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
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
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.
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
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