Please explain how a large house with superinsulation and ultimate windows can be adequately heated and cooled with an HVAC system matched for the appropriate heat loads. Using a made-up example of 5000 sq ft and a Man J load of 24,000 Btu, how could the standard blower on a 2 ton unit deliver enough air (and at the proper velocity) to keep the house comfortable? PS: I've been a member of the forum for several years but had to re-enroll with a new ID.

Turns out that with proper insulation levels - and with a very airtight house - you don't need "velocity". These houses simply loses heat very slowly, so you can add heat in areas where it will naturally circulate, and the house will stay comfortable at a constant temperature. At least that's been my experience building such houses in northern New England and heating them with a few air source heat pumps.

Well if you have super insulated, and invested in good quality Windows, you will also have payed as much attention to air sealing the envelope... In this situation you will want a HRV to enable proper healthy air exchange right? That system drawing from and returning to different spaces in the house will assist in distribution of air gently in a house that, as Bob alluded to, Is probably not loosing heat very quickly.

This sounds like a situation where radiant heat would also work well. It delivers the heat to every room of the house without the need to pump larger quantities of air throughout the house...

Good to know that it works, but it must violate the old rules about maintaining a certain fpm velocity - which I always thought was to prevent dust from settling in the ducts. Has that requirement been waived? Were there other reasons for having a velocity component in the design specs? Another aspect of the old ways was "throw distance" or something to that effect -- meaning that the supply air would be projected into the room rather than just fall slowly out of the registers. You both referred to heating; do the superinsulated houses also feel comfortable during cooling season with low air flow? Being in Texas, we've always ramped up the blower during summer to get even more air moving. Ceiling fans would likely accomplish the desired effect. Sounds like the new houses require a new mindset.

A HRV or ERV doesn't move much heat. But leaving doors open does a pretty good job on some very well insulated/sealed houses. But this depends on the layout of the house, how cold it is, window heat gains, # of levels, etc. You probably need to use more units (perhaps multi-splits), circulate conditioned air (a furnace) or use hydronic.

"it must violate the old rules"
Those rules were for leaky old houses which required that you pump in tons of heat so that a little stays long enough to heat something. "the new houses require a new mindset." Yes. Plus, with the HRV, you are bringing in fresh air, exhausting stale air. Heating with electricity instead of burning something, and not blowing air around the houses also means that there is less dust.

I recently received a letter from someone who moved into their new house a year ago - he said with all my talk about the efficiency and comfort, I didn't make it clear enough how quiet the house would be! So that's another benefit.

Our house is quite small (1400 sq ft) ICF and very tight. The 9K mini-split in the master bedroom does a pretty good job of keeping the whole house comfortable if the doors are left open. But then, we often go for long periods without using any HVAC. Thanks to a mild fall, we went from mid-September to January 4 without using it. Our ICF basement has no HVAC, but stays at a 'livable' temperature year-round here in SC. It's truly amazing living in an efficient home after having lived 60 years never having lived in an insulated house, including CT and FL.

Looks like "throw" is an issue and lack of it causes more stratification (even in this case with highly improved (80K->25K) insulation and not-that-low velocity). Also see here.

There may also be an issue with a smaller, low velocity, high surface area to volume ratio duct loosing too much heat through the duct walls before it reaches the destination (ie, would require insulated ducts).

Perhaps the solution is a booster fan.

Stratification is not an issue with a really well insulated house, so you don't need "throw". If the top of the room gets hotter than the floor, radiant heat transfer from the ceiling to the floor will be enough to even it out. If the floor's poorly insulated and/or there are air leaks near the floor, it will get cold faster than the radiation warms it.

Hi garyWEL0222, I'm in a cooling dominated climate too, with a large space to cover, two floors, and just one efficient 2.5 ton unit and blower in a steel SIPs house. The blower is centrally located in the plan with the longest run probably about 45 or 50 feet to the end of the duct line. I had my concerns about air distribution, but they turned out to be unnecessary. It's very comfortable and the air temp seems pretty even throughout the house. With tall ceilings and an open stairwell the second floor does get a bit warmer, but only by a little bit. If you're in east Texas you may consider adding a whole-house dehumidifier for the spring and fall, because the humidity still needs to be addressed even though the temps are mild enough not to need heat or air.