Adding Central Air to a House With Radiators or Boiler Heat: 4 Options

A home with hot water or steam heat has no ductwork to reuse, so adding cooling means choosing how the cold air will get there. Four routes work: ductless mini splits, a high-velocity small-duct system, conventional ductwork run through an attic and closet chases, or replacing the boiler with a ducted heat pump that heats and cools. Your attic, your basement, your ceiling heights and whether you want to keep the radiators decide which.

On this page:

• Why there is no single answer

• Option 1: ductless mini splits

• Option 2: high-velocity small duct

• Option 3: conventional ducts through chases

• Option 4: replace the heat too

• How to choose, and how it gets sized

• FAQs

Radiators and baseboard deliver heat through water. Air conditioning delivers cooling through air. There is no shared infrastructure between the two, which is why this question is really about where you are willing to put ducts, or whether you are willing to skip them entirely.

None of the four options below is the right answer in general. Each one is the right answer in a specific house.

Why There Is No Single Answer

This is extremely common housing stock in our service area. Philadelphia and Wilmington row homes and twins, Delaware County and Montgomery County colonials, South Jersey ranchers with baseboard, and Baltimore rowhouses were all built to be heated by water and never anticipated cooling.

Window units in the bedrooms and a hot second floor is the default outcome. It does not have to be.

What varies between these houses is not the heating system. It is the access: whether there is a floored attic above the top floor, whether the basement is full or a crawl, how tall the ceilings are, whether the walls are plaster over lath or masonry, and how many closets you are willing to give up. Those details, not the boiler, decide the answer.

Option 1: Ductless Mini Splits

Wall-mounted or ceiling-mounted indoor heads connected by a small refrigerant line set to an outdoor unit, with no ductwork at all.

Best when the house has no attic or basement path for ducts, when rooms are used very differently from one another, or when you want cooling in a few specific spaces rather than everywhere at once.

Trade-offs: the indoor heads are visible and some homeowners dislike how they look. You get one head per zone rather than one per room, so a small back bedroom may share with the hallway. Each head needs a line set route and a condensate route, and in a masonry row home those routes take planning.

Worth knowing: every head also heats. A ductless system is a heat pump, so it can carry the house through mild weather and take real load off the boiler in spring and fall.

Option 2: High-Velocity Small Duct

A compact air handler, usually in an attic or a closet, feeding small-diameter flexible tubing through wall cavities and joist bays to round outlets a few inches across.

Best when you want central air with a conventional look and minimal damage to plaster, and the house has no room for standard trunk-and-branch ductwork. This is the system that made central air practical in historic row homes and twins, and it is the option most homeowners have never heard of.

Trade-offs: a higher installed cost than conventional ducts in houses where conventional ducts actually fit, and somewhat more audible air movement at the outlets because the air is delivered faster through a smaller opening. The tubing still has to reach every room, so a house with awkward interior framing can still fight it.

Worth knowing: the high delivery velocity produces strong mixing, which tends to even out the temperature between floors. In a three-story twin, that is a real comfort advantage.

Option 3: Conventional Ducts Through Attic and Chases

A standard air handler in the attic or basement, with a trunk run through an attic, a closet chase or a built soffit, and supplies dropped or run out to the rooms.

Best when there is a usable attic above the second floor and a basement below the first. That combination allows the two-zone approach: an attic unit serving upstairs and a basement unit serving downstairs, each sized for its own floor. Many suburban colonials in Delaware County and Montgomery County are laid out exactly this way.

Trade-offs: you lose some closet or ceiling space to the chases and soffits, there is more construction and more patching than the other options, and an attic air handler needs proper insulation and a well-planned condensate route so it does not become a ceiling stain later.

Worth knowing: once conventional ducts exist, zoning, filtration and humidity control all become easy to add. Ducts are infrastructure, and they keep paying off.

Option 4: Replace the Heat Too

Once ductwork exists, a ducted heat pump can supply both heating and cooling through it, and the boiler and radiators can be removed or kept as backup.

Best when the boiler is already near the end of its life. You are paying for the duct access either way, and doing both at once avoids paying twice for the same walls to be opened.

Trade-offs: it is the largest project of the four by a wide margin, it usually involves an electrical service review, and the boiler removal adds its own work. And there is an honest non-technical trade-off: some homeowners simply prefer radiator heat, which is steadier and quieter than forced air, and that is a legitimate reason to keep it. 

Worth knowing: keeping the boiler as a backup is a real middle path. The heat pump carries most of the season and the boiler covers the coldest stretches, which also means you are not betting the whole house on one system.

The Four Options Side by Side

Same four options, compared on the things that actually decide it.

Horizon installs all four across Delaware, southeastern Pennsylvania, South Jersey and parts of Maryland, and the first step is always a walkthrough rather than a quote. Call 1-800-642-4419 to book one.

