You can get real air conditioning into an old house without ductwork, and you don't have to gut the plaster to do it. The realistic options, roughly from least to most invasive, are window and through-wall units, ductless mini splits (wall-mounted, ceiling cassette or floor console), compact ducted mini splits that sit in an attic or closet and feed a few rooms through short ducts, high-velocity small-duct systems, and full central air with new ductwork.
If you're cooling a house with radiators that was built before the Second World War, a ductless system, a short-run ducted mini split, or a combination of the two gives the best balance of comfort, cost and disruption. High-velocity systems suit owners who want the look of central air with very little visible equipment, and a full ducted retrofit makes the most sense when a renovation already has walls and ceilings open. This is a common situation close to Boston: by Census estimates, 53 percent of the housing in Everett, 49 percent in Newton and 44 percent in Watertown was built before 1940.
The options, with their trade-offs
Window and through-wall air conditioners
Window units are the cheapest AC for a house without ducts, and they need no contractor. They are also loud, block light and a window you may want to open, have to go in and come out every year, and leave gaps that are hard to seal. Through-wall units end the seasonal hauling but require cutting a permanent opening in the wall, which in plaster and lath or a brick house is not a small job. For one bedroom or a short-term fix, they're reasonable. As a whole-house plan, the noise, the hassle and the lost windows add up.
Ductless wall-mounted mini splits
A ductless mini split pairs an outdoor unit with one or more indoor units mounted high on the wall, with no ducts anywhere, which is why it adapts so easily to an old house. The connection between them, a refrigerant line set, a drain line and a power and control cable, passes through a hole about three inches across. They are quiet, efficient and zoned, so each room or area runs at its own setting. The trade-offs: you see the indoor units, a closed-off room needs its own unit to cool well, and every line set needs a route outside or through the house. Several indoor units can share one multi-zone outdoor unit, or rooms can get separate single-zone systems, a choice covered in the sizing section below.
Ceiling cassettes and floor consoles
These are ductless units in other shapes. A ceiling cassette sits recessed in the ceiling with only a grille showing and spreads air in several directions. It needs room above the ceiling, so it suits a top-floor room with an attic overhead far better than a first-floor ceiling under a finished bedroom. A floor console sits low on the wall, roughly where a radiator would, which makes it a good fit for attic knee walls or a wall full of windows, and it is particularly good at heating. Both usually cost more than a wall unit of the same capacity.
Ducted mini splits and slim air handlers
A compact ducted air handler can sit in an attic, a closet or a basement soffit and feed a few nearby rooms through short duct runs, with only small ceiling or wall grilles showing. In an old two-story house, one in the attic can cool all the second-floor bedrooms from above without a single unit on a bedroom wall. The rooms on that system share one thermostat, so a sunny front bedroom and a shaded back one may not land at exactly the same temperature. Attic installations also come with requirements of their own, covered below.
High-velocity small-duct systems
High-velocity AC systems push air at high speed through flexible tubing roughly two inches across, which can be fished through wall and ceiling cavities with far less demolition than standard ducts. The room outlets are small and round, so the system is close to invisible. Because the air leaves the outlet fast, it mixes well with room air, and these systems tend to do a good job of pulling moisture out. The trade-offs: they still need an air handler in the attic or basement, the outlets can make an audible rush of air, the equipment and installation usually cost more than a comparable mini split system, and as a specialty product it's worth asking who will service it down the road.
Full ducted central air
Conventional central air in an old house without ductwork means building a duct system from scratch: supply ducts to every room and a path for return air, which takes real space. The attic can usually reach the top floor and the basement the first floor. The hard part is getting between floors, which means borrowing closet space for chases or opening walls and ceilings. In a house going through a gut renovation, that is very doable. In a finished house with good plaster, it is usually the most expensive and disruptive route. Our comparison of mini splits and central air goes deeper on that choice.
