Maximum duty cycle value for boiler/hydronic systems

finiuch

I've noticed that during long periods of set-back recovery, the thermostat is calling for heat non-stop for a long time.



This runs the boiler to the point where the pressure is 30psi and the pressure relief valve starts dripping or sometimes the aquastat limit is reached and the burner turns off.



I've found the setting in the thermostat that allows for changing maximum duty cycle rate (100% now) and of course the cycle rate.



What are the proper settings for this type of system? I've also seen all kinds of suggestions for cph for hydronic systems from 6cph down to 2cph. Is there a definitive guide for these settings? I'm now curious to know for forced air and heat pump types as well.



thanks!

Jean-David Beyer

pressure relief valve starts dripping

"This runs the boiler to the point where the pressure is 30psi and the

pressure relief valve starts dripping or sometimes the aquastat limit is

reached and the burner turns off."





That is not a problem with the thermostat. That is a problem with something else. Either the expansion tank is waterlogged, the automatic water feeder is broken, or leaking of domestic water into the system from a leaky heat exchanger (e.g., an indirect water heater).



So do not frustrate yourself with your thermostat. It is OK if the aquastat limit is reached, if it is the operating one, not the safety one.

finiuch

pressure relief valve starts dripping and boiler aquastat limit hit

Thanks replying.



There is only one aquastat and when it reaches the limit (180F right now) it cuts out power to the transformer the thermostat.



It's possible the expansion tank for being waterlogged (how do I check this?).



I have noticed that the quarter turn shut-off vale that suppliers cold water to the system is fully open. After that there are two more 'things' on the pipe. I assumed one prevents water and pressure from the boiler system to back pressure the cold water pipes in the house and the other is the automatic feeder you mentioned. It seems to have a small angled lever at the top of it.



So these boilers are ok to run continuously if the load demands it? (i.e. 100% duty cycle is ok?)

Jean-David Beyer

ok to run continuously

"So these boilers are ok to run continuously if the load demands it? (i.e. 100% duty cycle is ok?)"



Sure.



BUT it is not OK for the pressure relief valve to weep or the boiler pressure to get to 30 psi.  And whatever you do, to not block the PRV from operating correctly.