To those of you recommending a dual system - let me see if I understand what you mean. Basically, I would have either an air/air or geothermal system WITHOUT electric resistance backup - and then have a new (maybe smaller??) oil boiler to act as backup heat for when the temps dropped too low.
Correct.
An air-air heat pump becomes very inefficient once temps drop to 10F or so. That's when a backup heat source has to kick in to keep the house comfortable. The reason most use resistive heat strips is because it is cheap to install and allows the builder to sell the house as 'fully electric'. You can use either gas or oil for the backup, you just have more investment than a simple heating strip. Typically, the backup is installed in-line with the heat pump air-handler and it uses the ducts to distribute the heat. If you have baseboard heat or radiators, you could keep those for backup (I don't believe that a baseboard or steam system likes to sit around unused for prolonged periods, you may develop reliability issues).
For geothermal, there is less of a penalty for low ambient temperature. The wells or loops don't really care how cold it is outside. You do have a need for more heating capacity as the house will lose more heat when it is cold, but your heat-pump itself is not losing efficiency. Now, if you have a long cold stretch, your geothermal system can start to freeze the loops. They are filled with brine or antifreeze but at some point the temp in the loop can get so low that you start to pump around an ice slushy. In that situation, you also have to go on backup heat to give the loops opportunity to thaw out. It is a lot less painful to get that backup heat from gas than from resistive electric.
To clarify, gas is not available where I am - my house is in farm country. It has steam radiators, and they do give out a great heat, so I have been kind of resistant to giving up the oil.
You can always get a 500 or 1000 gallon propane tank. Oil is a bit cheaper on a per-BTU basis, it just requires more maintenance and some don't like the smell.
Steam radiators are one of the more efficient ways of heating a house. I never minded the noises. I don't know if anyone still makes residential sized steam boilers if you just wanted to replace what you have.
The other thing I have learned over the last week is that in order to have electric resistance back up (and electric hot water -currently I have oil hot water,) I am going to need to change one of my electrical panels (for the hvac, I need a total of five two pole breakers for 2 hvac units, back up heat, and hot water. Did I say that correctly? I have no idea what I'm talking about!) I assume that this is going to cost me several thousand dollars - it wasn't called out separately in any of my bids.
Yes, for resistive heat you need a lot of juice. Make sure your main panel and the feed to your house are up for it.
