Old HVAC System wiring Problems

[ryguy85]

First off, hello to everyone ... this is my first time on here. I have a question regarding an old system I am trying to wire to a thermostat. The original owner of the home removed the thermostat 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 it is only helping to make things more 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 -
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 relay or something on the furnace 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 * 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

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[ryguy85]

... I also want to add a bit more information. Before the old thermostat was removed I remember the furnace fan seemed to always run on a low speed even when there was no call for heat or cool. 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 *

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.