Almost everything written about switching to a heat pump assumes a furnace and a set of ducts: pull the furnace, connect the heat pump to the same ductwork, done. Millions of homes, most of them in the Northeast, don't work that way. They have a boiler in the basement and hot water running to baseboard or radiators, and often no ducts at all.
A heat pump can still heat those houses. But there are three quite different ways to do it, and which one makes sense — and whether it saves you any money — comes down to two numbers you can work out yourself before talking to an installer.
First, Find Out How Much Heat Your House Actually Needs
Everything else depends on your heat load: how many BTUs per hour the house loses on the coldest night of a normal winter. Don't take it from the boiler. Boilers have long been installed far larger than the homes they heat, and sizing a heat pump to match one carries that oversizing forward — a bigger, more expensive unit that runs less efficiently.
The better source is your fuel bills. A year of oil, propane or gas, with the hot-water share removed, is a direct measurement of how much heat left the house over a real winter. Divide by how cold that winter was and you have your heat loss per degree; multiply by your design temperature and you have your load. Our heat pump size calculator does the arithmetic. For a Boston house burning 800 gallons of oil a year, with three people on the boiler for hot water, it comes out around 30,000 BTU/hr — about two and a half tons.
Option 1: Add Mini-Splits, Keep the Boiler
This is the most common route in American hot-water homes, and for good reason. Ductless mini-splits mount on the wall, heat the air in the room directly, and don't care what's happening in the boiler loop. You can put one or several in the main living areas, let them carry the house through most of the season, and leave the boiler and baseboard exactly as they are for the coldest nights, for rooms the heads don't reach, and for hot water.
- Pros: nothing in the existing system changes; you get air conditioning; the boiler stays as a full backup.
- Cons: wall units in every zone you want covered; bedrooms behind closed doors stay on the boiler unless they get their own head; two systems to maintain.
Option 2: An Air-to-Water Heat Pump on Your Existing Baseboard
An air-to-water heat pump makes hot water for the same pipes and baseboard the boiler feeds. The catch is temperature. Baseboard is rated with water averaging around 180°F, because that's what boilers supply. Air-to-water heat pumps run most efficiently at about 120°F and typically top out somewhere around 130–140°F.
At 120°F, a typical fin-tube element gives only about 37% of its rated output — less than the straight-line estimate many people use, because baseboard output falls off faster than the temperature difference. Whether that's enough depends on how much baseboard you have against your real load, and on the weather: a house loses roughly half as much heat at 35°F outside as it does on an 8°F design night.
So the honest answer is usually not "yes" or "no" but a switchover temperature: above it, the heat pump carries the house through the existing baseboard; below it, the boiler takes over. Our baseboard heat pump calculator finds yours from your heat load and the feet of baseboard in the house. With 80 feet of typical baseboard and the 30,000 BTU/hr Boston load above, the switchover lands around 34°F. With 120 feet, it drops to about 19°F, and somewhere around 150 feet the heat pump could carry the design night on its own.
Option 3: Replace the Boiler Entirely
Taking the boiler out works when one of two things is true: you have enough baseboard (or radiators) to heat the house at heat-pump water temperatures, or you've added mini-splits or extra emitters to make up the difference. Homes with radiant floors are the natural fit — radiant floors were designed for low-temperature water to begin with.
If you go this way, size the heat pump to your load at your design temperature, not to its headline rating. The capacity in a heat pump's name is measured at 47°F; what matters is what it delivers at 5°F or your local design temperature, which cold-climate models list in their spec sheets and in NEEP's cold-climate heat pump list. And plan for hot water: if the boiler was also your water heater, a heat pump water heater is the usual replacement.
Will It Actually Save Money?
That depends almost entirely on what you burn now. The useful number is the break-even COP — the efficiency a heat pump has to beat for its heat to cost less than your current fuel's. Using typical Northeast prices from our heating cost by fuel calculator (electricity 25¢/kWh):
| Current heat | Price | Cost per MMBtu delivered | Heat pump wins when COP is above |
|---|---|---|---|
| Propane, 80% efficient | $3.30/gal | $45.08 | 1.6 |
| Oil, 80% efficient | $3.85/gal | $34.75 | 2.1 |
| Oil, 85% efficient | $3.85/gal | $32.70 | 2.2 |
| Natural gas, 80% efficient | $1.95/therm | $24.37 | 3.0 |
| Natural gas, 95% condensing | $1.95/therm | $20.53 | 3.6 |
- Against propane, a heat pump wins almost all the time. NEEP's cold-climate specification requires ducted systems to hold a COP of at least 1.75 even at 5°F — already above propane's break-even.
- Against oil, it wins through most of the season. Heat pump efficiency is highest in mild weather, comfortably above 2.1 through fall and spring, and narrows toward break-even on the coldest nights. That's exactly the pattern a hybrid exploits: heat pump when it's cheaper, boiler when it isn't.
- Against natural gas, savings are slim at Northeast prices. A COP of 3 or more is a high bar on a cold day. The case for a heat pump over gas rests more on getting air conditioning, replacing equipment that's due anyway, or where gas prices go next, than on the heating bill.
Your own prices change all of this — Midwest oil and propane are cheaper, and electricity varies by utility. Plug your actual rates into the heating cost calculator, whose per-MMBtu figures line up with the table above, and check your heat pump model's COP at 17°F and 5°F in its NEEP listing.
Incentives in 2026
The federal 30% heat pump tax credit ended for equipment placed in service after December 31, 2025. Income-qualified households may still get a point-of-sale rebate through their state's HEEHRA program, and many Northeast states run their own heat pump programs through utilities — Massachusetts' Mass Save and New York's Clean Heat program among them — which often require a model from NEEP's cold-climate list. Check dsireusa.org and your utility before you sign anything.