Heat pump and existing radiators or boilers: keep the piping, or don't

The boiler is loud, the oil tank is a question, and the person in the driveway says a heat pump will “just tie into the radiators.” That sentence hides three different jobs. One is an air-to-water heat pump that heats the same water your radiators already use. One is wall units (mini-splits) that heat and cool the rooms while the boiler stays or gets retired. One is a new boiler, same piping, same 180°F habit. Those prices are not neighbors, and mixing steam, hot-water radiators, and fin-tube baseboard into one “hydronic” bid is how you buy a January where the living room never catches up.

If you have ducts and a furnace, you are on a different page: heat pump vs furnace is the climate decision, and dual fuel is the keep-the-furnace version. Stay here when the heat lives in pipes.

What are you actually buying?

An air-to-water heat pump is an outdoor box that heats (and, on many models, chills) water instead of blowing air. The Department of Energy lists it as a ductless option that sends hot water through radiant floors or radiators. Cooling, when it exists, usually needs a fan coil with a drain, because radiators are not meant to sweat.

DOE: heat pump systems

A mini-split (or a ducted air-source heat pump, if you add ducts) heats air. It does not care that you have radiators, except that you now have two heating systems in the same rooms unless you shut one off.

A boiler replacement keeps the fuel you already buy. HomeGuide’s 2026 boiler guide puts a typical install at $3,200 to $9,000, with the unit itself $1,400 to $6,500 and labor $1,200 to $3,200. Standard-efficiency replacement is $2,200 to $7,000. High-efficiency (AFUE 90% and up) is $4,000 to $10,000. A gas boiler in that table is $4,000 to $9,000. Oil is $3,300 to $7,200 standard and $6,300 to $9,000 high-efficiency, plus an oil tank at $1,000 to $6,200 if that tank is part of the job.

HomeGuide: boiler replacement cost

EnergySage’s 2026 marketplace averages are for air-to-air equipment, not boiler swaps: $15,393 across every type they see, $14,529 ducted, $25,957 whole-home ductless, $14,353 hybrid (heat pump plus furnace), before incentives. After the state and utility programs in their table, hybrids drop to $10,695. Those “after” numbers are local money, not a federal credit.

EnergySage: heat pump cost

Air-to-water is a smaller US product, so it will not land on EnergySage’s average by accident. Treat a bid that matches the $3,200 to $9,000 boiler band as a boiler, and a bid that matches the $25,957 ductless average as wall units, until the quote names water temperature, emitters, and whether cooling is included.

The IRS Energy Efficient Home Improvement Credit (25C) covered qualifying heat pumps placed in service through December 31, 2025, at 30% with a $2,000 annual cap. A 2026 install does not get that subtraction. Qualifying boilers were on a different, smaller line while the credit lasted. Do not let last year’s brochure knock $2,000 off either job.

IRS: Energy Efficient Home Improvement Credit

Why does the water temperature decide the job?

A radiator throws heat because the water inside is hotter than the room. Old boilers were sized to send 180°F (sometimes hotter) through relatively small iron or a short run of fin-tube. Most air-to-water heat pumps are happiest much cooler than that.

Caleffi’s hydronic design note is the number to put on the quote: distribution that can meet the heating load with supply water no higher than 120°F lets most current air-to-water units run well, and cooler than that is better. A panel radiator at 110°F average water, in a 70°F room, puts out about 27% of the heat it would at 180°F. That is physics, not a sales objection. Larger panels, extra emitters, or a smaller heating load (insulation, air sealing) are how you make up the missing 73%.

Caleffi: heat emitter options for air-to-water heat pumps

Traditional fin-tube baseboard was built for 180°F to 200°F. Caleffi’s line is that this product is not a good match for air-to-water as-is. Low-temperature fin-tube with more fin area exists; one example they publish is 272 Btu/hr per foot at 90°F entering water and 532 Btu/hr per foot at 120°F. Your existing skinny baseboard is probably not that product.

Cast iron is the exception that sometimes works. If those radiators were sized for a leaky, uninsulated house, and you later added attic insulation, new windows, or air sealing, they may already be oversized for today’s load. Oversized iron can run cooler and still heat the room. That only counts if someone measures the radiators and the load. “They’re big, so they’re fine” is not a load calculation (Manual J, the written heating and cooling load for this house).

Radiant floors are the easy hydronic case. Close tube spacing and a thin floor covering can run in the 95°F to 120°F band Caleffi describes. If you already have a slab on those terms, air-to-water is often the honest reuse of the piping.

