Hi - I am having difficulty replacing my old thermostat with a Noma brand programmable thermostat.

The old thermostat - an old manual mercury type - is connected to 5 wires labelled on the circuit board as G, R, Y, W, and 4. it looks very much like the model shown on this website:

http://home.howstuffworks.com/home-thermostat3.htm

The new thermostat, however, has only 4 connection terminals, labeled G, R, Y, and W.

After connecting the ones that match, I'm not certain what to do with the "4" wire. My furnace doesn't work if I leave it unconnected. I am thinking of connecting it also to the R wire. Is that correct?

Thank you kindly.

Likely not. The red wire connects directly to one terminal of the 24 volt transformer. You are just going to have to figure out what the ''4'' terminal did and how to make the new thermostat also do that function. There are some thermostat/furnaces that require the thermostat to activate the blower. If the gas is coming on, but the blower never does and the gas goes off again, that could be the problem. You might try the Fan on and A/C positions of the thermostat and see if they function without the 4. There may be instructions on the furnace or inside it explaining the function of the 4.

Thank you, labman.

It turns out that connecting R and 4 together to the same terminal did the trick. I'm happy.

New prog. Thermostat has 2 black wires. Old thermostat has 4 wires 2 red 2 black .how to connect?