Please let me stress that I still want to run the heat pump even when I am running the oil heat system... thanks.

I think I understand what you want. HVAC is not my trade, so others may chime in with better information. I think you want dual-fuel controls. The heat pump would do the heating alone until the outdoor temp dropped to the preset level and then the oil furnace would take over the heating. They would not both operate simultaneously. Mine with propane furnace backup is set at 28 deg. Propane heats at outdoor temps of 28 deg and below. I would argue that setting higher than 32 degrees is a mistake. Depending on the setup of the air handler, the heat of the furnace added to the air stream over the heat pump indoor coil while operating could send refrigerant pressure to levels that would damage the system. So the dual fuel controls operate one or the other, depending on the outdoor temps. The degree-day system for scheduling oil keep-fill service refills will not work as before and you may have to check the tank oil level yourself and request fillups as necessary.

Ok - why do you want to run both at the same time? That would be too much airflow as you realize. I am all about doing wacky things that make HVAC people cringe but I don't see the point. I mean I see the point but you can't do it.

Now you could build a seperate duct system. You could just beef up your current duct system but you have the possibility of overheating things if you do this.

What you really should do is get a heatpump that can work to low temps and remove the oil furnace. But I don't know your electric rates or your climate, but in much of the country, electric strips would be cheaper to run than an old oil furnace. If you aren't in the mountains, then a heatpump could probably work 98% of the time.

My assumption, and my comments were based on that, was that the furnace (blower) would serve as the air handler. The idea of running the furnace simultaneously could have been the hope that the heat pump would provide some heat even if the outdoor is 15 deg. and the oil furnace would supplement that. I already explained as best I understand why that might be a bad idea. If two separate systems not sharing an air handler or duct work, run them as you desire. But I would not want that for my house.

Run your new heatpump system most of the time. Hopefully your house isn't leaky and has some insulation in the attic. If that is the case, you should be able to run your heatpump down to 25 degrees and keep a pretty good comfort level the vast majority of the time.

At or below the level of comfort you would run the oil heat. You won't be running the oil heat and heat pump 'at the same time'.

Thank you all. I think I got my answer. agreed -- the heat pump won't be much of a use at 15 degrees anyway!

@david_cary: in my area, heat-pump only does not do the job in the months of December thru february.
@neohio: if someone from NE Ohio tells me that I can run heat pump down to 25 degrees, I should definitely keep it in mind!

-- Mohseng

Go for the highest SEER and HSCF(? the heating number) that you can afford.

Its very possible. I'm about 10 miles south of lake Erie. It gets cold hear. I have a high efficienc single stage heat pump and a gas furnace. I've run the heatpump down to 20 in the past. I prefer to run the heatpump at 30 and above for comfort reasons but they maintain comfy inside temperatures lower than that. It depends how good/bad your house is insulated.

In your situation of having oil as the backup, I would run the heatpump down as far as possible. Remember heatpumps are good at maintaining temperature but not bringing it up.

I am also purchasing a heat pump and replacing the central AC unit as primary source of heat with oil hot air as back-up. My question is can I use the present control board with the oil furnace or do I need a new control board to accomodate my new high efficiency heat pump and its interaction with the furnace blower? Thanks