High-temperature air-to-water machines exist. Daikin’s Altherma 3 H HT is sold as a boiler-replacement heat pump with leaving water up to 70°C (158°F) on heat-pump operation down to -15°C outdoor, and an operating range down to -28°C. That is how a contractor can claim you will not resize the radiators. It is also how you run the outdoor unit harder, because hotter water is more lift. Ask for capacity and efficiency at your design outdoor temperature and at the leaving-water temperature the radiators actually need, not only the brochure’s 70°C ceiling.

Daikin: Altherma 3 H HT

Chiltrix, a US air-to-water line, publishes heating of 33.81 kBtu/h at COP 3.92 with 95°F water, and installation notes that radiant is normally around 95°F leaving, fan coils around 105°F, with a supported leaving temperature around 120°F (target 111°F). If Bid A assumes 180°F radiators and Bid B assumes Chiltrix-class 95°F to 120°F water, they are not the same machine.

Chiltrix: CX34

Cooling does not come for free on the radiators. Caleffi’s warning is that chilled water in the 45°F to 60°F range will condense on panels, fin-tube, and radiant surfaces. Fan coils with a drip pan are the cooling emitter. If the bid is “heat pump, keep the radiators, and you get AC,” find the fan coils or you found a summer without cooling.

Is this steam, or hot water?

Look at the radiators. Steam units have a single pipe (or a two-pipe steam layout) and vents that hiss. Hot-water radiators have supply and return and stay full of water. HomeGuide prices a hot-water boiler unit at $1,200 to $4,000 and a steam boiler unit at $1,800 to $6,000. Steam runs at boiling. An air-to-water heat pump that tops out near 120°F to 158°F water is not a steam boiler.

HomeGuide: boiler replacement cost

On a steam house, the honest heat-pump jobs are usually wall units (or a ducted air handler if you add ducts), with the steam left as backup or retired. Converting steam to hot water is a piping project, not a condenser swap. If the quote says “heat pump on your steam radiators” with no conversion scope, keep shopping.

What do the three jobs cost when you put them next to each other?

Job 1: new boiler, keep the piping. HomeGuide’s $3,200 to $9,000 typical, $4,000 to $10,000 high-efficiency. Removal is $200 to $500. Permits $50 to $250. A written load is $200 to $500. New near-boiler piping, if the old near-boiler piping is a spaghetti of air and sludge, is $450 to $1,800. Zoning, if they sell you room-by-room control, is $1,700 to $4,500. Oil tank work, if the tank is done, stacks $1,000 to $6,200 for a new tank or $400 to $3,400 to pull the old one.

That job does not cool the house. If you also want summer cooling, you are buying a second system.

Job 2: mini-splits (or a new ducted heat pump), boiler stays or goes. EnergySage’s whole-home ductless average is $25,957 before incentives. Ducted air-to-air is $14,529 if you have or add ducts. This is the path that actually puts AC in a radiator house without fan coils on the hydronic loop. Ducted vs ductless and mini-split for one room vs whole house are the layout versions. You can keep the boiler as hydronic dual fuel: heat pump above a written outdoor cutoff, boiler below it, same idea as keeping a furnace. Write the cutoff. A boiler that still fires every time a radiator thermostat calls is not dual fuel.

Job 3: air-to-water on the existing loop. Price this as outdoor unit, indoor hydro box or buffer tank, glycol if the outdoor heat exchanger needs freeze protection, pumps, controls, and a radiator-by-radiator (or baseboard-by-room) output check at the leaving-water temperature the unit can hold on your coldest night. If that check fails, HomeGuide’s radiator replacement is $700 to $1,300 installed per unit, or $7,000 to $15,000 to replace 10 to 15 units in a house. New hydronic radiators are $350 to $1,700 installed each in their heat-source table. Swapping every radiator is how an “keep the piping” bid becomes a remodel.

HomeGuide: radiator replacement cost

If Job 3 is only a few thousand above Job 1, someone left the buffer tank, the electrical circuit, or the emitter math off the page. If Job 3 is $20,000 above Job 2, find whether you are paying for new panel radiators and wall units.

A 100-amp panel plus a heat pump is the 200-amp upgrade conversation, same as any other outdoor unit. Weatherization still pays before you oversize the machine. Weatherize before the heat pump is the envelope order, and it is how oversized old iron becomes usable at 120°F.

What should three real-looking bids mean?

Bid A, boiler only. New condensing boiler, same radiators, outdoor reset maybe, no cooling. HomeGuide-shaped money. Accept this when the emitters need 160°F+ water, the house is steam, or you are not ready to electrify heat. Reject the version that calls itself a heat-pump plan because the brochure mentioned hybrid.

