HVAC for a gut job remodel
I’m rebuilding a 1600 sqft ranch in Zone 5 central NY. The wall insulation will end up being a little better than R30 (R19 between 2×6 stud wall with 2.5″ polyiso on top of the OSB sheathing) and pushing R60 in the ceiling (most likely chopped fiberglass) with 2.5″ foam on the inside of the basement walls and 3.5″ polyiso blocking in the rim joists. The house has a lot of glass, close to 20% of total wall area and a relatively open floor plan with a contiguous kitchen/dining/living area of approx 900sqft. and the balance of the house in three bedrooms, 2 baths and a laundry room.
Originally, I was thinking of using either two minisplits to heat and cool, or one minisplit with either three or four interior units, but recently I have started to see recommendations for a ducted system on this site, rather than the minisplits.
Given that the house is a framed and uninsulated shell right now, so installation of ducting wouldn’t be super difficult what are the plusses and minuses for the three options?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Great question Andy!
Pros: 1. Best matches heat load to capacity for smaller rooms. 2. Best distribution 3. Best air filtering 4. Ductwork lasts, so replacement just involves air handler/outdoor unit. 5. Flexibility as technology changes. Cons: 1. ductwork expense 2. distribution energy is higher 3. Top of the line efficiencies aren't as good as top ductless options.
Ductless system (1 outdoor-to-1indoor unit)
Pros: 1. great modulation 2. High efficiencies 3. redundancy 4. initial cost isn't terrible
Cons: 1. distribution can be tricky 2. Dehumidification isn't always good 3. Air filtering is awful
Ductless system (1 outdoor-to-many indoor units)
Pros: 1. One outdoor unit. 2. Sometimes installation costs. Nothing else really. IMO worst option available for a gut remodel by far.
Cons: Two cons unique to multi-splits are: 1. low efficiency from oversizing 2. poor sizing due to bad modulation AND too much capacity/room. The other ductless cons also apply.
+1 on Paul's response. You can also consider mix-and-match: A single one-to-one ductless system for the large open space, and a one-to-one ducted minisplit for the bedrooms. The first step with any system is getting a well-done Manual J heat load calculation by someone who has no interest in overselling your equipment. Depending on how well you insulate and air seal a house that size, you're bumping against the minimum available minisplit sizes if you install two separate systems, but it's certainly worth running the numbers.
You can also "game" the minisplits a bit in the shoulder seasons by leaving the bedroom unit off so that the living area unit does all of the heating or cooling, making it more right-sized. The disadvantage with this approach is somewhat less control of temperatures in the bedrooms when that zone isn't running. Leaving bedroom doors open will minimize this, though at an obvious loss of privacy.
If going with a ducted system, make sure to get the air handler and ductwork inside the thermal envelope even if that means losing some interior space.