Warning: Do not confuse "B" Common

(GE, York and Trane)!

With "B" Energize reversing valve to heat (Rheem/Ruud/Weatherking)!

1819 light bulbs and sockets avaliable from Radio ShackClamp om ac ammeter Sperry Digisnap avaliable at Lowes or The Home Depot

    If you need to see the 24 volt control signal this lamp is an easy way do it. This 1819 lamp and sockets are available from Radio Shack. This clamp on ac ammeter is valuable for measuring ac current. The built in "Non Contact Volts" feature is worth the price. About $80 at Lowes or The Home Depot.

    I cannot say it enough times: You should never go after your thermostat as a cause of the problem until you verify with test lamps or a meter. To do so you are not solving the problem but creating more and risk blowing a fuse or the control transformer. On most systems you can get to the control wiring at the air handler or furnace and take your reading there. You should only replace or change a thermostat after you verify that it does have a problem.

    If your system does not have a fuse protecting the secondary side of the transformer add one before proceeding! An automotive ATC fuse holder and a 3 amp fuse should do the job. If you have a Trane unit and you blow the control fuse, replace it with and automotive 3 amp ATC fuse.

Link to White Rodgers that has a good look up table of different manufacturers. You will notice that Lennox's color codes are TOTALLY WEIRD to put it gently.
Link to White Rodgers Clip art that has pictures of their different control products.

    I would highly suggest identifying each wire by connecting the hot lead to each function wire with clip leads and a 3 amp fuse and recording the function before installing your new thermostat. While this is more trouble it may save you from ruining your day. 

Related pages:
    Electric furnace trouble shooting-and repair.
     Thermostat stuff.
    Heat anticipator setting.
    Oil furnace trouble-shooting and problem solving.
    Blower and fan controls.
    Heat pump defrost cycle
    Information about Heat pump repair stuff
    Defrost control images
    Contactor explanation
    Gas furnace intermittent pilot images
    Outdoor reset added to honeywell 594 thermostat
    Trane Heat pump thermostat wiring question. With more terminal codes.
    Millivolt gas valve electronic thermostat fix.
    Wiring Scheme for a Wood Burning Furnace.

To read about fan motors and furnace blower controls follow these links.

Did you consider Solar Energy?

 If your outdoor unit turns to a block of ice in the heating mode read this.

Thank you for visiting our site Scott Meenen N3SJH

Contact us

Email us at: jsmeenen@toad.net

To ask us to help solve a furnace problem use our heating help fill out form

To  ask us to solve a cooling or heat pump repair problem use our cooling form

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Written By:  Scott Meenen N3SJH of:
Specializing in Mechanical, Controls and Electrical Modifications Of
Heating, air conditioning, Refrigeration, Cold storage,
Ice Production and Food preservation.
Anything having to do with Heat and Energy.
Serving MD, DC, and Northern VA

We service and repair the following brands:
American Standard, Amana, Arco, Arco-Air, Bryant, Carrier, Coleman Evcon, Comfortmaker, Day/Night/Payne, Dunham-Bush, Fedders, Fredrich, Goodman, General Electric, Hotpoint, Heil, Intertherm, Janitrol, Kenmore, Lennox (Armstrong, Johnson Air-Ease), Miller, Modine, Nordyne, Rheem/Ruud, Sears, Stewart Warner, Trane, Williams, White-Westinghouse, Whirlpool, Weil Mclain, York, (Frasier Johnson/Borg Warner) and others.