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Opinion on setup controls

Looking for opinions on which way the setup for boiler controls should be in my set up for my semi-detatched bungalow in Central Ontario.

10:1 modulating condensing 85k BTU boiler with built in circ pump and 3way valve that controls indirect HWT via tank sensor (set to priority).
8 panel rads with TRV's on each
Buffer tank between boiler and heating circ pump (e008 pressure differential pump) followed by manifold with individual runs to each rad.
Room thermostat on main floor in hallway furthest point from all radiators.
Boiler is set to maximum heating supply of 150 (max target on coldest days when running on outdoor reset)

1st setup: run heating circ pump from boiler with boiler controlled by room thermostat and outdoor reset. (Don't like this because it defeats the purpose of the TRV's as they may call for flow in some rooms but will not get any if thermostat is not calling for it)

2nd setup: same as above except heating circ pump has power 24/7 with valve between supply and return providing .25gpm flow for pump when all TRV's are closed (which brings pump to lowest operating speed at 8w) (good except a room may need heat but boiler won't activate until thermostat calls for heat)

3rd setup: boiler is controlled by temperature controller on buffer tank with 30 degree differential (currently set at 120 target) heating circ pump 24/7. (Problem is I can't use outdoor reset so boiler just aims for target which I set to 2 degrees above buffer tank temperature controller which currently would be 122) (boiler gets decent run time and stays in condensing zone due to temperatures set, will have to adjust manually when temps drop if rads can't keep up when we get colder days)

4th setup: boiler thermostat terminals are jumpered. Boiler has heat call 24/7 and runs on outdoor reset targets and if supply temp goes over the target by 4 degrees it shuts down the burner and won't start back up until the supply is 30 degrees below target (I selected these stop and start points in boiler program) (built in circ pump keeps running so it gets constant read on system temperature to know when to activate burner again)I tried this and it modulated down to its lowest burner output(5%) when It reached 117 (118 was the target it came up with using the outdoor reset) and kept running for a good 30 minutes maintaining this temp even though the only flow was via the bypass i put in for the heating circ pump to not dead head. (Not sure if this is good and/or actually efficient. I assumed it would end up running over its target and shutting down the burner but it just maintained)( i did test this shutdown function by offsetting the target for a few minutes then returning it to 0 degree offset in which it was over its target and shut down till if went 30 degrees below its target and fired the burner again as I wanted)

Is the 4th setup a realistic way to operate and is there safety concerns?

How about wear and tear if it would run for long periods at 5% burner output even 24/7 if it didn't end up going over target?

Would it be efficient staying at 5% and always at temp and ready for TRV's to open vs say option 3 and the boiler having to run at 100% burner output to get up to target when the buffer tank controller calls for it?

Which setup or any setup suggestions welcome. 

Side question: Does it make sense to say if the burner is at 5% for 20 hours it would be equal fuel consumption to the burner running at 100% for 1 hour?

Thanks to all who took the time to read this and even more thanks to those who post their opinions/suggestions (good or bad I'm learning)


  • Jamie Hall
    Jamie Hall Member Posts: 22,212
    Or none of the above? Remember that the whole idea behind outdoor reset is to run the boiler at just the right temperature so that the installed radiation can just compensate for the heat loss of the house. Theoretically no indoor thermostat, though one is usually provided to get the house a little warmer or cooler as needed. Closing a small number of the TRVs -- perhaps to get a cool bedroom at night -- won't throw things off much, but having a TRV on each radiator pretty much presumes that the output from the boiler is always hot enough to satisfy the demand from however many radiators are open at a time.

    This suggests using the outdoor reset to control the boiler to maintain a fairly high target temperature in the buffer -- with a narrow say 10 degree spread -- high enough to satisfy the radiators, however, many may be calling. Then use the TRVs to control the various space temperatures. If you use a constant pressure pump on the heating loop side, it will vary its speed to correspond with however many TRVs are open at the time.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 20,836
    A delta P circ would be better instead of the pressure bypass. The pressure bypass is a parasitic device. Delta p and TRVs is a great match
    Also, mod cons are most efficient running at low turndown as you expose a lot of HX surface to a small flame and lots of condensation operation..

    A sensor in the tank for the boiler call, the boiler on ODR is how I see it

    Does the boiler control include a boost function? This increases SWT if the boiler runs a set time, if the ODR is set too low for example and it isn’t keeping up with the load
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream