Here's a quick laymans summary of the difference between AC and HP, and why they're not directly interchangeable ...
Quick explanation of the systems:
AC and Furnace combo: uses one system to cool and a different one to heat. The only common element is the blower in the furnace, as it has to move the air in the house for both cooling and heating. But the heat exchangers are separate units and completely isolated to their seasonal task. Typically in an upflow system the furnace sits below the indoor evap coil.
HP system: uses the outdoor compressor and coil in conjunction with the indoor coil for both heating and cooling; there is no separate heat exchanger for each season. The indoor unit has only one heat exchanger for both heating and cooling, and the blower again does all seasons.
First, you have to understand about heat loss and heat gain as it relates to the house. It is VERY important to think of it as a heating (or cooling) rate, and not only a capacity. It's not just "30,000 BTUs", but it needs to be understood as a load/time (ex: 30,000 BTUs/hr). The BTU loss per hour of your dwelling is based on construction items such as insualtion, shading of trees, number and quality of windows and doors, etc. A good HVAC tech will always do his own load-loss calculations based on many standardized tools in the HVAC industry. (pretty much anyone can estimate it with today's internet tools, but a good HVAC tech will do a better job than Joe Average).
Assumptions for the example:
temperate zone: average expected high of 85F in summer and low of 10F in winter
preferred indoor temp: 70F year round
Thefore the HVAC system must be capable of providing BTU/hr make up in summer with enough energy to satisfy the 15F max expected delta, and 60F max delta in winter. You don't choose a system based on capacity, you "size" a system to accomplish holding your desired setpoint against the expected max normal temps. The capacity isn't an input, it's a result of saying:
* If I want to hold a 15F delta in summer, I need XX,xxx BTUs per hour. (house loses 22k BTUs/hr; a 24k BTU cooling system will suffice with a small buffer of excess capacity)
* If I want to hold a 60F delta in winter, I need YY,yyy BTUs per hour. (house loses 68k BTUs/hr; a 75k BTU heat system will suffice with a buffer of excess capacity)
Your house has a smaller delta to overcome in summer versus winter; pretty typical for all of North America. The ratio (cooling to heating) may be different (FL versus ND), but the concept is the same. Heating loads are often larger than cooling loads for comfortable indoor living.
In the assumed example, using a traditional AC and Furnace system, the summer max expected load will be an average temp delta of 15F (85-70). If it's cooler than 85F outside, the AC will run intermittently as it can satisfy the load. If it's warmer outside than 85F, the house might rise inside above the 70F setpoint because the system cannot keep up with the cooling loss; in theory if it's 90F outside and you have capacity/hr for a 15 degF delta, then you'll get 75F inside despite your desire to have 70F. So in the warmer months, the system should be sized to overcome a thermal loss of X BTUs/hr up to the 15 deg F delta. The outdoor coil might be a 2.5-ton unit and the indoor coil will be similar. This is always adjusted a bit depending on humidity averages, etc. But overall, the AC indoor coil will be sized for that cooling load loss and a 15 degF delta. The furnace, on the other hand, will have to make up a 60 degF delta (70-10) at that same BTU/hr loss. So the furnace has to supply many more BTUs to overcome the loss rate. Whereas the indoor coil may have to provide up to 22k BTUs/hr, the furnace may have to provide 68k BTUs/hr. These are generalizations for the example, but you get the idea. Also, the condensing and evap processes move from side to side in HP whereas they are set in AC. With a HP system, you evaporate inside in summer, but you condense inside in winter. There are all kinds of nuances that are different between the two concepts and the application of different coils and their operational parameters.
Now, if you take away the furnace system and now decide you want a HP system, that indoor AC coil which only had to supply 22k BTUs/hr in summer, now has to supply 68k BTUs/hr in winter. Suddenly that smaller indoor cooling coil is not large enough to be an indoor heating coil. So in HP systems, the HP indoor coil is typically larger because it has to make up a larger delta-T loss.
Soooooooo ....
My point in the OPs quest is that he had already purchased a heating system based on a traditional Furnace/AC concept, and therefore the indoor coil is undersized for the heating need (especially in ND !!!). Add to that, we understand the fact that he now has made us aware that he had bought a new indoor coil along with the furnace. So if he wants to change to a HP concept after the fact, he not only is wasting the use of the furnace, but he also is going to have to toss out a brand new undersized indoor coil in favor of an indoor coil that is large enough to do heating in addition to cooling.
Now do your cost analysis. Yes, often electricity can be cheaper in a "per hour" use factor. But if you now throw in a furnce that won't be used much at all (wasted money spent) and a brand new indoor coil tossed in the trash (lots more wasted money spent), and the purchase of a larger indoor coil (more money spent), and the need to modify the ductwork for the new larger indoor coil ... welll ..... you get the picture now?????
If you want a dual-fuel or HP system, you have to commit to that concept up front. You cannot do it after the fact; you'll NEVER recoup the costs associated with modifying the systems in retrospect with the supposed "savings" of using a HP. When you toss thousands of dollars in the garbage because you bought a system that won't provide what you need, the ROI of the HP savings is urinated away very, very quickly.
- HP systems make excellent sense in warm and moderate temps (above 40F) as they are very efficient
- HP systems can work decently in cold temps (above 25F) as they are moderately efficient
- HP systems struggle in ever colder temps (below 25F). (Its not that they don't work; it's that they defrost often and use heat packs which are not nearly as efficient as the HP process; the colder it gets the more they defrost and the more they rely on the "kicker heat" aka toaster coils).
- HP systems will NEVER make sense when you try to bastardize a new Furnace/AC system into something it wasn't designed to be.
That's my take on it.
