv has a fast fill lever on it dont use it - use the prv as a normal prv and allow it to fill the system slowly only allowing 12psi through it...isolate teh expansion tank by turning the valve off to the tank - drain the tank - when the tank is drained close the tank drain - open the valve to the tank from the system - open the prv and allow the water to enter the system slowly - not with the fast fill - go the the highest point tin the houe start venting there - working your way to the lower units- run the pump a few times - go back to the highest point - vent - leave the tank alone there is enough water in the system if you are just venting one at a time and there is a big enough air pocket in the tank - to allow for water expansion...do a favor and if you find that the prv is leaking...buy a regular prv with out the fast fill feature.

fast fill cause problems - can run past 12 lbs and will not return to it.-Jake

jsmith · #7 01-11-2004, 11:12 AM

thanks a heap - your insight fixed the problem...

BF_JC230 · #8 01-12-2004, 12:07 PM

great
glad to help - Jake