Heat Pump Blowing Cool Air in Londonderry, NH

Last checked: August 6, 2026  •  Capacity and efficiency figures on this page come from manufacturer published performance data and from the U.S. Department of Energy's Cold Climate Heat Pump Challenge. Londonderry HVAC is a referral service that matches homeowners with licensed, insured local contractors; it does not test equipment.

The Air Is Cooler Than Your Hand. That Is the Entire Problem.

A heat pump moving heat into your house sends supply air out at roughly 85 to 100°F. A propane or natural gas furnace sends it out at 120 to 140°F. Your palm sits at about 98.6°F. So air that is genuinely twenty or thirty degrees warmer than your living room lands on your hand as cool, sometimes as cold, even while the thermostat is climbing toward setpoint exactly as designed. This is the single most common false service call in heat-pump neighborhoods, and Londonderry now has a lot of heat-pump neighborhoods.

The clustering is the local wrinkle. Streets here were built in waves - the 1980s subdivisions, the 1990s cul-de-sacs, the 2000s colonials off Route 102 - and when systems on a street age out together, replacements happen together too. Half a cul-de-sac converts from propane inside two or three winters, and then the same question arrives from four houses in the same week. Past the gas lines, where the fill-up bill has been setting the thermostat for years, that conversion is often the first time a household has ever felt heat that isn't scalding coming out of a vent.

None of that means a cool register is always fine. Sometimes it is a real fault, and a few of those faults are quietly expensive. This page separates the two: what is normal physics, what current cold-climate equipment actually delivers in a New Hampshire January, and the specific symptoms that mean it is time to get a licensed contractor out to the house.

Why 95-Degree Air Feels Cold

A furnace makes heat by burning fuel, which is why its output is so hot and so obvious. A heat pump doesn't make heat at all - it collects heat that already exists in the outdoor air and moves it inside, which produces a larger volume of gentler air rather than a short blast of hot air. The house ends up at the same temperature; it just gets there on a longer, quieter runtime. Two checks settle the question in about ten minutes without a service call: put a thermometer in a supply register and read it against your room temperature, and watch whether the thermostat actually holds its setpoint over an hour. If the register reads meaningfully above room temperature and the house is holding, the equipment is doing its job, and your hand is simply a poor instrument for the task.

The Defrost Cycle - Normal, and Alarming to Watch

• Every 30 to 90 minutes in cold, damp weather - the outdoor coil accumulates frost, and the system briefly reverses to melt it off
• Five to ten minutes per cycle - short, scheduled, and self-ending; it is not the system struggling
• Steam or vapor rolling off the outdoor unit - melted frost hitting cold air, and easily the most common reason a neighbor knocks on your door to say your unit is smoking
• A whoosh at changeover - the reversing valve shifting direction, normal on every heat pump ever built
• Genuinely cooler indoor air for a few minutes - during defrost the indoor supply really does drop, which is why so many homeowners happen to check the register at exactly the wrong moment
• Light frost on the coil between cycles - expected in a wet New Hampshire cold snap; a coil buried in solid ice is a different story, covered further down

What Cold-Climate Units Actually Produce When It Is 5°F

Vague reassurance is worthless here, so here are published numbers. Manufacturer performance data for a Mitsubishi MUZ-FH24NA shows 27,400 BTU/h at 5°F with a COP of 2.4, and the unit still delivering 18,000 BTU/h at -13°F. Broader than any one model, units qualified under the DOE's Cold Climate Heat Pump Challenge are built for efficient operation down to -15°F while retaining more than 70% of rated capacity, with COP generally in the 2.0 to 3.5 range. At the bottom of that envelope, -15°F, COP falls to about 1.2 to 1.5. That is the honest floor, and it is worth reading correctly: a COP of 1.3 means the system is still delivering more heat than the electricity it consumes, so even at its worst it is beating electric resistance heat rather than failing. For reference, Londonderry spends only a handful of hours per winter below that mark.

Be Honest: Not Every Heat Pump Is a Cold-Climate Heat Pump

• Standard heat pumps drop to 45-60% of rated capacity at 5°F - real equipment, sold in volume, and it genuinely does run out of room on a New Hampshire cold snap
• True cold-climate models hold 70-80% at the same temperature - the gap between those two lines is the whole argument, and it is decided by which box is sitting outside your house
• Age is a fair proxy - the inverter-driven equipment that made cold-climate performance routine is a relatively recent arrival, and an older unit installed as a shoulder-season machine was never asked to carry January
• Read the nameplate, not the brochure - the model number on the outdoor unit leads to a published extended-capacity table showing output at 17°F, 5°F, and below; that table is the actual answer for your house
• An undersized cold-climate unit behaves like a standard one - correct technology, wrong capacity, same disappointed homeowner in February

Auxiliary Heat: What It Is, and Why Your Bill Notices

Auxiliary or emergency heat is the backup the system leans on when the heat pump alone can't hold setpoint - electric resistance strips in the air handler on most installs, or the existing propane furnace on the dual-fuel setups that are common in Londonderry's tank-fed streets. Electric strips convert electricity to heat at a ratio of one to one. The heat pump they are backing up is running at two to three times that. So every hour AUX carries the house is an hour billed at roughly double or triple the cost of the same comfort from the compressor. Brief AUX operation during defrost and on the coldest mornings of the year is normal and intended. AUX running steadily at 25 or 30°F is not, and it is a leading cause of the winter electric bill that arrives looking like a typo - the usual culprits being refrigerant charge, a failed outdoor sensor, a thermostat configured with the wrong balance point, or a defrost board that has quit cycling properly. All four are diagnosable in a single visit, and all four keep costing money until someone looks.

Sizing, and Why Rules of Thumb Deserve Suspicion

Capacity is sold in tons, where one ton equals 12,000 BTU. The common cold-climate rule of thumb runs 30 to 35 BTU per square foot, while some guides push 50 to 60 for poorly insulated housing stock - a range wide enough to describe two entirely different pieces of equipment for the same floor plan. That spread is the point. A tight 2015 colonial and a 1987 raised ranch with original insulation and a bonus room over the garage do not share a heating load just because they share a square footage. The honest answer is a Manual J load calculation, run on your house, accounting for insulation, windows, orientation, air sealing, and the ductwork actually in place. Nearly every heat pump that disappoints its owner in a New Hampshire January was sized by arithmetic instead of by measurement.

When It Actually Is Broken

• The outdoor unit is encased in solid ice - not frosted, not steaming, but locked in a block of ice that defrost is clearly not clearing
• Continuous defrost that never completes - the system cycling into defrost and staying there instead of returning to heating
• Air that is genuinely cold - at or below room temperature on a thermometer, which is a different finding than air that merely feels cool on your hand
• Running constantly and losing ground on a moderate day - a system that can't hold setpoint at 35 or 40°F has a real problem, whatever it does at zero
• AUX or emergency heat running constantly at mild temperatures - the expensive failure, quietly running up the bill while the house still feels fine
• Ice blocking airflow through the coil, or persisting for hours - restricted airflow makes every other symptom on this list worse the longer it sits

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