Ductless mini-split or high-velocity central air for a house without ducts

Why a radiator house needs a different kind of central air

The Philadelphia Inquirer, reporting Census figures, found that most homes in the city were built before 1950. A large share of houses from that era were designed for radiators, not ducts. They have plaster walls, tight stairwells, and nowhere obvious to hide the bulky ductwork a standard central air system needs.

Window units are the usual stopgap. They are noisy, block light, and do little for the rooms they are not in. The two real alternatives are ductless mini-splits, which skip ducts entirely, and high-velocity systems, which use ducts small enough to thread through walls and floors with far less opening up. Both leave the radiators in place for heat if you want to keep them.

How a ductless mini-split cools one room at a time

A ductless system has an outdoor unit connected to one or more indoor heads by a line set: two refrigerant lines, a control cable and a condensate drain bundled together. Most heads mount high on a wall, though ceiling and low-wall styles exist, and each one cools or heats the room it is in. One outdoor unit can serve a single room or, as a multi-zone system, several.

Because each head has its own control, you cool the rooms you use and leave the rest alone. That makes ductless a natural fit for a hot third floor, an addition or a finished basement. Each head needs one small hole through the wall for its line set, and the condensate needs a path to drain, by gravity or through a small pump.

Ductless units are heat pumps, so they heat as well as cool. That lets them carry the house through mild spring and fall days while the boiler handles the coldest weather, if you choose to run it that way. More on ductless installation.

If you have tried living with window units, you already know the limits: one room cool, the hallway hot, and an unpleasant job hauling them in and out every year.

How high-velocity central air fits inside plaster walls

A high-velocity system works like conventional central air with much smaller ducts. An air handler goes in an attic, a closet or the basement, and air moves at higher speed through flexible tubing a few inches across. That tubing snakes through wall cavities, closets and floor spaces, usually with only small openings in the plaster.

In the rooms, all you see are small round outlets in the ceiling or wall, which tend to disappear into older trim and molding. The whole house is cooled from one system and one thermostat, though zoning can be added. These systems are also good at pulling moisture out of the air, which counts for a lot in a humid Philadelphia summer.

Heating depends on how the system is set up. Paired with a heat pump outside, it heats as well as cools. Some installs add a hot-water coil fed by the existing boiler. Others are cooling only, with the radiators carrying the winter. See high-velocity central air for how an install comes together.

Expect the install to take longer than a ductless job of similar size, mostly because of the time spent finding and opening tubing routes. The air handler also needs a condensate drain and its own electrical circuit, which affects where it can go.

Ductless versus high-velocity, point by point

Neither system wins across the board. They trade off in predictable ways.

For a house where only one or two floors are a problem, ductless usually means less disruption and a shorter install. For an owner who wants the whole house cooled evenly with nothing hanging on the walls, high-velocity usually comes out ahead, provided there is somewhere to put the air handler.

• Looks: ductless heads are visible on the wall; high-velocity outlets are small and round.
• Control: ductless gives each room its own setting; high-velocity is usually one zone.
• Install: ductless means little work inside walls; high-velocity needs tubing routes and an air handler spot.
• Sound: ductless heads run quietly; high-velocity outlets make a soft rush of air.
• Upkeep: ductless filters need regular washing; high-velocity has one filter at the air handler.

Where the outdoor unit goes, and why it matters in Girard Estate

Both systems need an outdoor unit, and on a rowhome that usually means a flat roof, a rear yard or a back wall. Line sets have to run from there to the equipment inside, and on a block of attached houses with party walls on both sides, the route usually goes out the back or up through the roof.

Girard Estate is a designated City of Philadelphia historic district. If your house is there, or in any other historic district, check with the city about what applies to exterior equipment before you settle on where the condenser and line set will go. It is far easier to plan the route around that than to move equipment after it is installed.

When a line set or a condenser stand goes through a flat roof, the penetration has to be sealed and flashed correctly. Done carelessly, a cooling upgrade turns into a roof leak over the top floor, so ask who handles the roof work and how.

Questions that decide it for most homeowners

Start with the rooms. If the trouble is one hot floor and the rest of the house is fine with fans, a single-zone or small multi-zone ductless system is the direct fix. If you want every room cooled, think about where an air handler could live: a roof space, a top-floor closet, or the basement with a route up through the house.

Then think about the walls. Owners who would rather not see equipment on the plaster, or who have rooms with ornate trim, often lean toward high-velocity. Owners who want to avoid opening walls at all lean ductless. And consider heat: if the boiler is aging, a ductless heat pump or a high-velocity system on a heat pump gives you a second heat source.

If you go high-velocity and the floors of the house run very differently, ask whether zoning should be part of the install from the start.

What a site visit should cover before you choose

A tech who installs both should walk the whole house with you before recommending either one. Expect them to look at the following.

The recommendation should come with a load calculation, not a guess based on square footage. If the visit ends with a price but no explanation of where everything goes, ask for the layout before you agree.

• Each room's size, windows and sun exposure
• Possible air handler spots and tubing routes
• Where outdoor units and line sets can go
• Whether the electrical panel can take new equipment
• Which rooms you actually use, and when