Central Air for a House Without Ducts: Your Real Options

The four routes, in plain terms

Central air means one system cooling many rooms by sending air through ducts, which are the channels inside walls, floors and attics. A house with no ducts cannot simply be connected to one. Either a duct path is added, or the system is chosen so that it needs little or none. Four routes follow from that choice.

The first is a new conventional duct system. A contractor builds supply and return ducts and sets an air handler, the indoor cabinet that holds the blower and the cooling coil, in the attic or a closet. How a split system and its parts fit together is covered in central air conditioning systems.

The second is a small-duct high-velocity system. Lennox, a manufacturer, describes it as an air handler that pushes air through narrow flexible tubing, typically 2 to 3 inches across, at higher pressure than a standard system. Lennox says the tubing can pass through existing wall cavities, under floors and above ceilings.

The third is a ducted mini-split. A 2013 Pacific Northwest National Laboratory review for the Department of Energy says some mini-split models have indoor units designed to be concealed within short runs of ductwork.

The fourth is the ductless mini-split, with an outdoor unit and one or more indoor heads on walls or ceilings. Its parts and operation have their own mini-split overview, so this page does not repeat them.

Where a duct path can run in a finished house

The first practical question is where air can travel. The Florida Solar Energy Center's builder guideline from 2011 says homes in hot climates are still typically designed and built with duct systems in the attic. The same guideline says an interior duct system, one placed inside the home's air and thermal barrier, should be planned before interior framing. It adds that retrofitting one is difficult but not impossible.

The guideline also says an interior duct system is not often suitable for retrofit unless a sealed attic or crawlspace is created. A sealed attic is an attic whose roof deck is air-sealed and insulated so that the ducts sit inside the house's boundary. Our reading: in a finished house without ducts, the realistic duct paths are the attic and any closet or soffit space that can be opened. Each one has to be checked in person.

The manufacturer literature adds one more point. Lennox says a high-velocity air handler is compact enough for an attic, a closet or a crawl space, which suits tight spaces. Our reading: a floor plan with no attic access and no closet near the middle of the house narrows the choice toward the mini-split routes. Ask any contractor to show the planned route for supply air, return air and the air handler on a sketch before you compare bids.

Why a Florida attic changes the choice

A duct in a Florida attic runs through a hot, humid space. The Florida Solar Energy Center guideline calls a hot or cold vented attic an extreme environment. It says the first air delivered when equipment starts can be uncomfortably warm or cold, because air sits in the ducts between runs and gains or loses heat. It also says equipment life is improved by removing the system from the dirt, contamination, moisture and heat of unconditioned attics.

The Department of Energy's job aid for flex ducts says all exposed ducts in unconditioned spaces should be sealed, insulated and supported. How much insulation Florida's code form lists is covered in duct insulation, and why attic ducts sweat in humid air is covered in ductwork in Florida. This page does not repeat either one.

Our reading: an attic route means paying attention to leaks, insulation and condensation on every foot of new duct. A ductless route moves the air delivery into the rooms, so less of that exposure applies. The refrigerant lines still have to be run, and that is work for a certified technician.

A sealed attic is the other answer to the heat. The Florida Solar Energy Center guideline warns that foam insulation is unlikely to meet fire resistance standards on its own, so it may need a second coating or drywall cover. It advises consulting the local authority before choosing a sealed attic.

Small-duct high-velocity: what its maker says it trades

Lennox is a manufacturer, so its homeowner guide is the maker's own description, not an independent test. With that in mind, the guide is open about the trade-offs. It says high-velocity systems usually cost more to install than a standard central air installation. It says their AHRI efficiency ratings are typically lower than those of top-tier variable-capacity central systems. AHRI is the Air-Conditioning, Heating, and Refrigeration Institute, which publishes equipment ratings.

Lennox also says improper installation is the primary cause of noise complaints with these systems. It says the technician tests static pressure and airflow before the job is done. Static pressure is how hard the blower has to push air through the duct system. Our reading: with narrow tubing, a measured result matters more than the sales description, so ask for the test numbers in writing.

Lennox lists adding cooling to an uncooled addition or converted space as one of the strongest uses for the system. Our reading: whether it beats a mini-split in your house depends on the attic or closet route, the noise limit you can accept, and the bids you receive.

Mini-splits: ducted or ductless

ENERGY STAR says mini-splits are increasingly used in older homes with no existing ductwork that have never had central air conditioning. It also lists additions and outbuildings where extending ductwork is difficult. For the parts and indoor unit styles, see the mini-split overview at /learn/ductless-mini-split-systems. Pipe length, condensate, power and the outdoor unit are on the mini-split installation page.

The ducted version sits between the routes. The 2013 national laboratory review says the installation cost of the concealed-ducted versions is higher than the wall-mounted units, and that extra fan energy is needed to move air through the duct and distribution grille. Our reading: a ducted mini-split suits a house where a short hidden duct run is possible but a full duct network is not.

Two cautions apply to any mini-split in a multi-story house or in Florida humidity. The same review says warm air from a downstairs unit rises but cool air does not, which left an upstairs hot in summer in a home with only a downstairs indoor unit. It also found no field data on mini-split moisture removal in extremely humid climates. The mini-split overview linked above reports what a later Florida field study measured. For indoor humidity in general, see the page on damp air despite a running system.

Sizing comes before the system type

ENERGY STAR warns that equipment sized too big or too small for the space struggles to keep a home comfortable. It says your contractor should verify the right size using a calculation called Manual J. Manual J is the industry method for working out how much cooling and heating a house needs.

The load calculation matters more when there are no ducts, because the load also helps decide which route is realistic. A house with a small load may suit a few mini-split heads. A larger or more closed-in house may need an air handler and a duct path. Our reading: ask every bidder for the Manual J result for your house, so the routes are compared on the same load. The load calculation and tonnage are explained on the tonnage page, and room-by-room mini-split sizing is covered in what size mini-split you need.

Permits, approvals and who may do which work

Adding a first system to a house is not a swap, so expect the building department to be involved. The Florida permit page explains that new HVAC equipment generally needs a permit from the local building department. The exact steps vary by jurisdiction, so confirm them with the building department for your address.

If a sealed attic or a new air handler location is part of the plan, the Florida Solar Energy Center guideline advises getting design features approved before construction. It says failing to pre-approve them can mean slow permitting and failed inspections.

Work on the refrigerant circuit is for certified technicians, as the Section 608 rules page explains. Electrical connections are for a qualified electrician. Check the contractor's license with the Florida Department of Business and Professional Regulation before you sign.

Questions to put in writing before you choose

These questions come from our reading of the sources above. They help you compare bids for different routes on the same terms.

1. Which routes did you consider for my house, and why did you drop the others?
2. Where exactly will the air handler, supply runs and return path go? Please mark them on a sketch.
3. What is the Manual J load result for my house, and how was each room handled?
4. If ducts go in the attic, how will they be sealed, insulated and supported, and who tests the finished system?
5. If you propose small-duct high-velocity, what static pressure and airflow numbers will you record at the end?
6. Which permits are needed, who pulls them, and what inspections follow?

A contractor who answers these in writing has made the choice about your house, not about a product.