What Indiana January Cold Snaps Do to Heat Pumps

NWS Indianapolis recorded a −27°F low in January 1994 — still the all-time record for the metro. January 2025 didn't hit that mark, but Boone County still got 11.5 inches of snow and windchills down to −25°F over a 72-hour stretch. For homeowners with heat pumps, those days are a stress test for a system that was not designed with Indiana's extremes as its primary use case.

Heat pumps have become the default new-construction choice in central Indiana subdivisions — Anson, Walker Farms, Harvest Park — because they're efficient in the 35-to-55°F range that covers most of our shoulder seasons. But January isn't the shoulder season. Here's what actually happens to a heat pump when the temperature tanks.

How a Heat Pump Works in Heating Mode

A heat pump doesn't generate heat the way a furnace does. It moves heat — specifically, it extracts heat energy from outdoor air (even cold air contains usable heat) and transfers it inside using a refrigerant cycle running in reverse of an air conditioner.

The outdoor unit acts as the evaporator in heating mode, absorbing heat from outside air. That energy gets compressed and moved to the indoor coil, which releases it into your home. The system is genuinely efficient down to about 30-35°F because it's moving energy rather than creating it. Below that, physics starts working against it.

Defrost Mode Is Normal — Here's What It Looks Like

When outdoor temperatures drop below 40°F and humidity is moderate, frost accumulates on the outdoor coil. This is normal and expected. Modern heat pumps automatically run a defrost cycle every 30 to 90 minutes to clear that frost.

During defrost, the system temporarily reverses — it runs in cooling mode for 2 to 10 minutes to heat the outdoor coil and melt the ice. What you'll see and hear:

• Steam rising from the outdoor unit (melting frost evaporating — not smoke)
• The outdoor fan may stop
• Airflow from indoor vents may feel slightly cooler or stop briefly
• An audible "thump" or "whoosh" as the reversing valve shifts

None of this is a malfunction. If defrost runs for more than 15 minutes without clearing, or if it never seems to finish, that's a problem.

Real Failures: What Actually Breaks in Indiana Cold

There are three failure modes we see repeatedly after Boone County cold snaps:

Base-Pan Ice Damage

When defrost cycles fall behind — or when the drain holes in the base pan are blocked — ice builds up under the outdoor unit. Thick base ice can lock the fan blades and crack the drain pan itself. It can also force the refrigerant lines into abnormal positions. If your unit is buried in a snow drift and has been there for 24+ hours, base-pan ice damage is a real possibility.

Reversing Valve Stuck

The reversing valve is the solenoid that switches the refrigerant flow direction between heating and cooling mode. In extreme cold, this valve can stick in cooling position, meaning the system runs but blows cold air inside — exactly what you don't need. A stuck reversing valve is a refrigerant-system repair that requires an EPA 608-certified tech. There's no homeowner fix for this one.

Auxiliary Heat Failing

Every heat pump paired with an air handler has an auxiliary heat strip or a gas furnace backup. When the outdoor unit can't keep up, the system is supposed to automatically engage aux heat. If those heat strips are burned out — and we see failed strips frequently in systems where the aux heat runs heavily every winter — you end up with a heat pump that's running but can't maintain setpoint, and no backup to cover the gap.

Outdoor Unit Buried in Snow: Clearance Requirements

Your heat pump needs airflow to extract heat from outside air. Most manufacturers require a minimum of 18 to 24 inches of clearance on all sides and at least 12 inches above the top of the unit. When Boone County gets a 12-inch snowfall followed by drifting, a unit sitting at grade on a slab can be nearly buried.

If your unit is submerged in a drift, turn it to emergency heat at the thermostat and clear the snow by hand — never with a metal shovel against the coil fins. Allow an hour for any accumulated base ice to drain before switching back to heat pump mode. Do not pour hot water on the unit to speed melting. Thermal shock can crack the base pan and damage refrigerant lines.

Balance Point Temperature: Indiana's Is 25–35°F

Every heat pump has a balance point — the outdoor temperature at which the system's heating output exactly matches the heat loss of the home. Below that point, the system loses ground and needs backup heat to maintain setpoint.

For most standard heat pumps installed in central Indiana, the balance point falls between 25°F and 35°F depending on the unit's capacity and the home's insulation. Cold-climate heat pumps (Mitsubishi Hyper-Heat, Carrier Greenspeed) can push that balance point down to 5°F or lower, but those are premium units and not the standard install in most Anson or Walker Farms builds.

What this means practically: when Boone County hits 15°F, your standard heat pump is at roughly 60-70% of its rated capacity, and your aux heat strips are doing real work.

When Aux and Emergency Heat Take Over — and What It Costs

Auxiliary heat on a standard air handler is electric resistance heat strips — the same technology as a toaster, scaled up. They're effective but expensive to run. Electric resistance heat has a coefficient of performance (COP) of 1.0: one unit of electrical energy in, one unit of heat out. A heat pump at 35°F has a COP of roughly 2.5 to 3.0, which is why your electric bill spikes noticeably after a stretch of single-digit temperatures.

If your system is in Emergency Heat mode — meaning you've manually switched to it at the thermostat — the outdoor unit is completely bypassed and you're running 100% on electric strips. Emergency heat is for when the outdoor unit is damaged or inoperable. Running it unnecessarily is expensive. Switch back to normal heat pump mode as soon as the outdoor unit is operational.

When to Call Us

Call (765) 894-0047 when you see any of these:

• Ice has built up above the coil fins or on the refrigerant lines — this is not normal frost
• The outdoor unit is running but the air inside is not warming up after 30+ minutes
• The unit runs in what sounds like a continuous defrost cycle (fan off, running long cycles) without clearing
• Aux or emergency heat has been on for more than 24 hours straight without the outdoor unit recovering
• You hear a constant mechanical grinding or loud humming from the outdoor unit — possible seized fan motor or compressor issue

Diagnostic is $129 flat, applied to the repair. We stock reversing valves, heat strips, defrost control boards, and defrost termination sensors for the common heat pump brands in Boone County.

Frequently Asked Questions