Can a Heat Pump Work With My Radiators?

If you live in an older Long Island house with radiators or baseboards, you have probably been told by at least one contractor that a heat pump will not work for you. Until recently, that was basically true. The story has changed in 2026, and the change is big enough that it is worth understanding before you replace your boiler.

Here is the honest answer to whether a heat pump can work with your existing radiators, why almost every cold climate heat pump on the market falls short, and what is actually different about the air-to-water heat pump that just hit the Long Island market.

Why Most Heat Pumps Can't Drive Radiators

A radiator works by sending hot water through it. The water comes out of your boiler at somewhere between 150 and 180 degrees, depending on how cold it is outside and how your system is set up. That high water temperature is what lets a relatively small piece of cast iron throw enough heat into a room to keep it comfortable.

The standard air source heat pump that goes in most Long Island homes is built to make warm air, not hot water. The few cold climate heat pump systems that do make hot water (sometimes called air to water heat pumps) have historically topped out around 120 degrees of flow temperature. That is plenty for underfloor heating, where the entire floor is the heat emitter and the surface area is enormous. It works fine for fan coils, which use a small radiator and a blower to move the heat around. But 120 degree water hitting a cast iron radiator that was sized for 170 degree water just does not produce enough heat. Your house gets cold.

This is why for most of the last decade, the standard advice for homeowners with hydronic heat has been: if you want a heat pump, you also need new radiators (much larger panel radiators sized for the lower flow temperature), or you need to tear up your floors for radiant, or you need to convert the whole hot water distribution system to ductwork. All of those are real projects on top of the heat pump itself.

What Changed in 2026

A new generation of heat pump technology has finally produced a unit that hits the same flow temperature as a boiler. The Daikin Altherma 3 H HT is a high temperature air source heat pump that puts out water up to 158 degrees, down to 5 degrees ambient outside, all on the heat pump alone with no electric backup needed. It keeps producing usable hot water at 150 degrees even at 18 below zero. Suffolk County has never been close to that cold.

That number, 158 degrees, is the number that matters. It is hot enough to drive your existing radiators or baseboards without resizing them. You do not have to swap your cast iron radiators for larger panel radiators. You do not have to rip out the floor. You keep the heat emitters you already have and you lose the boiler.

The system uses an outdoor unit, a wall mounted indoor box about the size of a combi boiler, and a separate hot water tank. One outdoor unit replaces your boiler, your air conditioner, and your water heater. The same machine that heats your radiators in February cools your house through fan coils or a hydronic air handler in August, and makes your domestic hot water year round. All electric, no fossil fuel in the house at all.

What Kind of Radiators Will Work

Pretty much all of them. Cast iron radiators are the classic Long Island case, and yes they work fine. Steel panel radiators work. Hydronic baseboard works. Existing fin tube baseboard works.

What changes is the efficiency, not the comfort. A heat pump pushing 150 degree water into cast iron radiators is operating at a coefficient of performance (COP) of around 2, which means roughly two units of heat for every one unit of electricity. That is still meaningfully cheaper than oil at current prices on Long Island, and it does not require any retrofit work on the radiators themselves.

If you are willing to mix and match, the same heat pump can also run a few of the more efficient emitter types in different zones at lower flow temperatures. Underfloor heating in a finished basement gets to a COP of 5 or 6. Fan coil convectors in bedrooms run around COP 3 to 4. The system can drive zones at different temperatures simultaneously, so you do not have to pick one. Existing radiators on the main floor, radiant in the basement, convectors upstairs, all on one outdoor unit.

What This Actually Looks Like in a Long Island House

The retrofit is simpler than most people expect. The hydrobox bolts to the wall where your old boiler was. Your existing radiator piping connects to the hydrobox. The outdoor unit sits where your central AC condenser sits today (or where it would sit if you do not have central air yet). The hot water tank goes wherever your old water heater is.

Most of the existing radiator piping in older homes is fine to reuse, especially if it was a forced hot water system rather than steam. Steam radiators are a different story and need to be converted to hot water, but on a steam system most of the iron is reusable. The compressor sound at the outdoor unit is about 38 dBA in normal mode and 35 dBA in low sound mode, measured at 10 feet. For comparison, a typical AC condenser is around 60. You can stand next to it during a phone call.

Cold climate performance is the part people doubt most, and the part where this product separates itself from anything else available. At 5 degrees outside, the Altherma puts out around 40,000 BTU on the larger unit. The next best brand on the market in the same category puts out around 18,000 BTU at that temperature. That difference is the difference between a heat pump that quietly carries the whole house through a Long Island January and one that quietly hands the load off to backup electric strips.

The Rebate Picture in 2026

Worth being honest about this. The federal 25C tax credit that used to cover heat pumps expired on December 31, 2025 and is not available for new systems installed in 2026 or later. That changes the math for any heat pump conversion this year. The state and utility programs are still active though, and on Long Island they are the bigger numbers anyway.

The Altherma qualifies as a cold climate heat pump under the NYSERDA Clean Heat program and PSEG Long Island's residential heat pump rebate. It is also a heat pump water heater, which has its own separate PSEG rebate. The open question that even Daikin has not pinned down yet is whether the same machine can stack incentives in both categories on one install, since it is doing both jobs at once. That could meaningfully change the total. We are tracking it and we will tell you exactly what stacks on your specific project before you sign anything.

Is This The Right Move For Your House?

If you have radiators and an aging oil or gas boiler, this is the first time in a long time that a real heat pump conversion has actually been on the table for you. We are one of the few Long Island contractors authorized to install the new Altherma at launch, and we are happy to walk through what it would actually look like in your house, what your stack of incentives lands at, and whether it is the right move for your situation or whether something else makes more sense.

If you want the full breakdown of the system itself, we wrote a whole page on it: the Altherma hub covers what is inside the system, how cold weather performance actually plays out here, and what the project looks like start to finish. For the broader rebate landscape, rebates and savings has every program we currently see homeowners stacking on a heat pump install in Suffolk County.

Or just give us a call or text at 631-209-7090. We will come look at what you have, run the numbers, and tell you straight whether this is the right swap for your house. No pressure, no oversell. If a different system would actually serve you better, we will tell you that too.