I currently have central air in the attic and furnace in the basement for the heat (steam radiator heat). I have both wires run to the same location but have two thermostats. Can I wire them both to one thermostat? I have a picture of the thermostat that is currently used for the AC only. Any help or suggestions would be helpful. 
Controlling separate heat and AC units from a single thermostat
thermostat
Related Solutions
You can use any heat/cool thermostat on the market to do this. All is required that you run 2 separate 18/4 wires from each system to the thermostat.
R is what carries continuous 24v power to the thermostat. RH is continuous power for heating, and RC is contentious power for cooling. As long as you have constant 24v to these terminals, your system will work. Many modern thermostats have these built in to one terminal on the thermostats (will only show R). All the thermostat does is split the 24v continuous power to RC and RH at there thermostats control board.
From Furnace:
Connect R to R - If your thermostat has RH and RC you can either keep the jumper in, or attach it to just RH for heating. This does not matter as long as you have 24v constant from the unit.
Connect W to W For Heating
Connect G to G for the Fan
From A/c Air handler:
Connect R to R - you can use both R wires at same connection on thermostat if there is only an R terminal. If there is RH and RC like above, you can either leave the jumper in, or connect it to RC.
Connect Y to Y for Cooling
Connect G to G for the Fan
The reason for connecting the G terminal for only the air handler is because you will only really want to have the fan continuously running for the cooling season. Additionally, depending on the type of air handler you have, some control boards require the Y and G terminals to be energized together to run the Cooling and the Fan speed on high. If older units with older style control boards have just Y energized on a call for cooling, the unit will only run the outdoor condenser, which will cause issues with the system. Almost all thermostats will energize G and Y together to avoid this happening if there thermostat is used on a older system.
You should check the wiring in the furnace. There are no standards for thermostat wire colors, so you can't always trust the color of the wires. Seeing how the wires connect to the furnace, is a sure fire way to figure out what's what.
The O (cool mode) and B (heat mode) terminals are usually used for a reversing valve, in heat pump systems. I'm not familiar with oil systems, so I can't say for sure what they would be used for.
Some thermostats have separate O and B terminals, like this.
Others have an O/B terminal, and use a jumper to select which should be used.
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Best Answer
Yes.
RandRcterminals.Rwire from the heating system to theRterminal.Rwire from the cooling system to theRcterminal.Then you should be able to follow the thermostats installation guide, to connect the rest of the wires for the heating and cooling systems.
Unfortunately, without seeing the wiring in the heating and cooling systems, it's impossible to tell you how to connect the rest of the wires. Typically
Wis for heat call,Yis for cool call, andGis for fan. However, there's no standards or requirements that make this universal.It appears that you're using a Honeywell Wire Saver device, which is why the cooling system uses the
Kterminal instead ofY. I'd leave that wired the way it is, and simply add the heating call wire (from the heating system) to theWterminal.