How to change from 2 wire to 4 wire furnace

Discussion in 'HVAC Heating & Cooling' started by snuffy47, Jun 2, 2009.

  1. snuffy47

    snuffy47 New Member

    Jun 2, 2009

    I have an old Newmac Wood/Oil furnace CL115B(35 years old) with a Honeywell R8239E transformer

    I would like to use my furnace for air circulation in the summer either in cooling mode or manual this would also aid me to run it in early winter when using the wood stove.

    Looking at the honeywell diagram is it just amatter of fishing the 4 wire and connecting the extra 2 wires on the transformer?

    Currently right now there are 2 wire to a digital thermo but the manul on does not work.

    My biggest fear is I change something and when I start a wood fire the fans donnt come on.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    This is simplifying things, but basically, power comes out of the transformer on one side of the coil, then goes through various switches (often in your thermostat) to supply power to activate the desired things. These things can be: heat request, manual fan request, or a/c request. Since many (but not all) thermostats require power to operate, they usually also get a lead from the other side of the 24vac coil. So, basically, the switches in the thermostat apply power to tell the system to generate heat/cooling, or to turn the fan on manually. Usually, the switch will not allow both heat request and cooling requests to occur simultaneously. Some fancier thermostats have multiple staging for heating, and those used for heat pumps may have an additional lead.

    Now, what your HVAC plant does with those signals will vary depending on the model. Not all can support all of the possible inputs. On a two-wire system, it's just like a light switch, power is on one lead, and when the thermostat switch closes, it applies it to the heating plant. For cooling or fan, it needs to branch that signal out to additional switches and wires to get back to the HVAC system.

    Most heating units automatically turn the fan on when required, but you can force it to stay on, if your system supports it, and you have a remote switch (often found on the thermostat).
    Last edited: Jun 3, 2009
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  4. hj

    hj Moderator & Master Plumber Staff Member

    Aug 31, 2004
    Cave Creek, Arizona

    IF you connect those two wires to the transformer you will burn out the transformer or thermostat or maybe both. You only connect ONE wire from the transformer to the thermostat. ALL the other wires have to be connected, according to the wiring diagram, to the various devices it controls.
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