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Condensing combi boiler with constant system circulation

bob eck
bob eck Member Posts: 930
Can you do constant system circulation with a combi boiler.
Combi Boiler would be piped P/S with one or two system zone pumps that would run constant during the heating season.
System is cast iron radiators with 2” steel pipe mains in the basement.
When the boiler stops after a call for heat so do the system pumps. There is a lot of hot water already heated and just sitting in the 2” black steel pipe in the basement. With constant circulation that hot water would end up in the radiators.
If the radiators do not have thermostatic radiator valves would the radiators over heat with constant circulation?
Or is there a way to keep system pumps running another 10-15 minutes after the boiler turns off?


  • SuperJ
    SuperJ Member Posts: 609
    edited October 2019
    Constant circulation and post purge are both good options.
    It depends on your controls setup. No reason it couldn't work on a combi.
    With two zones, You might need a 3 way valve on each zone to allow recirculation between heating calls so that heating calls from opposing zones don't influence each other and cause overheating.

    Constant circulation works best with some sort outdoor reset. Possible with indoor reset as well. If your boiler is producing 180deg water all winter constant circ might overheat you on a high mass system due to a lot of residual heat, but if the outdoor reset is matching your load profile pretty closely, constant circ can add a comfort.
    Constant circ (delta P) + ODR + TRV is bliss, nice smooth constant heat without cold spots or overheating (it's what I use).

    Here is a good article written by a heatinghelp member.