Have you had an HVAC pro do the appropriate calculations and make the plans already?

Not yet. We are at preliminary design level with architect and builder to establish a baseline for estimated costs. The builder will provide modeling to size HVAC based upon inputs from each room, including orientation, insulation, glazing, latitude, etc. However, this won't happen until design for wall, floor, and roof assemblies and glazing specs are complete.

However, you raise a good point, apple pie. Is it typical to bring in HVAC pro early on into the design? I assumed architect would do HVAC design. So far we have only discussed HVAC in general terms. We have a good team. Meanwhile, I seek to be a super informed client.

Bring an HVAC pro in now. He should be involved at this stage to make recommendations and design input. He may even be able to recommend a more efficient system I think that a high efficiency HRV with a heat pump could do what you want it to do, with respect to zones (not positive though). Find a company who is familiar with passive solar homes and Passivhaus and they'll be able to get you sorted out. But, again, now is the time to do this while there is still time to plan around the equipment, rather than try to cram the equipment into a set design that didn't plan for it.

The challenge for home owners is that we have to be extraordinarily informed about building science to understand the choices and be comfortable with the decisions that pros make when they often differ on the right solution. Consider the passionate disagreement amongst pros on spray foam insulation here on Houzz.HVAC strikes me as complex. Our small cabin will have 4 different systems: heat pump, electric furnace, ventilator, and wood stove. All of these systems have to be integrated perfectly to meet heating, cooling, and ventilation needs, not the least of which is comfort. The HVAC system itself must integrate successfully with the building envelope, and all the variables that go into its successful execution. Moreover a passive solar cabin has to successfully interact with the sun throughout the seasons at the specific latitude and orientation of the building site in addition to all the system components of a traditional high performance building.My question to this page about multi-split heat pump vs. dumping a single coil into an electric heat plenum duct system seemingly is a straight forward question. However, I appreciate the complexities involved and hope to hear from pros here so that I may better understand our choices.

I am unaware of HRV having capability to heat a home. Reviewing notes from design team, it may be that the intent is to use heat pump as ventilation.

One HVAC consideration I'll ask the design team about is to drop the electric furnace backup from our plan. Instead buy LED fireplace and invest savings (no duct work and smaller electric resistance unit) in more insulation in roof assembly and potentially windows. Triple pane are currently slated only for southern and western exposures.

I am curious why you are worried about efficiency, yet not going with a ground source heat pump. The efficiency of using the ductwork vs multi split is insignificant unless you are putting the ductwork in an uninsulated crawl space with cold floors, or other rediculous senario, especially compared to ground source vs air to air. Passive solar should have floors that absorb the heat, so why wouldn't you be using in floor radiant heat rather than mini-splits?

You might try your question over at GreenBuildingAdvisor.com

http://www.greenbuildingadvisor.com/qa

or other forums more aligned with high performance mechanical systems.

The duct work is necessary for the HRV, although it could be much smaller if that is all it is used for. It also doubles as the air conditioning (cooling) system. If you don't want cooling capabilities in summer, it opens up more options. The electric backup will come on when the temp is around zero outside (air-to-air) quite often and supplement the heat pump unless the system is oversized to the point of being less efficient. If you are interested in comfort, I would not eliminate the electric backup, especially with an air-to-air heat pump, radiant floor heat would be a wiser option at that point. Unless you constantly feed the fireplace, not having the electric backup will cost you more in electric bills in the long run usually.

PPF greenbuilding is a favorite source. I didn't realize they had a forum. Fred S. Good points. Thanks all.

Fred S. Ground source vs. air source: I haven't seen compelling studies that warrant the cost of ground source compared to Mitsubishi's, for example, less expensive variable refrigerant flow air source heat pumps. Moreover the complexity of ground source installation further reduces its appeal for me.

"variable refrigerant flow" doesn't mean anything in cold temperatures. Maxed out is maxed out. "Effective" down to -13 doesn't mean efficient. Your design temperature is probably -15, and it gets much colder than that. Of course, Mitsubishi's "studies" are going to brag up their newest technology, but the physics doesn't support it in that cold of a climate. You are also in a more humid area than what they test them in. If they count the energy used during the defrost cycles and the lack of air getting to the coils due to ice buildup in your climate, it is probably not going to be much more efficient than baseboard heat much below zero very often. Then, factor in the snow and how you intend to keep it far enough away from the outside unit all winter so that it has at least some chance to perform well in temperatures above 0. I have been putting in heat pumps for 30 years in cold climates. While the initial cost of ground source is higher, it also pays for itself faster. Those that have chose ground source wouldn't have it any other way. Those who choose air to air, not so much. The physics doesn't support their claims. Just replaced a 30 year old air to air at my mother's house with a new state of the art with all the bells and whistles model. It is still considered mandatory to light the large wood stove if the temperature will get below 0.

The only time the "variable" nonsense does any good is in the spring and fall when I don't usually have the system turned on anyway. If you think about it, the fact that you are going passive solar pretty much negates the need for that marketing gimic all together. By the time you need the heat, the heat pump will be running full on by virtue of the outside temperature anyway. Save money by not buying the comics that you won't use, and design the system for the cold temperatures you will actually need it for. In that "study", the ground source wins hands down. And without an outside unit, the ground source is actually less complex. The second best option is an air to air with a propane backup, if you fill the tank in the summer when prices are low.

Fred S thanks for your comments. I might have to hire you! Finding HVAC pro with experience to customize a ground source heat pump is part of the challenge for any homeowner.. I don't know HVAC science and am merely a homeowner seeking to be informed. Air source vs ground source. HRV vs ERV, ductless vs duct. You name the HVAC component and there are likely those who will argue passionately for one approach over another. Some arguments spring from science, evidence, and experience. Some viewpoints have their roots in bias, like someone who will always buy Chevy over Ford. No matter what. I welcome all comments and sort through them the best I can.

Yes, I get that finding the people to do the ground source part is the biggest problem, unfortunately. But, once it is done, everything is easier. The air to air might pay for itself after 10 years, and the ground source may take 15. But if you look further down the road at the expected life of the heat pump, then at 20 years you start all over (almost) with the air to air. It will then take another 10 years to get back to even again. With ground source, the 20 year mark and replacing the compressor etc. will most likely take 5 years or less to pay for itself.

Here's the feedback that I got on greenbuidingadvisors.com
http://www.greenbuildingadvisor.com/community/forum/mechanicals/38886/best-hvac-1100-sq-ft-passive-solar-house-46-n-latitude

Thanks for posting feedback link.