Two-wire furnace/AC & programmable t'stat

Discussion in 'HVAC Heating & Cooling' started by 480sparky, Oct 13, 2007.

  1. 480sparky

    480sparky In the Trades

    Messages:
    149
    I have a customer in a mobile home that has a furnace/central air system with a 'heat/cool' switch on it, and the t'stat has only two wires to it.

    He wants to install a programmable stat to lower his energy bills, but I, being an electrician, haven't been able to locate a decent prog'le stat that can be installed on this system.

    Any help would be appreciated.
  2. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,304
    Location:
    Cave Creek, Arizona
    t'stat

    I cannot figure out how it works with just 2 wires. You would normally need 3 wires, (a hot wire, one for the heating, and one for the air conditioner), at the very least.
  3. 480sparky

    480sparky In the Trades

    Messages:
    149
    There's no On/Off/Fan or Heat/Cool switches on the stat, just the temp adjustment.

    So the switch on the furnace must do something as the stat can only open and close the circuit. So my assumption (being an electrician, mind you) is that when it's on heat, it looks for the stat to close. On cool, it looks for it to open. The 'call for heat' or 'call for cool' is based on the switch on the furnace.
  4. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,304
    Location:
    Cave Creek, Arizona
    t'stat

    In that case you may never find a t'stat that works. Without a heat/cool at the thermostat, it would not know whether to raise or lower the temperature to set it back, even if you could figure out how to get the electronics to work.
  5. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    Well, I think you need a wiring diagram of the furnace. But, if your existing thermostat is a typical 2 wire, what we would call a "heat only", then possibly that switch on the furnace is accomplishing the switching regarding activating the system on temp drop for heat, activating on temp rise for A/C. IF that is the case, you could use any T-stat, and just connect as if it was "heat only". But you really need to confirm if that is how your system is set up.

    The White-Rogers 1F80-240 is a heat-only programmable that might work for you. Check your system, and if necessary contact white rogers for assistance.

    http://www.white-rodgers.com/wrdhom/pdfs/06_Cat_pages/Cat_06_pg0012_13.pdf
    http://www.white-rodgers.com/wrdhom/pdfs/instruction_sheets/0037-6325.pdf
    http://www.white-rodgers.com/wrdhom/pdfs/06_Cat_pages/Cat_06_pg0178.pdf
  6. 480sparky

    480sparky In the Trades

    Messages:
    149
    Thanks! I'll post my results when I get them.
  7. tman67

    tman67 New Member

    Messages:
    1
    Cheap easy fix...

    Just buy a cheap programmable thermostat at local store.
    RH and RC terminals will be jumpered and hooked to the red wire.
    White will be hooked to white.
    Done.
    The rest of the terminals will not be used.
    You still will have to open the furnace and switch over in the changing of the seasons.
    But you will have the energy savings you were looking for.

    I have the same situation, maybe later will look into rewiring.
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,824
    Location:
    New England
    It might just be easier to run a new thermostat cable! If the furnace-a/c unit supports it, it would give manual control of the fan as well then. Not sure how jumpering leads would work...on heating, it's normally open and closes when it gets colder. On cooling, it closes when it gets hotter. Seems incompatible to me...
  9. Lonewolf92c

    Lonewolf92c New Member

    Messages:
    3
    Location:
    Webb City, Missouri
    NEWBY looking for clarification...Thanks!

    :confused:hey all!!

    Looking at answer below I could use some clarification please. I too have mobile while I build new home. Pulled off old thermostat and only two wires. I see at furnace where you switch blower to heat/cool manually and AC unit wired from furnace and not used yet obviously.
    I have red and white wire in wall (not labeled) and a new thermostat with posts labeled:
    G-RC-RH-Y-W with a jumper wire between the RC and RH.

    Do I read response below to say put the white wire in the W and red wire into RC or RH WITH THE JUMPER LEFT IN PLACE, AND IT WILL BE GOOD TO GO? Then all I have to do is swith manual heat/cool switch on blower on furnace in summer?

    I have tried 6 million combos (although none utilizing the RC or RH posts) and I can get furnace to kick on, but never kicks off even when temp set is exceeded.

    Thanks in advance and I appreciate the help.


    Semper Fi
    Bill P.


  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,824
    Location:
    New England
    Try removing the jumper and putting the red on the RH terminal for heating.
  11. Lonewolf92c

    Lonewolf92c New Member

    Messages:
    3
    Location:
    Webb City, Missouri
    So you're thinking white wire on the W post and red wire on the RH and remove jumper altogether?
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,824
    Location:
    New England
    Yes...should work for heat.
  13. MaintenanceMan

    MaintenanceMan In the Trades

    Messages:
    54
    Location:
    Indiana
    On a lot of older mobile homes the furnace and A/C T-stats are separate. The outside units a lot of times were actually package air conditioners with their own T-stats. If he in fact has central air/a split system then the a/c needs at least 3 wires to work. Power from the furnace transformer ( usually red, sometimes with split terminals at the stat), one for the blower (usually Green), and one for the condenser (usually marked yellow but a lot of the times the wire is blue). The white wire is designated for heat.

    You need to make sure the customer has a split system and not a package A/C outside. In any case, any T-stat is going to work with his two wires for heat. But if he in fact has a split system it would be best to just run the proper wiring. 4 wire from furnace to t-stat, 2 wire from furnace to condenser contactor.
    Last edited: Dec 17, 2008
  14. Lonewolf92c

    Lonewolf92c New Member

    Messages:
    3
    Location:
    Webb City, Missouri
    Hey Jim!!

    That seemed to work great. Furnace comes on and turns off like clockwork. Thanks a ton for the help!
  15. pcranger

    pcranger New Member

    Messages:
    1
    Location:
    Belvidere,Il
    Two-wire A/C T'Stat

    There are many older mobile homes that use a two-wire (heat only) thermostat to control both heat and A/C.

    They are wired to an: {A/C CONTROL BOX FOR USE WITH HEATING ONLY THERMOSTAT (2 WIRE)}

    You can google 7900-6021/B for an Evcon/Coleman wiring diagram.
Similar Threads: Two-wire furnace/AC
Forum Title Date
HVAC Heating & Cooling Water? in the bottom of furnace/AC cabinet Jul 22, 2011

Share This Page