Bid B, mini-splits, boiler as backup. Indoor heads (or a ducted air handler), outdoor unit, written switchover, boiler inspected. EnergySage ductless or hybrid neighborhood. This is often the cleaner 2026 path in a steam house, or in a fin-tube house that would need every baseboard replaced to run at 120°F.

Bid C, air-to-water, keep the radiators. Leaving-water temperature on the page, capacity at your design outdoor temperature at that water temperature, each room’s emitter output at that temperature, buffer tank volume, freeze protection, and a yes/no on cooling fan coils. Daikin-class 158°F water can make Bid C honest without touching the iron. Chiltrix-class 95°F to 120°F water is honest on radiant and on oversized cast iron after weatherization. It is not honest on short 1980s baseboard unless they add emitters.

You cannot compare A to C until C has water temperature. You cannot compare B to C until you know whether C includes summer cooling.

Failure modes that show up on the second January

The radiators were sized for 180°F and nobody derated them. Rooms at the end of the loop never catch up. The outdoor unit runs wide open, then the installer “fixes” it with electric strips or by firing the old boiler every night.

It was steam. Two-pipe hot-water assumptions on one-pipe steam is a flooded, hammering mess. Conversion has to be in the scope.

Cooling was implied. Radiators do not dehumidify. Without fan coils (or a separate AC or mini-splits), you bought heat.

The high-temp heat pump was sized on cooling stickers. Reading SEER2, HSPF2, and EER2 still applies to air-to-air. For air-to-water, the number that matters is capacity at your winter outdoor design and at the leaving-water temperature the radiators need. Cold-climate spec sheets are the air-to-air version of that question.

Sludge and leaky valves. Old iron holds rust. A heat pump with a plate heat exchanger is less forgiving than a cast-iron boiler. Flush, dirt separator, and new valves belong on the bid, or they belong on a change order in week three.

The oil tank was “later.” HomeGuide’s tank line is large enough to flip the comparison. If the tank is failing, put it on this bid or exclude it in writing.

A home warranty was taped to the heat-pump brochure. Read what the contract excludes before you treat that as coverage for a mixed hydronic system.

What has to be on the quote

Three bids, same written scope. Same rooms, same emitters kept or replaced, cooling yes or no, boiler kept or removed.

Ask for, in writing:

1. Name the distribution. Steam, hot-water radiators, fin-tube, radiant slab, or mixed. Mixed is two jobs.
2. Show the heating load for this house, room by room, not square-footage rules of thumb. HomeGuide prices that load calc at $200 to $500.
3. Show leaving-water temperature at design outdoor temperature, and each room’s emitter output at that temperature. Caleffi’s 120°F guideline is the default test unless they are selling a documented high-temp unit.
4. Show the outdoor unit’s capacity at that pair of temperatures, not only at 47°F and 95°F water.
5. Show cooling. Fan coils with condensate, mini-splits, or “heating only” in those words.
6. Show the boiler’s fate. Removed ($200 to $500 in HomeGuide), abandoned in place, or kept with a written outdoor cutoff.
7. Show electrical. Breaker, wire, disconnect, and whether a panel upgrade is included or excluded.
8. Show freeze protection, buffer tank, dirt separator, and glycol if the design needs them.
9. Show radiator work as a line, not a vibe. HomeGuide $700 to $1,300 per replaced unit.
10. Show who files which rebate, and a 2026 date. Do not bake in 25C.

The heat pump quote checklist is the air-to-air line-by-line. Use this list when the heat is in pipes.

How do I decide this week?

1. Walk the basement and name the system: steam, hot water, or a mix, and whether you have iron radiators, fin-tube, or a slab.
2. Decide whether you need cooling in these rooms. If you do, radiators-only air-to-water is incomplete.
3. Get a room-by-room load and an emitter check at 120°F (or at the high-temp unit’s real leaving water) before anyone pulls the boiler.
4. Put the same rooms on three bids: boiler-only, mini-splits with the boiler as backup, and air-to-water with the emitter list attached.
5. If Bid C needs a houseful of new radiators at $7,000 to $15,000, compare that stack to whole-home mini-splits at EnergySage’s $25,957 average and stop pretending C is a cheap boiler swap.
6. If the house is steam, treat air-to-water-on-the-existing-mains as incomplete until conversion is a line item, and price wall units as the heat-pump job.

Keep the piping when the water can be cooler, or when a documented high-temp unit still heats every room on your coldest night. Replace the boiler when you want the same radiators and the same fuel. Add wall units when you want cooling and the emitters will not run at heat-pump temperatures. Write that choice on the quote, then hire the job you actually have.