Changing upstairs from forced air to mini-split?
April 11, 2018 1:25 PM   Subscribe

I've got a second story with ducts running to it from the basement, through the main level. Supply and return. The ducts are in a central spot and if we could remove them to re-work the floorplan, we'd get much better use from the space, adding a new bedroom. I was thinking about cutting off the ducts and going with mini-splits. Anyone with experience living with mini-splits or who went from ducted to mini-split?

It's not without a dedicated cost and once done, wouldn't want to be un-done. The new space would have three bedrooms and a bathroom. Right now all the heating vents are in the ceiling and it's a vaulted space. A bonus would be that the ducts on the main level are also in an annoying spot, it would be great to just be rid of them!
posted by amanda to Home & Garden (4 answers total) 3 users marked this as a favorite
This is a really common thing back in the NE, but mostly with folks replacing their radiators with mini split systems. They work really well, and have improved pretty drastically over the past decade (or so said the hvac place I was an admin at in Maine said). Pair that with their higher efficiency ratings, and he fact they can cool and heat a space, they’re prerty neat. Lots of people use them for additions that would otherwise require ducting. We have Lon term plans to add a room, and will be using a mini split to save room taken up by ducting. This is a legitimate use for sure.

Functionally, I doubt you’d see any difference, and depending on how the rest of the house is heated, you might even see some (operating) savings.
posted by furnace.heart at 1:45 PM on April 11, 2018 [1 favorite]

We put in mini splits last spring, and couldn't be happier. Caveat: Northern California, temperate climate where heat pumps work really well.

The main thing that drove us to them over forced air was that zoned climate control works a lot better; just heating one room at a time, when we need it, seems like a way better use of resources.
posted by straw at 2:05 PM on April 11, 2018

HVAC engineer here (but not your engineer, not licensed in your state, etc.) First of all, mini-splits are great! If you can afford them, they can be an awesome option for spaces like you're describing.

But I'm worried about what's going to happen to your existing furnace/AC unit. Are you going to keep running it for just the ground floor and basement? Are you planning to keep all your other ductwork the same and just cap the ducts serving the upstairs? In that case you're basically asking your furnace/AC system to provide the same amount of heating/cooling energy to half as much air, so the air leaving the unit will be much hotter in the winter and colder in the summer. This kind of thing can lead to a cooling coil getting clogged with ice and causing major damage to itself in the summer. In the winter, your furnace would notice that its discharge air temperature is too hot and lock itself out on a high-limit condition.

Even if you give all that extra air somewhere to go, you could still end up with a system that's oversized, making it likely to cycle on and off more frequently. You might have humidity control issues in the summer, where the air is cold enough but still carrying a lot of moisture, giving you lots more condensation and potential for mold growth.

You might be better off switching to mini-splits for your whole house. Is that an option?

Feel free to memail me if you'd like to nerd out about HVAC!
posted by beandip at 2:28 PM on April 11, 2018 [5 favorites]

I'd be much happier with mini splits, personally. I have never had a forced air system that didn't have an annoying temperature difference between rooms that makes it impossible to make everybody happy during at least some conditions.

The only mini-split I've seen that I didn't like was one installed to cool a server room that became oversized when a bunch of equipment was replaced and the cooling load dropped by half or more. After that the only options were hotter than desirable or colder but overly humid since it could never run long enough to fully dehumidify the air before hitting its setpoint even when we tried 60F. It was fairly old and almost certainly predated the variable speed inverter models, though. Those can deal with much more variation in load since they can run the compressor slower if need be.

Point being new mini splits are almost always great, but beandip has a good point about your old unit becoming oversized. That said, if it hasn't been cooling the upstairs effectively anyway, it might turn out ok. I once lived in a house where the upstairs registers had so little flow it would have hardly made a difference to the unit, and that is sadly common in houses that have a single unit for multiple floors.
posted by wierdo at 8:59 PM on April 11, 2018

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