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Forced air wood furnace-need some help with terminology

I'll start with some background:

The house I just bought had a brick corner hearth pad in the kitchen, and an unlined masonry chimney. After lining the chimney (Ventinox SS liner and Thermix), I installed a used, older air tight forced air wood furnace I bought from a co-worker. Very simple furnace, meant to be installed in a basement or furnace room and piped into the air ducts and return air ducts. I'm going to run some duct work in my overhead crawlspace to feed the extended rooms for more even heat, but that I've got figured out...Where I come across the snag is here:

The furnace is set up with a simple spdt switch to select low fan speed or high, with a thermal switch that kicks the blower on when the firebox is hot enough, and the furnace just plugs in to an ordinary wall outlet.

   What I want to do, is use a 24v thermostat of some kind in the bedroom down the hall to control the fan speed, on at call for heat (if the thermal switch is satisfied that the firebox is hot enough), high speed fan at a 5 degree or greater difference, low at a 1-4 degree difference, and off once the thermostat is satisfied.

    I worked as a residential electrician for 4 years before I started my current job, but my exposure to hvac control terminology and devices has been limited, what would I need to make this work? I know I'll need a 24v transformer, and i'm guessing a 24v/120v fan relay, but is there anyway to operate at different fan speeds automatically? I guess I need to dig through the furnace wiring and determine how the blower speed is determined by the spdt switch, I'm guessing it closes pole 1 on the blower in low, and closes pole 1 and 2 on high, but I've not dug that far into it yet.



  • lchmb
    lchmb Member Posts: 2,997
    the problem

    with this idea is the furnace will not be able to get rid of excess heat. It is designed to protect itself using the thermal switch's and what your doing may actually cause the unit to over heat causing a fire hazard. It's not like an open style fire box that can move the heat naturally away from it...
  • Werewolfx
    Werewolfx Member Posts: 16
    Already have that worked out

    With a higher cut in limit switch, that will bypass the thermostat and run the blower on high to cool the top half of the firebox until it drops back below the threshold. Ideally, it would also operate a set of dampers on the outbound ducts (one 8" round, two 6" round, and one 4" round) that would close the ducts to the rest of the house and open a duct that runs to the crawlspace under the floor or outside, either recycling the heat or getting rid of it. Either way, If I install a thermostat, either via a manual line volt that shuts the unit off after its satisfied, or a 24v unit, it will have a safety override that will force the blower back on before it reaches damaging temps.
  • meplumber
    meplumber Member Posts: 678
    Your home insurer might not like this.

    I have seen a few instances where the home insurance company dropped customers for modified solid fuel systems.

    Be careful.
This discussion has been closed.