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  1. #1
    Join Date
    Oct 2011
    Posts
    6

    Control Wiring and Fan Control

    First off, hello to everyone ... this is my first time on here. I have a question regarding an old non-condensing system I am trying to wire to a newer thermostat. The old thermostat had been removed by someone else but did not label the wiring so I do not know exactly how I should wire things. I figured that obviously looking at the wiring at the furnace itself would help but a few things are confusing.

    So, first off my plan is to use a basic digitial Honeywell RTH5100B thermostat which has the standard R, RC, Y, W, and G terminals.

    The system is an old mid 80's forced air natural gas system (model HGD-75). It is manufactured by ICG (became keeprite i think) in canada. Most of the wiring seems straight forward. The ac unit is connected to the C and Y terminals of the furnace. The following connections from the furnace to thermostat also make sense:
    Furnace - Wire color to thermostat
    R - Red
    W - White
    Y- Yellow
    G - Green

    So far everything makes sense but this is where i get a bit confused. The gas valve is a white-rodgers 36C03 model and as far as wiring is concerned, there are two black wires that appear to come from a black transformer or relay or something on the furnace (you can see it in the pictures) and are connected to the th-tr and the tr terminals of the valve. There is a also a wire from the th-tr terminal to the W terminal of the furnace. Finally (and what is confusing me the most) is there is a blue wire from the th terminal to the thermostat.

    So how should i wire the new thermostat ... is the blue th wire really like an RH wire? This is what the current wiring is to the thermostat location

    Furnace Terminal - Wire color to thermostat
    R - Red
    W - White
    Y- Yellow
    G - Green
    TH - Blue


    I have attached a quick drawing i did of the wiring as well as two pictures of the furnace wiring.

    Also, I noticed that before the old thermostat was removed I remember the furnace fan always ran on a low speed even when there was no call for heat or cool or fan. When there was a call then the fan would go to a higher speed. Is this normal fan operation for older units like this with a standing pilot?

    The reason I am asking is I am wondering if the old thermostat was wired wrong or something. I know the unit has been serviced before and none of the service people have said anything about that.
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  2. #2
    Join Date
    May 2004
    Location
    Frederick, MD
    Posts
    947

    Unfortunately,

    we are not allowed to give DIY info here. It shouldn't cost you that much to have a pro come out and install the stat for you and do it right. Giving the wrong info here could cost you more in the long run.
    GO NAVY, BEAT ARMY!

    A DECADE OF DOMINANCE! +2

  3. #3
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,619
    your best bet would be to get a contractor to come and inspect that furnace for safe use and if it is ok install a thermostat for you

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