I'm replacing an older Honeywell thermostat with a new programmable Noma/UPM model THM501. There are 4 wires - red, yellow, white and blue. On the Honeywell the blue wire was on the "G" terminal - all worked well. I wired up the new one the same way. However, although the new thermostat shows the heating mode, the A/C comes on and blows cold air - not something desirable with snow on the ground. Can anyone help? Much appreciated,

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. 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.

You must identify and match functions between the thermostat and furnace. To the furnace, it doesn't make any difference how the terminals are marked or the color of the wires. If both follow the convention above, it is easy. Otherwise instructions or a schematic are a big help. As above the G terminal usually has a green wire and switching power to it runs the fan only. I have to run now. Post back if you need more help figuring it out.

Thanks Labman. Wow, it seems complicated. I had to reinstall the old thermostat (with the blue wire on the G terminal) when the new digital programmable one didn't work. I'll try to sort it out tonight.

You wrote "Digital or programmable thermostats may need the blue wire connected to them" - which terminal would you connect the blue wire to the thermostat if there is no readily apparent connection - The new thermostat has Y,RC,RH,W,G,C,H2, and H1. I'm stumped as to why the old thermostat functions perfectly with the blue wire on the G terminal but the new one won't...

One side of the transformer is common to all the devices controlled by the thermostat. Often it is labeled B or C and connected with a blue wire. Before connecting the blue wire to the C terminal of the thermostat, make sure it is connected to the common side of the transformer at the furnace.

Have no idea why the old thermostat used the G for heat. Before you can connect the wires to the new thermostat, you must figure out what they connect to at the furnace.