I already read thru possible answers from the list...

An older home just purchased. Oil burner, house water heat on demand when switch is 'ON'. Heat is hot water run from a pump that is controlled by a simple thermostat - 2 wires - that connect below the transformer on front of the burner with heat exchanger coils inside hot box. Baseboard is an open loop that goes through the entire house in series - nice crawl space. Seems the thermostat is bad cause it is constant heat when unit is on. I tried running the thermostat up and down several times and it seemed as no switching took place - temp in house was close to 80F at the time and I could not shut off the heat. Simple mercury switch with coil... I'm hoping it is the switch.

Some years later they installed an AC unit in the overhead crawl space - compressor outside on slab. Completely separate everything. Has digital readout. Fan switch is AUTO/ON, cooling switch is OFF/ON, two arrows to set temp up/down buttons.

The AC is ducted to an overhead circular vent in center of every room in the house - one level - and the return is a rectangular open screen located at the end of a central hallway.

I need to replace heat switch - in dining room 50' from AC controller that is near the AC return vent opposite end of house. I'm thinking about combining them so that when heat is called for, the circ pump will come on, and my belief is that by adding the central fan running it will be more efficient in heating the house.

Anyone have a thought if this is wise? And then, with nothing more than 2 wire go-no go wiring system for the heat, can I combine this and the AC to a common controller such that the central fan runs when heat is called for?

sheesh, I need a 'tag' but it won't accept any...

  • What make and model is the boiler? Also, where are you on this planet? Commented Aug 29, 2017 at 11:39
  • Schuylkill County, PA Didn't get the make and model- owner cannot afford upgrade/ replacement at this time. I need to investigate make next time I get up there - hour+ away from me.
    – Lane M
    Commented Aug 30, 2017 at 11:43

2 Answers 2


You could add a relay that is powered by the same terminals as the circulating pump.The fan would run when the pump runs and it would be isolated by the relay to not run the hot water pump when the A/C is running. If you are good with wiring you could utilize a low speed for heating (if the motor is a multi-speed) so you would reduce the air movement and the noise when on heating. I have to ask a question " you said that you have a series loop heating system which would mean that almost no piping would be in the crawl space. But you mentioned that you have a nice crawl space. If you have piping in the crawl space you may have a zoned system or a "monoflo" system. If it is a "monoflo" system you could zone every room with non-electric zone valves. Just a thought. There are other possibilities to up date the system, depending on the lay-out.

  • Pardon me; I missed the part about controlling both systems from one thermostat. You can control both systems with one thermostat but I can't explain it here. If that is what you want I would hire an HVAC company to accomplish this since it is probably more complicated than a normal home owner can handle. Then wiring in relays as I said in my original answer would also be appropriate. If you close the doors to the rooms and they don"t feel cool enough you may have to trim the bottom of the doors to get adequate return air from those rooms since you have a single return in the
    – d.george
    Commented Aug 29, 2017 at 23:25
  • I like the idea of adding a relay from the pmp to the fan and bumping it down. I peeked in the overhead there is 2' of blown in insulation and it is everywhere. Looks like they made a dam out of plastic sheet around the AC unit not keen on going up there. I need to replace thermostat either way. The issue would be routing the wiring clean Person is moving in next week I will discuss options with her. Not my line of work but was in refrigeration/HVAC 25 yrs ago at major food co. Been away a long time. wrk in nuke field 25+ yrs, know I&C, electric, mechanical, cooling fitting... not a normal guy
    – Lane M
    Commented Aug 30, 2017 at 12:26

Decided to table this until spring. She's moving in this weekend. Just gonna replace the mechanical thermostat for now and see how the oil burner works over the winter. It is 40 years old and maybe replacing it next year is the correct answer - then tie it all together. Too bad no gas here. Thanks

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