Hi all. Was reading a similar thread earlier, and decided to post mine to see if someone can help. The post I was reading was sort of similar, but not quite, so the instructions got muddled with aspects that were not the same. Lots of connections and letters. I have an oil fired furnace, OHB5-F059-085-3. It had an old 2-wire thermostat, and this furnace was connected to that when it was installed. I am replacing the old 2 wire with a simple Honeywell RTH2510B. The thermostat works fine in the two wire mode, but the fan cannot be run without heat from the thermostat. I have to go to the furnace room and flip the fan switch. I want to be able to circulate air in the house without the heat on. Before I run a new thermostat wire, would like to know it can be done. I can detach the thermostat from it's location and jump wire it at the furnace, but need to know which wires go where. By the way, the color tape on the old thermostat wire is not accurate. at the thermostat there is white and red, to correspond with the letters in the back of the thermostat. White and Red from W and R to W and R on the thermostat is the way it is currently.

 

I would pull a 8 conductor cable and have 3 spares. Let us know when your ready to wire it.

 

i have 5 conductor. here in seattle, there is little need for air conditioning in my small house, so even 5 is overkill. how much is needed for heat pump (the only logical upgrade that can happen here in our neighborhood without natural gas)?

 

i will have all my jumpers here tomorrow. left them at my studio.

 

4 will support a heat only digital stat that also uses a common wire and G.
5 will support most AC applications and 6 is the minimum for most heating applications. It is always better to have extra wires.




R = 24 volts AC
C = 24 volt common ( required with many digital thermostats)
G = fan
W = heat (or axillary heat with a heat pump)
Y = compressor
O = heat pump reversing valve (B with Rheem/Ruud heat pumps)

 

i have not purchased wire yet. perhaps i will look at 8. always better to have too much. i apologise for not getting back to this, got swamped from all four directions the past few days. i do have my jumpers here at the house tho. will take the thermostat off and play downstairs with the posted info.

 

CYGRWP is what is on my junction box. the same letters plus RC are on the thermostat pin sockets. pics are below.

 

C,G,R and W are what you will use to add fan and common.

 

i'm gonna add some more pix... it looks like the terminals "y" and "p" on the furnace are not used. blank on the connections on both sides. i put a pic with the terminals marked on the thermostat, which currently uses "w" and "r" only. at the furnace the old tape shows the w with red tape, and the r with green tape, only because the guys who replaced the furnace didnt take it off.

 

well, i have uploaded lots of pics, i have my alligator clip jumper wires, and i am ready to pop the thermostat off and lug it downstairs to the furnace. i just need to know what terminals on the thermo jump over to what terminals on the furnace. i wanted to do it as a test jump to make sure it all worked before going up into my nasty attic and running wires. in your last post, are you saying just add the c and g to the r and w?

 

what connects to what? there is no 'c' on the thermo, only 'rc'. what does the furnace 'c' connect to? there are 'g' terminals on both.

 

If your thermostat uses batteries then there will be no C terminal. The C connection would normally run the clock section. You won't need to use C at all.

Rc/Rh (leave jumper in place) goes to R on furnace.
G goes to G on furnace.
W goes to W on the furnace.

That should be it.

 

i have no rh, so i assume r-rc on the thermostat? i see the jumper from r-rc. so to review (assuming rc-r correction) i would leave jumper rc to r on the thermostat, then to r on the furnace terminal block. g to g, and w to w. is that correct? so the only addition would be the g to g.

 

well, that worked! with the thermostat on cool, and fan on auto, it comes on automatically. fan only switch works when thermostat is set to off. frosts me that i did not run new thermo wire when i had the kitchen walls open, as they back up to the thermo location. i read somewhere about add-a-wire for making 2 wires 3. i dont suppose that would work in this situation?

 

The fast stat 1000 would work for you. FAST-STAT - Wiring Extenders and Thermostats Controls

 

thanks! i will try this before donning my tyvec protection and digging into the attic rewire option. it is a cheap experiment.