Hi. I am trying to figure out how to wire the controls for a new central air conditioning system being added to my existing Hallmark furnace. I currently have an oil fired furnace with 2 wire running up to a programmable thermostat, which is capable of cooling and heating. I plan to run a 4 wire from the thermostat to the furnace and then a believe I need a 2 wire from the furnace to the condenser. On the furnace there is a terminal board with T2 T1 F2 F1 : The T2 and T1 look they run up to the thermostat, and the F2 and F1 have yellow wires going back into the housing. Anybody got any ideas on where I need to hook up the new wires for cooling? Thank you for any help.

I have to run. Your furnace seems to be set up differently, but this should get you started. More later.

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.

When you say gas valve, is that for just gas units or will my oil fired unit have a gas valve? Do you think if I take off the cover next to the terminal board on the furnace, I will find the R B C W Y G terminals?

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Originally Posted by BAMBAM

When you say gas valve, is that for just gas units or will my oil fired unit have a gas valve? Do you think if I take off the cover next to the terminal board on the furnace, I will find the R B C W Y G terminals?

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... will also be used in the oil fired units. Remove the cover and post your findings so Labman can have more info to work with when he returns.

OK ill look when I get home. Thanks

I took off the cover but there are no other terminals inside. I just have 4 screws T2 T1 F2 F1 with the two wires going from T2 & T1 up the R & W terminals in the thermostat? Anybody have any ideas as to how I can hook this up to run central air that I am adding to my furnace?

It would help a lot if you can find some instructions for your furnace. Often there is a schematic pasted to the furnace or in an envelop attached to it. Try their web site too. You need to make sure the wire that goes from T1 or T2 to the R at the stat is the one coming straight from the transformer. The wire that goes to the W terminal at the stat must go to the relay for oil pump, the limit switch, and anything else needed for heat in your system. The other side of the relay coils will both tie to the other side of the transformer. Gas and oil work much alike, except the stat controls the gas valve directly, but must operate a relay for the oil pump.

I am guessing the F1 and F2 correspond to G and Y, the G going straight to the blower relay, and the Y going to any limit switch or timer and possibly a different coil in the blower for a divergent blower speed for A/C. You may be able trace the wires inside the furnace and figure out which is which. One may also go straight to the transformer leaving no option for continuous fan. I am assuming you will have a technician charge the A/C. Perhaps he can help sort out F1 and F2. Most systems use limit switches or timers to have the blower run on a few minutes after the heat or cooling shuts off.

In cooling mode, the thermostat will switch the R terminal to the Y terminal. You need a wire running from it to both the proper terminal in the furnace to activate the blower in cooling mode and out to the A/C relay. Then a second wire needs to run from the other coil terminal back to the other terminal of the transformer. Try to sort this out and post back.

I found a shematic and it looks like the F2 & F1 are just wired to flame detector, which is factory wired. So the only filed wiring for the unit is the 2 wires from the T terminals to the R & W in the thermostat. In the back of the manual it had parts for all the models and there was something in there called a control center made by honeywell that had the normal terminals on it. Could it be that I have to add this control center to the furnace to accommodate cooling?

The control center could make a better job of it. From my own experience and the manual for my furnace, I have a good handle on the external wiring. The internals are a little more of a mystery.

Without it, I guess you could connect a third wire to the ones running to the Y terminal on the stat and the coil on the compressor relay. Run that wire to a relay coil. Run another wire from the terminal connected to the R terminal to one of the switched contacts of the relay. Then run a wire from the other contact of the relay to the blower relay coil. If you connect both the Y and W terminals directly to the coil of the blower relay, power will feed back to both the pump relay and the compressor relay. The control center may do some of this isolation in cheaper, more reliable solid state.

It may also give you options on different blower speeds. It the blower motor has more than 2 wires running to it, is has different speeds available.