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Thread: Two-wire furnace/AC & programmable t'stat

  1. #1

    Default Two-wire furnace/AC & programmable t'stat

    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.
    Just my 2 worth.

  2. #2
    Moderator & Master Plumber hj's Avatar
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    Default 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. #3

    Default

    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.
    Just my 2 worth.

  4. #4
    Moderator & Master Plumber hj's Avatar
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    Default 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. #5
    Plumber jimbo's Avatar
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    Default

    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/..._pg0012_13.pdf
    http://www.white-rodgers.com/wrdhom/.../0037-6325.pdf
    http://www.white-rodgers.com/wrdhom/..._06_pg0178.pdf

  6. #6

    Default

    Thanks! I'll post my results when I get them.
    Just my 2 worth.

  7. #7

    Default 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. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    Default

    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...
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  9. #9
    DIY Junior Member Lonewolf92c's Avatar
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    Default NEWBY looking for clarification...Thanks!

    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.


    Quote Originally Posted by tman67 View Post
    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.

  10. #10
    Retired Defense Industry Engineer jadnashua's Avatar
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    Default

    Try removing the jumper and putting the red on the RH terminal for heating.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  11. #11
    DIY Junior Member Lonewolf92c's Avatar
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    Default

    Quote Originally Posted by jadnashua View Post
    Try removing the jumper and putting the red on the RH terminal for heating.
    So you're thinking white wire on the W post and red wire on the RH and remove jumper altogether?

  12. #12
    Retired Defense Industry Engineer jadnashua's Avatar
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    Default

    Yes...should work for heat.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  13. #13
    In the Trades MaintenanceMan's Avatar
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    Default

    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 by MaintenanceMan; 12-17-2008 at 11:54 AM.

  14. #14
    DIY Junior Member Lonewolf92c's Avatar
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    Default

    Quote Originally Posted by jadnashua View Post
    Yes...should work for heat.
    Hey Jim!!

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

  15. #15
    DIY Junior Member
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    Default 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.

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