How to Choose, and How It Gets Sized

Start with the building, not the equipment.

Walk the attic, the basement and the closets with a contractor and find out where a duct or a line set can physically go. That one walkthrough eliminates at least one option and usually points clearly at another, and it takes an hour.

Only then talk about equipment, because every one of these four routes is sized the same way and a guess is a guess in all four.

What Size System You Actually Need

Not the size of whatever window units you have now, and not a number derived from square footage. Square footage rules of thumb are why so many homes in this region are cooled by systems that blow cold for fifteen minutes and leave the house damp.

Proper sizing comes from a Manual J load calculation that accounts for insulation levels, window area and orientation, air sealing and infiltration, ceiling heights, duct location and the shading around the house. Two 2,000 square foot homes on the same street can have materially different loads.

Sizing a 1900s Philadelphia Twin

The twin is the clearest example of why the calculation matters. Masonry walls with high thermal mass, tall ceilings, large single-pane or retrofitted windows, one shared party wall with a conditioned neighbor, and frequently no wall insulation at all.

That combination produces a load profile no rule of thumb predicts. The shared wall alone can move the number meaningfully, because a neighbor's conditioned space is not an outdoor surface.

Insulation Changes the Answer

Improving the envelope lowers the load, so a system sized before insulation and air sealing will be oversized afterward.

If attic insulation is on your list, either do it first or tell the contractor it is planned so the calculation reflects the house you are going to have rather than the one you have now.

Staging Beats the Headline Efficiency Number

In a Mid-Atlantic summer, how the system stages matters more than the rating on the sticker. A two-stage or variable-speed system at a moderate efficiency tier will usually leave a house drier and more comfortable than a single-stage unit at a higher rating, because it runs longer at lower capacity and removes more moisture per cycle. 

Buy the staging first, then the efficiency tier your budget supports. Our explainer on SEER, AFUE and HSPF ratings covers what each number measures.

Permits and How Disruptive It Is

Permit requirements for this work are set by your township or city rather than at the state level, and most jurisdictions across Pennsylvania, Delaware, New Jersey and Maryland require a permit and an inspection. Horizon pulls the permit and schedules the inspection.

On disruption, be realistic about which project you are doing. A straightforward equipment changeout where ducts already exist is typically a single day and you do not need to leave the house, with the power and system off for most of the working day.

Adding ductwork to a house that has none is construction, not a changeout. Expect multiple days, drywall and plaster patching afterward, and dust control as part of the plan. A contractor who tells you otherwise has not walked your attic.

Why Choose Horizon Services

A contractor who installs only one of these four options is not neutral about which one your house needs.

Horizon installs ductless systems, high-velocity small-duct systems, conventional central air systems and ducted heat pumps, and we also service the boiler you may be keeping. That means the recommendation can follow the building instead of the truck inventory.

We cover Delaware, southeastern Pennsylvania, South Jersey and parts of Maryland, with free estimates and financing options available on installation.

Call 1-800-642-4419 to schedule a walkthrough.

Adding Central Air to a Radiator-Heated House FAQs

Adding central air to a house with hot water heat, what are my options?

Four options work: ductless mini splits with no ductwork at all, a high-velocity small-duct system using narrow tubing through wall cavities, conventional ducts run through an attic or closet chase, or replacing the boiler with a ducted heat pump once ductwork exists. Your attic and basement access usually decide it.

My house has radiators and window ACs, what are my options for central everything?

If you want one system doing both, you need ducts and a heat pump, or enough ductless heads to cover the house. If you want to keep the radiators, add cooling only, using ductless or high-velocity. Start by finding out where a duct or line set can physically run.

How do ductless mini splits handle both heating and cooling?

A ductless system is a heat pump. It cools by moving heat out of the house and heats by reversing that cycle, and modern cold-climate inverter units hold usable capacity well below freezing in our winters. Each indoor head is controlled separately, so rooms can be conditioned independently.

Sizing a heating and cooling system for a 1900s Philadelphia twin, where do you start?

Start with a Manual J load calculation, not square footage. A twin has masonry walls, tall ceilings, large windows, one shared party wall with a conditioned neighbor and often no wall insulation, and that combination produces a load no rule of thumb predicts. The shared wall alone changes the number.

What size air conditioner do I need for a 2000 square foot house?

Square footage alone cannot answer it. Correct sizing comes from a Manual J calculation covering insulation, window area and orientation, air sealing, ceiling height, duct location and shading, and two 2,000 square foot homes on one street can differ materially. Oversizing leaves a house cold and damp rather than cool and dry.

Does adding attic insulation change what size AC I should buy?

Yes. Insulation and air sealing lower the cooling load, so a system sized before that work will be oversized afterward. If insulation is planned, either complete it first or tell the contractor so the load calculation reflects the improved envelope rather than the current one.