Large pressure swings in secondary heating loop?
Large pressure swings in secondary heating loop?
(OP)
We have a school with a primary heating loop off the boilers and a secondary glycol loop via a converter. Our plumbers are telling me that in the shoulder seasons when the temperatures are warmer they have to add fluid beacause the pressures in the glycol side are too low but when the outdoor temp gets very cold the loop gets to full heat (180F) the pressure gets too high and they have to drain some fluid out of the system.
My first thought was too small an expansion tank, but I checked and rechecked and the sizing is OK. What else could cause such large pressure swings. Perhaps the initial charging of the expansion (diaphragm) tank was incorrectly done. Is this a possibility? I don't think incorrect pump sizing would be the culprit?
Thanks for any tips.
My first thought was too small an expansion tank, but I checked and rechecked and the sizing is OK. What else could cause such large pressure swings. Perhaps the initial charging of the expansion (diaphragm) tank was incorrectly done. Is this a possibility? I don't think incorrect pump sizing would be the culprit?
Thanks for any tips.





RE: Large pressure swings in secondary heating loop?
What is the total volume of fluid in the secondary loop? And what is the maximum volume expansion? From the literature I found that the volume expansion of a 50% aqueous solution from 50 F to 180 F is approximately 4%
You can check for a failed bladder in the expansion tank by draining the secondary loop and checking the pressure at the tank as it drains. Bladder integrity is evidenced by slowly falling pressure during the draining. A ruptured bladder will will bwe immediately evident after a short loss of fluid-- pressure will drop to 0.
RE: Large pressure swings in secondary heating loop?
A.
RE: Large pressure swings in secondary heating loop?
Regards,
RE: Large pressure swings in secondary heating loop?
3782 U.S. gallons (50/50 mix) is the closed system volume.
Diaphragm tanks are 264 U.S. gallons each (quantity of 2) for a total of 528 U.S. gallons. 12 psi precharge and acceptance volume is 81% of tank volume.
RE: Large pressure swings in secondary heating loop?
IF that means that at system fill pressure the expansion tanks have only 100 gallons of capacity "left" (less than 3% of system volume), then the 4% thermal expansion zeke calculated is going to give you grief...
Would be easily remedied with slightly more fill pressure to give better volume balance.
RE: Large pressure swings in secondary heating loop?
One more question. The proper maximum pressure to use for the primary side is about 5 psi below boiler pressure PRV...but on the secondary side, what determines the max operating pressure? You probably wouldn't want to exceed the pumps max inlet pressure..correct?