Mini-duct air conditioning for an old house
March 1, 2017 12:57 PM   Subscribe

Retrofitting an old house for air conditioning. Does anyone have experience with retrofitted high-velocity mini-duct AC systems?

As I understand it, the mini-duct system is a new way to install central air in a house without the ductwork taking up as much space -- it has ducts but they're thinner tubes, so they take less space running around and they require smaller holes for the vents. So it's easier to install in an old house without being hugely cumbersome or disruptive.

The house is two floors of living space, with unfinished basement and attic. Radiator heat. Plaster walls. Until now we've used window unit ACs.

What should we be thinking about? Pro's and con's, pitfalls of choosing an installer, installation tips, etc? If you have one of these systems, do you like it?

Should we be thinking about a ductless mini-split system (no ducts, and a wall unit in one or two rooms) instead?
posted by LobsterMitten to Home & Garden (8 answers total) 3 users marked this as a favorite
The ductless mini-split systems have a significant negative in that you have to have wall space where you can actually put the inside units, and then you have that big box sitting on the wall, which might or might not be okay. However, you can get multi-zone mini-split systems that allow for independent control, which can give you much more precise control over cooling. The high-velocity mini-duct systems don't seem to do that, and you still have to find a larger pathway to each area you wish to service than you would with a mini-split.

There are so many factors here that it would be hard to reach a solid conclusion.

If you have two living space floors and then unfinished basement and attic, that suggests that you might have less trouble installing a high-velocity system.

If your house has lots of individual rooms with exterior walls, especially ones that are exposed to the sun, it will be a lot of trouble to get a conventional mini-split system that adequately addresses cooling, because the inside units are really intended to cool a single room. With two floors, that could be a lot of trouble if your house has a lot of sun exposure, because you might end up needing cooling in four or five rooms. With a mini-split, you're either putting units in each room, or figuring out how to cause air to circulate (usually with box fans).

However, if the exposed-to-sun walls are in large rooms where you spend most of your time, you might be able to address that with a mini-split more easily, and concentrate your cooling efforts in frequently used parts of the house. That may be more energy-efficient, because there may be a tendency to want to design a high-velocity mini-duct system cool more areas of the house, which takes more energy to do.
posted by jgreco at 3:50 PM on March 1, 2017

Hi, we basically have the same house. I'm still in the process of figuring out how this is going to work as we get away from window units.

I've brought in three contractors now specifically to ask about high velocity mini duct systems and they've all steered me towards a traditional air conditioning system. In every instance I've asked for bids for both and the mini duct systems almost invariably cost 50-75% more, and I'm usually told it's because the mini duct equipment costs more and is more labor intensive - they will still be cutting out parts of your ceiling drywall to run the mini ducts through. You'll also need some sort of closet that will allow the duct work for the main vein of the mini duct system to run through to connect the two floors to the system.

Other cons I've heard: More energy intensive, depending on how the system is designed, potential for cold spots.

On Mini splits - I liked the idea of these as they're everywhere in asia where I've traveled quite a bit and I've liked them there, but those were also in asian style apartment units, not houses. We've gotten quotes that average about $4k a unit here, and you should think about where you would ideally place them in each of the rooms you want and where the closest access to an outside wall is in those rooms - if it's not the wall where the unit is hanging, you'll have tubing that will need to be run along the wall outside to the condenser. I've seen units advertised that sit on the floor, but I've never been able to get an A/C installer who has experience putting one in. I've also been told by numerous real estate agents that buyers tend to dislike the mini-split units if you're thinking about adding value and selling the house one day.

One thing I'd think about doing is asking for detailed plans from the contractors as to how they are planning on designing the ductwork (regardless of which system you get).
posted by Karaage at 4:13 PM on March 1, 2017

High velocity is great - IF installed correctly. Most companies do not install correctly. Ask for references - at least two seasons worth, check references. Get 3 quotes for the high velocity. Get 3 more quotes from DIFFERENT COMPANIES for a traditional AC system. I live in an old, old, old house with plaster walls and cast iron big honkin radiators, and paid $$ to shoehorn in a traditional AC system. It's absolutely amazing.

Skip the minispits, you live in a house, not a motel.
posted by Geckwoistmeinauto at 5:51 PM on March 1, 2017

Response by poster: Thank you!
jgreco and Kaarage, thanks for the info about the mini-split. Our first floor and the staircase is all open (no doors to close) so it would be tricky to do zoned heating there. It sounds like it wouldn't really be a step up from the window unit.

Kaarage, it's really interesting contractors are steering you away from the more expensive system -- I wonder what the deal is there, you'd think it would be the opposite. (And it's odd that it's more labor intensive, since isn't it basically running ducts along roughly the same pathways, just narrower ducts?)

Geckwoistmeinauto, do you have a sense what I'd look at, in evaluating installers -- are there typical/obvious ways the systems are installed wrong?
posted by LobsterMitten at 7:44 PM on March 1, 2017

I think it should be pointed out that mini-splits can have ceiling cassettes in lieu of or in addition to the wall units.

The thing you really need to think about is wall construction - probably no insulation unless you have it put in, and no vapor barrier. If you live in a humid in summer area, the inside of your wall is likely to go below dew point when you run the A/C, causing water to form inside the wall. Not good for framing, good for mold.
If you have no insulation, the same thing may happen, or the exterior of your wall may go below dew point and encourage mildew on the outside of the building. I'm living that one right now in the repo I bought. If I keep the A/C to where we don't hit dew point of outside air, it isn't a problem. If my 11 year old cranks it down for some reason, I get black mildew on the outside of the house. There are paints with zinc additives that will stop this, which I will use when I paint the building.
posted by rudd135 at 4:12 AM on March 2, 2017

Regarding problems with installations, one house we looked at with a high-velocity system had a strange problem that made the vent in one room sound like a helicopter – annoyingly loud. I guess the air was oscillating somehow. Another room was quieter but had a similar problem.
posted by exogenous at 8:23 AM on March 2, 2017

Contractors frequently push back on less common technology. They make money by quickly installing familiar equipment in the same old way. They see the high-velocity option and think long learning curve, less field experience, less established vendor relationship, more callbacks. Try to figure out if there's somebody who's making it their niche.
posted by werkzeuger at 9:23 AM on March 2, 2017 [1 favorite]

Another HVAC contractor here today who came by, who I got on referral as someone who specialized in traditional HVAC system in old homes and wouldn't require extensive renovations. He seemed to know what he was doing, and we talked extensively about the design. Ended up encouraging me to do a two zone system so no new soffits would have to be built - the basement and first floor is controlled in one zone and the ducts are run in the basement joists and registers/vents built into the floor of the first floor. The second floor is controlled by another zone, and the ducts (and blower) are run through the attic crawl space and down the ceiling.

This one seemed to know what he was doing and we talked extensively about mini-ducts as well. He pointed out that he didn't like them because he found them to be noisy (i.e. similar to the sound of blowing hard through a straw), and that they were less energy efficient - told me to look up the SEER ratings of the unit he was recommending and the spacepak.
posted by Karaage at 2:10 PM on March 3, 2017

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