Hello everybody,

I've been trying to replace an entry-level Honeywell thermostat - TH3100D - with a programmable model - RTH8500 - from the same company.

The TH3100D had five wires connected:

WYGORc[shunted with R via jumper] B and C were not used.

However, for the RTH8500, W and O use the same connection; according to their manual, there should be no O wire, unless you have a heat-pump (I don't - I have an gas-burning boiler and a York air-conditioner outside). The "typical" and "alternate" wiring schemes described in their manual show some other wiring (C, Y, etc) which I don't have.

The "typical" scheme (with O left out) works fine for heating - but the air conditioning doesn't pair up with the fan: the device itself starts functioning, but its effect is nil, because the fan won't kick in.

The "heat-pumps only" scheme, which makes O go into W's place and W into Aux (shunted with E via jumper) sets the fan ON permanently (with no possibility of stopping it), but doesn't start the air conditioning system.

What's the catch here? what am I doing wrong? where should the O wire go?

Did you do your installers setup??? The use the top set of letters, the white will go to the w. and remove the jumper from r to rc if you have a boiler and a airhandler for the ac blower.

Did you do your installers setup??? The use the top set of letters, the white will go to the w. and remove the jumper from r to rc if you have a boiler and a airhandler for the ac blower.

The manual says to ignore the colors if they don't match, label the original wires and then connect them accordingly to the new thermostat. That's what I did: the old thermostat, which functioned as intended, had Red > W, Black > Y, Orange > G, Green > O, White > Rc. There also was a jumper - in the old thermostat - connecting R and Rc (and the new thermostat had a jumper in the same position).

Just so i am on the same page as you. You had all the terminals hooked up on the old stat? the orange??? And you have a hot water boiler? or a gas furnace? Or a electric furnace and heatpump? sorry i am not 100% sure waht you ment on your original post.

Just so i am on the same page as you. You had all the terminals hooked up on the old stat? the orange??? And you have a hot water boiler? or a gas furnace? Or a electric furnace and heatpump? sorry i am not 100% sure waht you ment on your original post.

1. The old thermostat (TH3110D) had W Y G O Rc R B C
2. B and C were unused;
3. R and Rc were shunted (jumper);
4. The original wiring (i.e. the combination that actually worked) was at follows:
W: Red
Y: Black
G: Orange
O: Green
Rc: White

I have a gas furnace + an air conditioner

If you do not have a heat pump you do not need a wire on O, So you have it working no or still no?

If you do not have a heat pump you do not need a wire on O, So you have it working no or still no?

Please bear with me: I am a complete and total newbie when it comes to heating&A/C devices.

This is a brand new apartment I moved into. The developer left me two leaflets (NOT manuals), one for the furnace, the other one for the A/C; neither of those mention the words "heat pump" and, after googling their respective model numbers, I came to the (maybe erroneous) conclusions that, in fact, I do not have a heat pump. But, as you're saying, then why do I have an O wire? why did the original thermostat work perfectly? why does it have separate O and W terminals, while the upscale, programmable model has the same terminal for O and W - and no matter which combination (as per its manual) I chose, doesn't work properly?

[Bump - still seeking an answer to this]