Fresh air without ducts: options for mini-split, radiator and baseboard homes

A wall-mounted mini-split is a fan, a filter and a coil: only refrigerant lines and wires go through the wall, so it heats and cools the air already in the room. Radiators, baseboard heat and radiant floors don't move air at all. In a home without ducts, a bedroom's fresh air comes from the door, the window, gaps in the walls, or a fan you add.

None of that is a problem if the bedroom stays fresh. Measure first: three ordinary nights show whether the room reaches the 800 ppm goal, and how far off it is. A bedroom needs about 17 cfm of outdoor air for each person sleeping there.

The options, from free to built-in

Costs are from the sources below, read in October 2026. Your installer's quote replaces them.

Free: the door and the window

In Danish bedrooms in winter, opening the window lowered overnight CO₂ from about 1,820 to 760 ppm, and opening the door from about 2,360 to 1,290. The window costs heat, noise and sometimes security; the door shares the bedroom's air with the rest of the house. Free fixes has how to try them fairly.

Cheap: a bath fan that runs all the time

A quiet, continuous-rated bath fan pulls stale air out of the house, and outdoor air leaks in to replace it. It's the least expensive way to ventilate on purpose. Two limits:

• It barely reaches a closed bedroom. In Building America lab houses, a 45 cfm fan in an en-suite bathroom added only about 0.1 air changes an hour of outdoor air to the bedroom next to it.
• The replacement air comes from wherever it can. Passive inlets (small vents in a bedroom wall) are meant to guide it, but wind and the stack effect can push air through them either way, and they pass little: one maker's inlet (Lunos ALD) is rated about 7 to 15 cfm at the small pressures a fan makes.

If a furnace, water heater or fireplace vents up a chimney, a fan running nonstop can pull exhaust back into the house. Have combustion safety checked first.

Room by room: units in the outside wall

These go through an outside wall of each bedroom, bring fresh air in, take stale air out, and keep much of the heat. Room-by-room units lists the ones on sale in the US, with airflow, sound and hole size.

• Pairs that take turns. Two small fans in the room's walls (or the walls of nearby rooms) swap direction about once a minute, storing the heat in ceramic cores. Lunos, VENTS TwinFresh and Blauberg Vento work this way.
• One unit, in and out. A box with a fixed core moves both airstreams at once. Zehnder's ComfoAir 70 and VENTS' Micra are examples.

Compare the heat-recovery airflow, not the headline number. Units that take turns are usually sold by their ventilation-mode airflow. Blauberg's own data sheet shows its "30 cfm" unit moving 15 cfm in heat-recovery mode at top speed, so a pair gives about 30 cfm. A Lunos e² pair moves up to about 22 cfm. One person needs about 17 cfm and two about 34: often two pairs, or a unit with more air.

Expect less than the rating. In 60 units measured in Luxembourg homes, heat recovery averaged about 70%, and wind and the stack effect pushed the airflow around. In a lab, one reversing unit nearly stopped exhausting against 20 pascals of pressure. Owners on Green Building Advisor mention the click of the fans swapping, street noise through the wall, and, in one 600 sq ft home, a setup that didn't hold CO₂ down.

Walls and holes. Each needs an outside wall thick enough to hold it: Lunos asks for 12 in (7½ in for its short version), Zehnder's ComfoAir 70 at least 11 in. Holes run from about 6 to 10½ in across.

Cost. The units cost about $470 to $2,300 each before installing. One New York installer prices a single room at about $2,000 to $3,500 installed. Three or four sets can cost as much as a ducted system.

Certification. We found no HVI listing for a unit that takes turns. Where the energy code requires an HRV or ERV (climate zones 7 and 8, and 6 under the 2024 IECC), it must be listed with at least 65% sensible heat recovery at 32°F, so check with your building department.

A compact ERV with short ducts

A small ERV in a closet, soffit or ceiling, ducted a short way to two or three rooms, gives more air per hole in the wall, can be balanced and measured, and carries certified ratings. Panasonic's WhisperComfort 60 (20 to 60 cfm, HVI-certified) and Intelli-Balance 120 (30 to 120 cfm) are sold for this. One cost guide puts a 150 cfm ERV with about 30 ft of duct at roughly $2,300 to $6,200 installed. It needs somewhere to hide the ducts, and an installer who will measure each grille.

Mini-splits that say they bring in fresh air

A few do, with conditions. Ceiling cassettes and concealed-duct units can take a fresh-air duct, usually unfiltered and untempered. Pioneer's Quantum Fresh wall unit claims a selectable 10% outdoor air but states no airflow or heat recovery, and the Daikin fresh-air wall units we found are sold in Europe and Japan; we found none listed in the US. Ask any seller for the cfm of outdoor air reaching each bedroom, and measure it.

Questions for the installer

1. How much fresh air reaches each bedroom in heat-recovery mode, in cfm, against about 17 cfm per sleeper?
2. How many units or pairs, and where: high on an outside wall, away from corners, the bed and street noise, never in a closet?
3. How thick is the wall, how big is the hole, and how is it sealed to the house's air barrier?
4. How loud is it in the bedroom, at the speed it will run at night, at a stated distance?
5. How does air leave the room with the door shut?
6. What do the filters cost, and how often do they and the cores need cleaning?
7. Is a permit needed, and is the unit listed where your energy code requires it?

Plan it

The room planner works out each bedroom's airflow and lists the room units that cover it. In a condo or a rental, start with Fresh air in an apartment or condo: a hole in an outside wall usually needs someone else's yes.