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Ductless heat pumps offer a means of heating and cooling homes without ductwork. Common applications include energy-efficient new construction and retrofits of homes with electric resistance, hot water, or steam heat. In homes with ducts, adding a ductless head can sometimes resolve comfort problems in bonus rooms and additions not well served by the main furnace or air handler.
Three kinds of ductless heads
Ductless indoor units, or heads, come in three main types. The most common are high-wall units, which sit about seven feet above the floor. High-wall units are usually located on exterior walls so that refrigerant lines can run outside the house and condensate can drain to the ground by gravity.
Also popular in my area are low-wall units, also known as floor units. Their general shape and position mimic that of hydronic radiators, and their aesthetics work well in historic homes. They can be a good choice for the second story of Cape Cod-style homes, where higher wall space is limited. Refrigerant lines and condensate for low-wall units can run on the exterior or, in the case of first-floor units, through unfinished basements and crawlspaces.
Less common, at least in my area, are a third ductless type known as ceiling cassettes. These units are recessed into an attic or dropped ceiling cavity, leaving only the plastic trim with return grille and supply outlets exposed. Many technicians, myself included, are at first intimidated by ceiling cassettes. A few aspects of their installation—the carpentry needed to hang the unit and the piping required for condensate disposal—are more involved than with wall-mounted ductless types. But other key tasks like refrigerant piping and wiring are nearly identical.
Ceiling cassettes offer some interesting features, like built-in condensate overflow switches and…
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10 Comments
I installed a ceiling cassette (9 kBTU Fujitsu Halcyon) in a 750ft2 ADU application in zone 5, no other heating or cooling, two paired HRVs for fresh air control. The ceiling is slightly vaulted so part of the thought process was that the ceiling mount would be both innocuous and help with heat stratification. The thermostat is wall mounted and its sensor is used for the control. Has worked great even during multiple consecutive days at -10.
Nice. Is the ADU extremely tight? I am in zone 5 in NW NJ and heat with three Fujitsu wall mounted one ton units over about 2200 sq feet. But my house is leaky and full of large sliding doors. I have had them produce heat down top around -10F although I need to supplement the units with baseboard on really cold days .
Do the ceiling mounts circulate warm air well?
It is extremely tight. We did blower door testing twice, I believe we ended up at ~0.77ACH50. When opening and closing the front door you can feel the pressure vs if there is a window open. I've got Alpen windows (fairly local to me) and their seals are excellent.
That's a good question on the circulation. I think there is still some heat stratification (walls hit 9-1/2' at the peak where the cassette is, slope down to 8' on either side of the house) so I often leave a ceiling fan on low to mitigate that. I also did cellulose in the bedroom interior walls for soundproofing, which I think leads to that room staying a little cooler, but i like that for sleeping. The ceiling fan in the bathroom has a heater in it that i did as a backup, i never use the heater.
I wish there were options for smaller cassettes that didn't involve multi-splits or VRF.
6k-7k BTU 1:1 wall mounts are available, so why not 1:1 cassettes of that capacity range? Same with the lack of small 1:1 ducted units; multi-split or VRF only...
For anyone considering these, my recommendation is to have a long conversation with your installer and their equipment representative about where the air-barrier is meant to be.
Because these were originally conceived as being in drop-ceilings, but now are advertised for "cut through drywall and stick into the attic" there is a lot of confusion on this point.
Long story short we had a Mitsu diamond contractor install 5 one-way cassettes in our place. Cutting through the ceiling and fitting between joists (in an attic with lots of loose fill insulation). We had never ending problems with dust building up on the wheels, causing unbalanced operation, horrible noises and many visits from installer and Mitsu representatives. 15 month nightmare. We ended up replacing all of them with a central blower/ducted system (same outside compressors) (our contractor stuck with us throughout and ate costs as well).
It was never clear where the air-barrier was meant to be, lots of argument about how well the trim was fitting to the ceilings (another big challenge with these units if you have any slope in your ceilings at all). The one unit we kept (in the garage) is fine ... but I air-sealed that one in a box from above, properly.
Oh, and remember that you'll be paying around $200 per unit for a once a year (or maybe 18 months) cleaning, which involves a different spray hood than they use for high-wall units. Lots of mess and furniture moving.
So, voice of experience here: at the very least have a different contractor create a properly air-sealed recess for these units, tested with a blower door.
Added some pics of dust issues on the wheels. Also mold in the water tray (no issues with the pump, not sure if this is normal or related to the dust intrusion issues).
I don't understand why minisplits don't have high quality replaceable filters.
I've been looking at ceiling cassettes and I wish they made them smaller.
Specifically, I wish they made a 3000 BTU/hr cassette that would fit between 16" floor joists. That would probably be perfect for most bedrooms, which seems to be a major unserved segment of the market.
I agree. Mitsubishi's K series ("one-way") ceiling cassette fits between 16" joists but the smallest rated capacity is 6,000 Btu/h. An engineer who keeps up on the latest news said they are working on one with lower output but he didn't know when it would hit the market.
Those are the 6k multi-split-only head mentioned in the article; MLZ-K? That's the only one I've been able to find.
Curious about how those are different than the 6k wall mount heads with a 1:1 outdoor unit, and why they haven't done the same type combination for the cassette.
Yes, and I don't know why they won't work in a 1:1 configuration.
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