How do ductless mini-splits actually work?
A ductless mini-split is a heat pump that skips ductwork entirely: an outdoor unit connects to one or more wall-mounted indoor heads through a small line set — refrigerant lines, power, and a drain bundled through a penetration needing only a three-inch wall opening. Each head heats and cools its own zone with its own remote, and because refrigerant carries heat directly to the room instead of through leaky ducts, ductless systems avoid the 20–30% distribution losses typical of older forced-air homes in Greater Seattle.
The quick answer
- No ducts: refrigerant lines carry heat directly to wall-mounted heads in each zone
- Each head is its own thermostat zone — heat the rooms you use
- Inverter compressors glide up and down instead of blasting on/off — quiet and efficient
- Installation avoids the duct-loss problem entirely (typical ducts leak 20–30%, per ENERGY STAR)
Where ductless is the natural answer
Homes without ducts — Seattle's pre-war Craftsmans and Tudors, baseboard-heated mid-century houses, townhomes on wall heaters. Retrofitting ductwork through finished plaster walls costs thousands and tears the house open; regional installers put duct-retrofit costs at $5,000–$15,000. A line set through a three-inch hole skips all of it.
Also: ADUs and DADUs (the de-facto standard — one head, full comfort, no ducts), garage conversions, and the one problem room a central system never reaches. That's most of Seattle's housing stock, which is why ductless is the retrofit workhorse here.
The honest limitation: ductless shines in zone-by-zone comfort, not in moving huge volumes of air through a whole floor at once. Open floor plans share heads well; a closed-door culture of bedrooms needs more design thought — which is why load calculations and head placement matter more than brand badges.
The pieces and how they connect
Outside: a compact unit holding the compressor and outdoor coil. Inside: one or more heads — wall-mounted most commonly; floor consoles and slim ceiling cassettes where walls don't work. Between them, the line set: two insulated refrigerant lines, a control cable, and a condensate drain, bundled through a small penetration and run along the exterior in a channel.
It heats and cools exactly like any heat pump — moving heat rather than making it — but delivers it at the head instead of through ducts. Skipping ducts skips duct losses, which is a real slice of the efficiency story.
What 'inverter-driven' buys you
Traditional equipment is on/off: full blast or nothing. Inverter compressors vary their speed continuously — sprint to temperature, then glide at a whisper to hold it. That's why mini-splits are startlingly quiet (indoor heads in the low 20s dB range on low), why room temperature stays steady instead of sawtoothing, and why cold-climate models (Mitsubishi's H2i hyper-heat line we install holds 100% capacity at 5°F) manage Puget Sound winters without breaking stride.
Single-zone vs multi-zone
Single-zone: one outdoor unit, one head — an ADU, a bonus room, a small home's main space. Multi-zone: one outdoor unit feeding two to four-plus heads, each independently controlled — the whole-house pattern for duct-free homes. Zone count and head placement come from a room-by-room load calculation; layout and line-set runs drive most of the price spread.
Where ductless projects disappoint
Oversized heads. The smallest head often exceeds a small bedroom's load, so an oversized one never modulates down into its quiet, efficient range — it behaves like on/off equipment. Honest design sometimes means fewer heads than rooms, using door openings and a hallway head, or a slim-duct cassette serving two small rooms.
The other letdowns are maintenance-shaped: washable filters that never get washed (heads live in the room with you — dust is constant), and condensate drains that clog and drip down the wall. Both are five-minute fixes on a schedule and water stains when ignored.
In Puget Sound's damp winters, we also see outdoor units set too low on ground that puddles — defrost melt refreezes into an ice shelf that eventually reaches the coil. Raised pads and drainage are small install details that prevent the most common winter complaint calls on otherwise good equipment.
Questions Seattle homeowners ask
Do mini-splits heat as well as they cool?
Yes — they're heat pumps first. Cold-climate models hold full heating capacity at 5°F and operate far below anything Puget Sound delivers. In this climate, buying cooling-only equipment is usually the mistake; the same hardware should carry your winter. Seattle's long mild heating season is where inverter-driven mini-splits earn their efficiency advantage over resistance heat or aging oil burners.
How visible is the installation?
Inside: the head itself (about the size of a long duffel bag, high on the wall) — floor and ceiling options exist where that offends. Outside: the compact outdoor unit plus a paint-matchable line-set channel along the exterior. No soffits, no torn-open plaster walls, no ducts. In Seattle's pre-war homes, careful routing along less-visible walls and soffits is part of the craft — we've been placing line sets in these houses since 1938.
One head per room?
Not necessarily — that's often over-design. Heads are sized and placed from a room-by-room load calculation; open layouts share heads, and slim-duct cassettes can serve two small rooms from one unit. Right-sizing each head matters more than head count. A head that's too large for a small bedroom never modulates down into its quiet, efficient range and behaves like old on/off equipment.
What maintenance do they need?
Homeowner: rinse the washable filters every few weeks in heavy season — October through May for heating, plus summer if you're cooling. Professional: annual service with coil cleaning, drain flush, charge and electrical checks. Our maintenance plans cover ductless the same as any system, from $23/month. Skipping filter rinses is the most common reason heads get noisy and lose capacity in Seattle's dusty shoulder seasons.
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How we know this
Equipment behavior per the Mitsubishi Electric and Daikin product lines Glendale installs; duct-retrofit cost context from regional installer published guidance; design practice per ACCA room-by-room load calculation.
Reviewed by the Glendale Heating & Air Conditioning team · Last updated 2026-07-15
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