I wanted to share. This is the control for two Dunkirk 500MBh boilers. My interest in this is purely academic. I was in the room to trace the water piping (which is similar in organization to these controls). I could not resist so I spent a fair amount of time tracing the control wiring.
The two boiler control circuits are completely independent.
The relay top right center closes the two circuits on call for heat from any of eight pump zone control switches (see below).
Each boiler has an outside air temperature switch. It looks like the intent was for one boiler to operate below 65 deg and the second to operate below 30 deg.
Only boiler #2 control operates a combustion air damper.
As mentioned above there are eight zone pumps (pumping to the boiler of course). No main pump. The zone pump control relays enable the boiler controls. The two boilers are connected to the original building supply and return headers with 2 each 4” copper supply and returns (that’s 2 supplies and 2 returns each boiler). There is a lot of water volume right in the boiler room.
So….. let’s say on a mild day when the thermostats are satisfied, the boilers are off and that water is cooling down. Then a thermostat calls for heat and the boiler has to heat all that water up enough to satisfy the thermostat. Does this seem normal?
I have to say:
In this case three of the zones are recently installed air handling unit coils that perhaps run when the temperature is below say 60 and therefore enable the boilers. So it may work out. I didn’t check.
Based on the control board paint the controls have been changed so who knows what the original intent was.