Before anything else: heat pump air in heating mode is supposed to feel cooler than you expect. A furnace blows air at 120–140°F. A heat pump delivers air at roughly 90–100°F — warm enough to heat your house, but noticeably cooler than body temperature on your hand. If your system is running steadily and the house is getting warmer, there may be nothing wrong at all.
That said, there are real problems that produce genuinely cold air in heat mode. Here's how to tell the difference, and what to do about each one.
First: Normal Cold-Air Situations
Two things make a properly-working heat pump blow noticeably cold air for short stretches:
- Defrost cycles. When the outdoor coil freezes over (which happens normally during cold, humid nights), the system reverses into cooling mode for a few minutes to melt the ice. You'll feel cool air from the vents, possibly see steam rising from the outdoor unit. This is normal. It ends in 5 to 15 minutes.
- Low ambient temperatures. Below about 35°F, heat pump efficiency drops. The system still works, but output is reduced. On Jacksonville freeze nights, auxiliary electric heat strips should kick in automatically to make up the difference.
If the air is cold and the house is warming up, let it run. If the air is cold and the house is not warming up, keep reading.
Homeowner Checks Before You Call
Run these in order before picking up the phone — they resolve a meaningful share of "not heating" calls:
- Thermostat set to HEAT, not EM HEAT or COOL. "Emergency Heat" mode bypasses the heat pump entirely and runs only the electric strips. It works, but it draws two to three times the power and will crater your bill. COOL mode in winter is obvious but common — especially after someone "fixed" a thermostat setting.
- Filter. A clogged filter chokes airflow across the evaporator coil. The coil gets too cold, the refrigerant can't absorb heat, and you get weak or cool output. Pull the filter, hold it to a light source. If you can't see through it, replace it.
- Outdoor unit airflow. Check whether the fan on the outdoor unit is spinning. If the unit is iced over completely — solid ice, not just frost — that's beyond a defrost cycle. Turn the system to fan-only for 30 to 60 minutes to let it thaw, then call us if it ices back up.
- Breakers. A tripped breaker on the air handler or condenser can partially disable the system. Reset once. If it trips again, stop — there's a wiring or component issue that needs a tech.
The Reversing Valve
A heat pump has one component a straight AC doesn't: a reversing valve. It's a refrigerant-path switch that tells the system whether it's heating or cooling. When it sticks or fails partially, the system can run in cooling mode even when the thermostat calls for heat. The symptom is unmistakable — the house gets colder, not warmer, no matter how high you set the thermostat, and the outdoor unit is actively running.
A stuck reversing valve isn't a homeowner fix. It's a refrigerant-side component, and diagnosing it requires gauges and a trained eye. If the house is getting colder while the system runs in heat mode, that's the call to make.
Low Refrigerant
Heat pumps move heat by cycling refrigerant between the indoor and outdoor coils. When refrigerant is low — due to a leak, not evaporation — the system can't transfer enough heat. Output drops, runtime increases, and eventually the evaporator coil freezes. You may also notice ice on the refrigerant lines near the air handler.
Low refrigerant isn't a top-off job. Refrigerant doesn't get used up — if it's low, something is leaking. Our techs find the leak before adding charge, or you're back in the same situation in 6 to 12 months.
Auxiliary Heat Not Engaging
On Jacksonville cold snaps — mornings in the low 30s — your heat pump relies on electric auxiliary heat strips to maintain setpoint. If those strips have failed (a burned element, a tripped thermal cutout, or a wiring issue), the heat pump alone can't keep up with a hard freeze. The system runs constantly, the indoor temperature creeps down, and the air from the vents feels cold relative to how hard the system is working.
You can confirm aux heat should be running by checking your thermostat display — most stat screens show "Aux Heat" or "Em Heat" when strips are active. If it shows Aux Heat and the house still isn't warming, a strip element may be out.
"The reversing valve and a failed strip element can look identical from the thermostat: system running, house not warming. The difference comes down to what the gauges read. Don't guess — the wrong diagnosis on a refrigerant job costs real money." — Cory, lead tech
When to Call
Call for a diagnostic when: the house is not warming despite the system running, the outdoor unit is completely iced over and stays that way after a thaw cycle, the house is actually getting colder in heat mode, or the system is short-cycling (running 3 to 4 minutes at a time and shutting off). These are emergency repair situations in a freeze — don't wait them out.
For more on how heat pumps work and what they're rated to handle in Northeast Florida temperatures, the heat pumps page covers equipment specs and what to expect on extreme-cold days. And if your system has been struggling but you haven't kept up with service, read our post on heat pump maintenance in Florida — deferred maintenance causes more cold-air calls than any component failure.
The Short Answer
If the air feels mildly cool but the house is warming, you're fine — that's just how heat pumps work. If the house is cold and not recovering, check the thermostat mode, the filter, and the outdoor unit, in that order. If none of those fix it, there's a refrigerant, reversing valve, or auxiliary heat issue that needs a technician.
Don't spend the night in a cold house trying to diagnose a stuck reversing valve — just call us at (904) 379-8762. A real person picks up 24/7.
