Clement Boston Heat Pumps plans a heat pump for a triple-decker as three homes stacked on one footprint, because that’s what the building is.
Each floor has its own heat, its own bills and often its own owner, and the job works best when the heating does too. This guide covers how systems, outdoor units, rebates and approvals fit the building.
How common are three-deckers here?
Very. An estimated 15,000 triple-deckers were built in Boston and the surrounding cities between 1880 and 1930, nearly all wood frame.
| Place | Share of homes in 3-4 unit buildings |
|---|---|
| Dorchester (by ZIP code) | 35% to 44% |
| Chelsea | 27.3% |
| Somerville | 27.0% |
| Everett | 26.8% |
| Boston overall | 23.5% |
| United States | 4.3% |
Many heat with a boiler per floor: gas in much of Dorchester and Somerville, sometimes oil, sometimes old electric heat. That starting point sets the conversion and the rebate.
One system per floor
Each floor gets its own outdoor unit, its own air handler and its own thermostat, sized from a load calculation for that floor.
Why the floors size differently
The first floor sits over a basement, the middle floor is sheltered above and below, and the top floor sits under the roof. The top floor usually needs the most heat and far more cooling. Sizing each floor separately avoids one big unit that short-cycles in mild weather.
Where the air handlers go
- First floor: the basement, with short duct runs up through the floor.
- Top floor: the attic or the low space under the flat roof, feeding ceiling registers.
- Middle floor: the hard one, usually a closet air handler with ducts in soffits or a small-duct system.
Our ductwork page covers sizing and sealing new runs.
Where the outdoor units go
Three outdoor units need room, drainage and quiet, and a three-decker lot rarely has a big yard.
| Spot | Works when | Watch for |
|---|---|---|
| Back porches | The porch framing can carry the unit | Defrost water dripping onto the porch below |
| Flat main roof | Roof access and structure allow | Roof load, membrane, a crane day |
| Side yard | There’s distance to the lot line | Boston’s 50 dBA night limit at the neighbor’s line |
| Rear yard | The yard is shared or owned by one unit | Condo approval, snow piles |
Stacked porches put units close to bedroom windows, so a quiet variable-speed unit helps. Our snow stand guide covers stands, hoods and defrost drainage.
Rebates for three-deckers
Mass Save counts its whole-home heat pump rebate once per utility account, so separately metered floors each qualify on their own.
What each floor needs
- Cold-climate equipment installed by a contractor in Mass Save’s Heat Pump Installer Network
- Proof of weatherization for that unit
- The old heating system for that unit removed or disconnected
The whole-home rate is $2,650 a ton up to $8,500 for systems installed through 2026, and $2,350 a ton up to $7,500 from January 1, 2027. Income-eligible rebates are higher, and apply to individually metered buildings of one to four units. See Mass Save heat pump rebates.
Electrical service for three meters
Each floor’s heat pump runs from that floor’s panel and meter, so each floor’s service has to carry it.
Older three-deckers often have modest services per unit. Some carry a heat pump with careful backup heat sizing; others need an upgrade. Our typical range for an electrical service upgrade is $3,000 to $6,500 per service. The meter bank is in the basement, so the electrician coordinates all three with the utility.
Taking the old boilers out
Whole-home rebates need each floor’s old heating removed or disconnected, and in a three-decker that usually means three boilers in one basement.
- Gas boilers: removed, with each gas line capped by a licensed gas fitter.
- Oil boilers: the burner out, the fuel line capped, and the tank pumped and removed; in Boston that takes a Boston Fire Department permit.
- Chimney: with no boilers left, the flue can be capped or left alone; it no longer vents anything.
Our typical range for oil tank removal from an accessible basement is $600 to $1,500, plus the $110 Fire Department permit. Clearing the basement also frees room for the first-floor air handler.
Condo three-deckers and permits
Many three-deckers are condos. The porches, roof and yard are usually common area, so the association approves outdoor units and line set routes before permits are pulled.
Boston’s short-form permit rules also matter: a homeowner may pull their own permit only in an owner-occupied one- or two-family home. If you own one floor of a three-decker condo, the work has to be done by a contractor. The condo guide covers board approval in more detail.
What it costs
Each floor is its own system and its own line on the estimate, priced against the range that fits its ducts.
Our typical range for a whole-home heat pump on ducts we can reuse is $15,000 to $28,000 and up; with new ductwork or a small-duct system, $20,000 to $40,000 and up. A floor with radiators and no ducts falls in the second range.
When floors convert together
Shared crane days and permits can be split when floors convert together; the cost guide shows what moves each number.
Planning a three-decker job
Start with all three floors in view, even if only one converts this year.
We measure each floor, check each service and the basement meter bank, and map outdoor spots and line set routes for all three, so the first system doesn’t block the others. The old houses guide covers balloon framing and plaster, which nearly every three-decker has. To start with your building, call us or send the form.