What an old house throws at the installer
Plaster, lath and balloon framing
Plaster cracks and makes dust when it's cut, and it takes care to patch invisibly, so a good plan keeps openings few and puts them where repairs are easy. Balloon framing, common in houses built before about the 1930s, uses wall studs that run unbroken from the sill to the roof framing. That open cavity can help: a line set or a high-velocity tube can sometimes travel from basement to attic inside a wall without opening every floor. It also means a cavity opened on one floor connects to the others, which matters for drafts, pests and fire blocking, so it should be closed up properly afterward.
Line sets and condensate
Every indoor unit needs a refrigerant line set back to the outdoor unit and a drain for the water it pulls out of the air in summer. Line sets often run outside the house in a cover that can be painted to match the siding, or inside through closets and stacked cavities. The drain is often the harder part. Condensate needs a continuous downhill route to a suitable drain or to the outdoors, and where gravity won't do it, a small condensate pump will, at the cost of a little noise and one more part that can fail. Each manufacturer also sets limits on line set length and on the height difference between indoor and outdoor units, which matters in a three-story house.
Attic heat and insulation
Equipment and ducts in an unconditioned attic sit in the hottest air in the house all summer. Ducts up there need to be sealed and well insulated, or they will give back a good share of the cooling before it reaches the rooms. An air handler above a finished ceiling needs a secondary drain pan or an overflow shutoff switch, so a clogged drain doesn't come through the plaster. Installers should also leave the attic insulation intact rather than crushing or pulling it back, and keep a safe path for service. If the attic floor hasn't been air sealed and insulated, doing that first helps both the new cooling and your heating bills.
Electrical service
Older homes often have smaller electrical services and panels with no open spaces. Each outdoor unit typically needs its own 240-volt circuit and a disconnect nearby. Sometimes there's room; sometimes the panel or the service needs an upgrade. That work belongs to a licensed electrician, under a permit with an inspection, and it's worth settling before the equipment is chosen.
Historic exteriors and outdoor units
If your house is in a local historic district, changes to exterior features visible from a public way go to the town's historic district commission for review, and the Massachusetts Historical Commission lists air conditioning units among the structures those commissions review. Exemptions depend on each town's bylaw; window units are a common one. Check with the commission before the outdoor units are placed. Even outside a district, the usual approach is the same: outdoor units on a side or rear wall away from the street, line set covers run along corner boards and painted to match, and units set on stands or brackets above the snow and clear of a roof's drip edge.
The heating bonus
Most mini splits are heat pumps, so the system that cools the house in July can also heat it in January. In a house heated by an old boiler, that changes the math. A cold-climate heat pump can carry spring and fall on its own and much of the winter, leaving the boiler as backup for the coldest days. If the boiler burns oil, a cooling project can be the first step of a larger change; our guide to switching from oil to a heat pump covers what that involves.
Mass Save offers rebates on qualifying heat pumps that displace oil, propane, gas or electric resistance heat. The conditions include an integrated control where the boiler stays in use and an installer from the Mass Save Heat Pump Installer Network, so confirm network membership with any contractor before you sign. The amounts and rules are in our article on Mass Save heat pump rebates.
Zoning and sizing, room by room
Old houses rarely have even loads. A top-floor bedroom under a poorly insulated roof, a sunroom full of glass and a north-facing first-floor den can need very different amounts of cooling for the same floor area. That's why sizing should come from a room-by-room load calculation rather than a rule of thumb per square foot. An oversized unit cools a room quickly, shuts off and leaves it clammy, because it never runs long enough to pull out moisture. An undersized one runs all afternoon and still falls behind on the hottest days.
Zoning follows the doors. One indoor unit in an open living and dining area can cover both, but a bedroom with the door closed at night needs cooling delivered into it. With multi-zone outdoor units, watch the small rooms: a small bedroom unit on a large outdoor unit may cycle on and off in mild weather, and sometimes two smaller systems serve a house better than one big multi-zone unit. Top floors usually need the most capacity per square foot, so plan them first.
If you'd like help working out which rooms need what, our mini split installation page explains how we plan the zones and place each unit, and you'll get the price for the whole system before any work starts.
Sources: Massachusetts Historical Commission, Establishing Local Historic Districts; Mass Save air source heat pump rebates.