Can I see your heat loss/gain calculation?
A Manual-J heat loss or equivalent should be used to size any heating or cooling appliances. There are online calculators for heat loss and heat gain, and I have investigated many of them. My consensus is that the quicker and/or easier the calculator is, the less accurate the calculation tends to be. Unlike boilers, furnaces, and air conditioning systems, which should be sized based on heat-loss calculations, a heat pump may be best if slightly oversized by up to 20%. Use the manufacturer's information to determine how much. If this is done, use a modulating AWHP in your application.

Why is a heat loss/heat gain calculation so important?
Yes, because most heating appliances are oversized, they produce more heat than needed, which shortens the cycle. This short cycling will create higher operating costs.

Can't I size the AWHP as the same size as my current heat source?
No, most existing heating appliances are way oversized. If you size based on the current appliance size and add more to slightly oversize to achieve the most efficient operation, you will cause short cycling.

Will an AWHP reduce my heating costs?
This is a challenging question to answer directly for multiple reasons, given the moving targets. We would have to consider a few factors. We are not considering any electric or gas backup below.
1. Average outdoor monthly temperatures from year to year by winter months
2. Moving current fuel and electric prices year over year and area to area
3. Combined gas & electric or gas and oil bills per month
4. Was the AWHP oversized
5. Is the heat pump of the modulating type?
6. Are the settings on the heat pump set properly?
7. If using night setback, how much is too much?
8. How does your thermostat recover from a night setback? Does it recover from setbacks a couple of degrees at a time to avoid the backup devices from operating?
9. Are the circulators properly sized?
10. Was the AWHP properly installed?
11. Is the water temperature setting for the water to run through the pipes too high?
12. Is the AWHP operating on an outdoor sensor for the proper water temperature at a given outdoor temperature?
So, let me answer your question. Yes and no. Yes, it should be cheaper on a year-to-year comparison if the AWHP was sized and installed properly. I will warn you, though, that there may be a month or two where the cost is higher, but most months will be lower. The average annual cost will be lower.

With the higher cost of electricity compared to the cost of gas or oil, how can it be cheaper to run? It's all about the operating efficiency. Heat pumps have two efficiency ratings. The first is the Seasonal Energy Efficiency Ratio (SEER), which applies to the air-conditioning side of the heat pump. The higher the number, the more efficient the cooling cycle will be.
The second is the Coefficient of Performance (COP) for the heating cycle. The higher the number, the lower the operating cost. Electricity is billed in kilowatt-hours (kWh). To explain this, let's assume you choose an AWHP with a coefficient of 4. That means for every 1 KW of electricity you buy, you will get 4 KW of heat. The COP is a moving target as outdoor temperatures change. This is why it is a good idea to slightly oversize the system based on the manufacturer's heating curves and to use an outdoor sensor to automatically adjust the supply water temperature to the system. The cooler the system's supply water temperature, the higher the unit's COP. AWHP should run exceptionally long cycles to be efficient.

Will I have to add insulation to my home?
It is possible, depending on the heat-loss calculation and the appliance's heating curves. You could use a larger appliance, but the price and operating costs will be higher. It's usually a good idea, even if it would work as is. Regardless of how you heat your home, adding insulation and sealing air leaks will reduce your heating costs.

Should I remove my old cast iron radiators and install some other type of radiation?
No, cast-iron radiators are good at operating at low water temperatures. The supply water temperature must be calculated based on the square footage of radiation and the room-by-room Manual-J heat-loss calculation.

What type of heat emitter is best for AWHP applications?
While cast-iron radiation is ideal for an AWHP application, a radiant floor in a lightweight pour is the best option.

Is the heating response time the same as my gas or oil boiler?
No, AWHP works on much cooler water temperatures, so the heat-up time is slower. You will learn that, on warmer days in the fall and spring, it may be best to turn the thermostat down two or three degrees instead of completely off. Using an Outdoor reset will help maintain a lower water supply temperature, addressing this issue. As the outdoor temperature rises, the supply water temperature decreases. As outdoor temperatures cool, the supply water temperature increases.

I heard AWHPs do not work in colder temperatures.
Yes, most heat pumps will work down to 0°F or lower. As I stated earlier, you may need backup heat to raise the supply water temperature from the heat pump's output to the required supply water temperature. Remember, the colder it gets outside, the higher the required water temperature will be.

I have a hot water heating system and a separate ducted air conditioning system. If my A/C fails, can the AWHP do cooling in my house?
Yes, the AWHP will supply hot water for heating, domestic hot water, and chilled water for air conditioning. Your existing air conditioning coil will need to be removed, and a water coil installed in or near your existing fan unit. The outdoor compressor unit and interconnecting lines to the fan unit can be removed and discarded. Water lines will need to be installed from the AWHP to the new coil.

I have two a/c units. One for the upstairs, one for the basement, and one for the first floor. Can the AWHP do both?
Yes, it can do heat, domestic hot water, and air conditioning. The same application as above, and do multiple a/c units.