short cycling

dmsanborn

I have a system that was recently converted from oil to LP. The new unit is a Weil Mclain GV90+4 with a 40 gallon boilermate indirect fired w/h using a Taco priority 2 zone control SR502, the new tstat is a Honeywell pro 4000 series. The system is a mono flow with the in wall copper convector/radiators. the domestic hot water cycles perfectly the issue is with the heat side. The boiler is set at 160 limit and when there is a call for heat the boiler cycles on and comes to temp ok but the circulator only runs for about 3 minutes at a time and shuts down and 5 minutes or so later the call for heat is back and the process starts over. the radiation is not getting warm until this happens for about an hour or so. I have purged the radiators and have a clean flow of water so it doesn't seem air bound. Question is what could be causing the call for heat to short cycle like that?

STEVEusaPA

What is the water temperature doing throughout this cycle? Also what kind of aquastat is on the boiler and what is the LL (low limit) setting.
Are you using deep set backs? If so, it's probably acting correctly (not ideally) if it's coming up to temperature (high limit), shutting off burner, but letting circulator run until the water temperature drops, the firing the burner to raise the tempature....over and over.
Everytime the circulator comes on in this high water volume system, lots of cold water come back to the boiler, stopping the circulator.
Could still be air, could be the circulator is not working, or the control not turning the circulator on...could also be the new thermostats aren't set up correctly.

Zman

STEVEusaPA said:

What is the water temperature doing throughout this cycle? Also what kind of aquastat is on the boiler and what is the LL (low limit) setting.
Are you using deep set backs? If so, it's probably acting correctly (not ideally) if it's coming up to temperature (high limit), shutting off burner, but letting circulator run until the water temperature drops, the firing the burner to raise the tempature....over and over.
Everytime the circulator comes on in this high water volume system, lots of cold water come back to the boiler, stopping the circulator.
Could still be air, could be the circulator is not working, or the control not turning the circulator on...could also be the new thermostats aren't set up correctly.

I agree,
The circulator should be running continuously, it sounds like it is turning off with the boiler call.

KC_Jones

Might sound crazy, but did you change the thermostat with the new install? Is it possible the CPH setting is incorrect? If it's set for forced air this sounds like the type of thing it would try and do, run as much as 6 times over the course of an hour to get the room to temperature....if it's a digital thermostat anyway.