Heat pumps actually work in Vermont. Here's the engineering.

By Adrienne Beaulieu, Founder · NEEP-trained · BPI-Certified · UVM ME '07 · Mitsubishi Diamond Contractor
Published December 2025 · 12-minute read
Cold-climate heat pumps · Engineering · Vermont

A decade ago, the conventional wisdom in Vermont was that heat pumps couldn't handle the winter. That was true of the heat pumps a decade ago. It is no longer true. This essay walks through what actually changed, what the engineering data says, and what we have seen in 840+ Voltline installs since 2019.

What the old objection was.

Before about 2015, residential heat pumps used a single-speed rotary compressor. The compressor ran at a fixed speed, regardless of outdoor temperature. As outdoor temperatures dropped, the refrigerant cycle had to move heat from a colder source against a larger temperature differential, and the single-speed compressor could not move enough refrigerant mass to keep up. Capacity collapsed below about 17°F, and the system relied increasingly on supplemental electric resistance backup — which is to say, the heat pump effectively turned off below 17°F and the building was heated by an electric coil.

That was a fair objection. In Burlington, where the 99% design temperature is −7°F and the coldest nights of the season reach −20°F or below, a system that gave up at 17°F was not really a heating system for Vermont. It was a shoulder-season air conditioner that did some heating in October.

The conventional wisdom from that era persists. It is unfortunate. The engineering changed.

What actually changed: the inverter compressor.

The technical pivot point is the variable-speed inverter compressor. Instead of running at a single fixed speed, the compressor's motor speed is continuously variable, controlled by an inverter drive that modulates frequency to the compressor windings. This is the same technology that has been standard in commercial chillers since the 1990s and that became cost-competitive for residential equipment around 2014.

The variable-speed compressor lets the system match output to demand. At mild outdoor temperatures, the compressor loafs along at low speed. As outdoor temperatures drop and the refrigerant cycle needs to move more heat per unit time, the compressor speeds up. The relevant engineering term is "turn-down ratio" — the variable-speed compressor can run at anywhere from roughly 15% to 110% of nameplate capacity, where the old single-speed compressor was either at 100% or off.

Combine that with flash injection or enhanced vapor injection (EVI) — a refrigerant-cycle modification that improves vapor-phase heat transfer at low outdoor temperatures — and you get a heat pump that maintains rated capacity down to outdoor temperatures the old machines could not approach.

What the published COP curves actually say.

COP — coefficient of performance — is the dimensionless ratio of heating output to electrical input. A COP of 1.0 means the heat pump is delivering as much heat as it consumes in electricity (which is what an electric resistance heater does). A COP of 2.0 means the heat pump is delivering twice as much heat as it consumes. The published COP curves tell you, at a given outdoor temperature, what your real-world efficiency is.

For a Mitsubishi Hyper-Heat MUZ-FH-NA series (the H-spec hyper-heat units), the published curve at 47°F outdoor is around COP 3.5. At 17°F, it drops to COP 2.8. At 5°F, COP 2.4. At −5°F, COP 2.0. At −13°F, COP 1.7. Below that the curve continues but the manufacturer does not publish formal ratings — it depends on the specific unit and the install.

The Fujitsu Halcyon Extra Low Temp series has a similar shape with slightly lower COP at very cold temperatures (a few tenths of a point lower at −13°F) but slightly better capacity retention. The Bosch IDS Premium Connected is rated to −5°F for capacity retention; below that, supplemental strip backup is typically specified.

Capacity retention versus efficiency.

COP tells you the system's efficiency. Capacity retention tells you whether the system can keep up with the heating load. These are related but distinct.

A modern Mitsubishi Hyper-Heat retains 100% of nameplate heating capacity down to 5°F, drops to roughly 90% at −5°F, and roughly 80% at −13°F. As long as that retained capacity equals or exceeds your home's heating load at design temperature, the system covers the building without supplemental heat. Manual J load calculation tells you the load; the manufacturer's capacity curve tells you whether the system covers it.

For most well-insulated Burlington homes, a properly-sized 3-ton Hyper-Heat covers the full load down to the 99% design temperature of −7°F with no supplemental backup. For older or poorly-insulated homes — or for homes in colder pockets like Underhill or Bolton — we configure supplemental electric strip backup that engages below a controller setpoint.

The systems do what the manufacturers say they do. The published cold-climate COP curves are not marketing. They are AHRI-certified test results from the manufacturer's lab. The field data agrees.

What our field data shows.

Voltline has data-logged 47 of our installs in cooperation with Efficiency Vermont since 2022. The data loggers record outdoor temperature, indoor setpoint, energy consumption, and thermal output every five minutes for a full heating season.

Across that data set, the median measured COP at outdoor temperatures between 0°F and 10°F was COP 2.5 — slightly above the published value of COP 2.4. The median measured capacity at the −7°F design temperature was 92% of nameplate. The lowest outdoor temperature any of our installs operated at without engaging supplemental strip backup was −13°F, recorded in Charlotte on January 25, 2022.

Across the 47 systems, supplemental strip backup (where installed) ran for a median of 14 hours per heating season. Strip operation is the exception, not the rule.

What this means for your install.

If your house has a Manual J design load that the cold-climate heat pump's retained capacity can cover at your specific design temperature, the heat pump alone handles the building. Burlington's 99% design temp is −7°F; South Burlington is similar. Shelburne and the lakefront communities are warmer (around −4°F). Underhill and the deep-pocket valleys are colder (down to −15°F). Voltline runs the calc for your specific address.

If the design load exceeds the retained capacity at your design temperature, we configure supplemental electric strip heat. The strip is electric. It runs only when needed. The whole system stays on the electrical service — no fossil fuels.

What this does not mean: that a heat pump is a like-for-like replacement for an oversized oil boiler. Oil boilers are typically oversized by 50–100% versus actual heating load (because they were sized by a rule of thumb based on the home's footprint, not by a Manual J calculation). The right-sized heat pump is smaller than the boiler it replaces, but it covers the same actual load. The Manual J is the engineering basis for the sizing. We do one on every install.

The customer takeaway.

The cold-climate heat-pump technology that exists today is fundamentally different from the heat pumps that earned the technology its bad reputation in cold climates. The variable-speed inverter compressor, enhanced vapor injection, and the AHRI-certified COP curves are real. The field data corroborates the lab data. We have five years of installs across Burlington, Charlotte, Shelburne, Underhill, and the cold pockets, and the systems do what the manufacturers say they do.

If anyone tells you heat pumps don't work in Vermont, ask them what specific model they tested at what specific outdoor temperature, and what their measured COP was. If the answer is "well, my brother had one in the 1990s," you can discount the testimony. The technology changed.

The 840 Voltline installs since 2019 are a working data set. We are happy to put you in touch with several past customers who have specifically run their systems through a −15°F night. They will tell you what their heating bill looked like.

— Adrienne Beaulieu, Founder · Voltline Heat & Cool · Burlington, VT