I am replacing an old "General Controls" thermostat, model T-90H110A137, 24 volts max, 0.8 AMP.
The wires are labelled but not to the current standard of H1/H2/B/O/RC/RH/G/Y/W. how do I determine which wire is for what?
The old thermostat controls a furnace and has a continuous fan option. There are wires for A/C but that is not needed in the new setup.


Wall wire Code
---------- -----
White --/---- G
... \--- F

White ------- C
Red --------- H
Black ------- V

*** the first 2, "G" and "F", are shorted together.


I will be replacing it with a NOMA THM501, a 7-day programmable with a large LCD.
http://www.upm-marketing.com/products/ProductDisplay.cfm?CFID=118850&CFTOKEN=24325866&pt=%21%28%20%3AK%0A&rtRn=484%263O%5E%5F%3A29VU%5B%5ERP%3D%5BY6%226N%22 W%5EL%20%0A&item=%22%28%206K%0A

Thanks in advance

How are wires attached at furnace? It looks like the stat you are wanting to hook up is heat pump

Some thermostats can be used for both conventional systems and heat pumps. If so, there should be directions for both that came with the thermostat. If it didn't come with directions, take it back.

From there as you were told, see what the wires do at the furnace.
Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. Many have RH and RC to allow separate transformers for heating and cooling. With just one red wire, jumper them. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.


To do simple checks like this you do need some tools. A test light, a meter, or a voltage detector might be the best place to start with. I came across the niftiest gadget for trouble shooting, a voltage detector. They work through the insulation of wires. There are several brands. I have a GB Instruments GVD-505A, less than $15 at Home Depot. Touch it to a hot wire, and the end glows red. Find the doodad that lights it on one side, and not the other, and you have the culprit. You do not have to open up housings and expose electrical contacts. You are looking at where your hand is, not where the meter is. Most people are capable of doing repairs and will get it going and not get hurt if they use a little sense. The voltage detector makes it even easier.

Obiwan.. I am having the Same problem.. My wires are exactly the same as yours G, F, C, H, V but the colors are off so I know that the colors have nothing to do with this issue. G and F are shorted together. I have a feeling that those are the Rh and Rc but not sure. Have you or anyone else found a solution yet? My new thermostat has the Rh, Rc, W, Y, G terminals. PLEASE PLEASE PLEASE PLEASE help my freezing arse!

Obiwan if you haven't already found an answer, I was able to make the correct wire designation last night with the help of a site that gave previous naming conventions. You should hook the wires accordingly

G = fan = G
F = fan (jumper w/ G) = G
C = Cooling = Y
H= Heating = W
V= power = Rh, Rc

from here you should be able to find the solution for your thermostat better.

I have old honeywell thermostat has 3 conections 1,2,3, screws to connect to yellow, red ,black wire.how can I replace it to a new one?