My house in Massachusetts has forced-air heating with a gas furnace. I also have two heat pumps which take care of cooling and also help with heating. One of them is a multi-zone heat pump, the Mitsubishi “hyper-heat” model MXZ-3C30NAHZ, which is currently connected to two wall units, one MSZ-FS12NA and one MSZ-GL06NA. So this can accommodate a third indoor unit, and I am trying to decide what it should be and where it should be placed.

The easiest choice would be another 12k wall unit, but I see no good location in the house to put this. In a nutshell, it would end up either in a small closed room for which it is way over-sized, or else in a more open area that already has another 12k unit in it. Either way, some rooms would be served with excess heating capacity and others not at all. What I really need is just a little additional capacity on the (open-plan) 1st floor plus small amounts of heat in each of a few little rooms. The gas-powered forced-air system covers the whole house, of course, but the objective is to rely on that as little as possible.

So I got interested in the air handler SVZ-KP12NA, which in principle is compatible with the MXZ-3C30NAHZ and the two other indoor units it already serves. I naively thought that the SVZ could be installed alongside the gas furnace in the basement, tied into the same existing return and supply plenum, with electronic controls, of course, that prevent the two from ever running at the same time. This way, I could heat every part of the house with my heat pumps, except for the coldest times of the year, when I would switch to the furnace.

But this is not a set-up that is offered by any of the HVAC companies that I have talked to. According to them, the only way to utilize any of the pre-existing duct work is to remove the furnace and put the SVZ in its place. This, however, would result in an overall system that’s severely undersized for my house (by their calculations). If I wanted the SVZ while keeping the furnace, they would only install it with all new duct work of its own (down to even new floor grilles in the rooms). That seems crazy wasteful, not to speak of outrageously expensive (I was quoted close to $30,000).

So I am not sure where to go at this point. I am aware that I haven’t provided enough data here for people on this forum to come back with specific advice. I could certainly supply more information. At the moment, however, my main reason for posting this is plain curiosity. Why exactly is it that the heat pump and the furnace can’t share ducting? Are there technical obstacles? Would it violate building codes? Does it just not fit the companies’ business models? If any of you experts could be generous enough with their time to help a lay-person understand this better, that would be wonderful. If no contractor will build it for me, it may make no practical difference if I know the reasons, but I would still sleep better and be very grateful for your answers!