Published by Fairfax Mechanical · fairfaxmechanical.co · July 2026
More Northern Virginia homes are heated by heat pumps than any other system type — and every winter, homeowners call us concerned about behavior that turns out to be completely normal. Here's what's actually happening.
How a Heat Pump Heats Your Home in Winter
A heat pump doesn't generate heat — it moves heat. Even cold outdoor air contains thermal energy. The heat pump extracts that energy using refrigerant, compresses it to raise its temperature, and delivers it inside. For every unit of electricity consumed, a heat pump delivers 2.5–3 units of heating energy — far more efficient than resistance heat strips, which deliver exactly 1 unit per unit of electricity.
As outdoor temperatures drop, available thermal energy decreases and efficiency declines. At the balance point — typically 35–40°F for most Northern Virginia systems — heat pump output exactly matches the home's heating load. Below this, backup resistance heat must supplement.
The "Cold Air" Misconception
Heat pump supply air in heating mode measures 90–95°F. Gas furnace supply air measures 120–140°F. Both are warming the house, but the heat pump air feels lukewarm on your hand compared to furnace air.
This is the most common reason Northern Virginia homeowners call thinking their heat pump "isn't working" in January. The correct test: Is the house reaching and maintaining the thermostat setpoint? If yes — the system is working correctly. The air temperature is not the right diagnostic tool.
When to actually be concerned: the house is consistently 3–5°F below setpoint after running for more than an hour.
Balance Point and Backup Heat — Why Your Winter Bill Is Higher Than Expected
Outdoor Temp | Heat Pump Mode | Backup Heat | Cost |
|---|---|---|---|
Above 45°F | High efficiency | Off | Lowest |
35–45°F | Efficient + some backup | Periodic | Moderate |
20–35°F | Full capacity + backup | Frequent | Higher |
Below 20°F | At limit + heavy backup | Most of the time | Highest |
Northern Virginia winters average 15–30°F overnight from December through February. This is why heat pump electric bills often surprise homeowners — the backup heat strips running on cold nights cost 2.5–3× more per BTU than heat pump output. This is normal system behavior; it's not a malfunction.
AUX Heat vs. EMERGENCY Heat — Know the Difference
AUX heat: Engages automatically when the heat pump alone can't meet demand. The heat pump continues running alongside the strips. This is normal cold-weather operation. When you see "AUX" on the thermostat — do nothing, this is expected.
EMERGENCY heat: Completely disables the heat pump and runs on resistance strips only. Cost is 2.5–3× higher than AUX mode. Use EMERGENCY heat only if the outdoor unit has physically failed. Never switch to EMERGENCY heat just because the house feels cold — instead, leave it on normal heating mode and call for service.
Ice on the Outdoor Unit — Normal vs. Problem
Normal: A thin frost layer that clears every 30–90 minutes. The heat pump automatically reverses refrigerant flow (defrost cycle) to melt it. You may see a brief steam cloud. The fan may stop briefly. This is the defrost system working correctly.
Problem: Thick ice encasing the unit that doesn't clear. Ice on the refrigerant lines. Unit completely covered without defrost cycle engaging. If you see this — switch to EMERGENCY heat (which shuts off the outdoor unit), then call Fairfax Mechanical for defrost system diagnosis.
Warning Signs: Normal vs. Actual Problem
What You See | Normal? | Action |
|---|---|---|
Lukewarm supply air (90–95°F) | ✓ Normal | Check setpoint maintenance |
Thin frost that clears periodically | ✓ Normal | Nothing |
"AUX" on thermostat in cold weather | ✓ Normal | Nothing |
House 3–5°F below setpoint for 60+ min | ⚠ Investigate | Check filter; call for service |
Unit encased in thick ice | ⚠ Problem | Switch to EMERGENCY; call Fairfax Mechanical |
Electric bill dramatically higher than last year | ⚠ Investigate | Refrigerant or efficiency issue likely |
Grinding or banging noise | ✗ Problem | Shut off; call immediately |
Pre-Winter Heat Pump Checklist
September–October is the optimal tune-up window in Northern Virginia — before heating season demand peaks.
Schedule professional heat pump tune-up (includes refrigerant check and defrost testing)
Verify outdoor unit has 18" clearance on all sides
Clear leaf debris from fins and base
Replace air filter
Test EMERGENCY heat mode so you know where the setting is
Never cover the outdoor unit itself — this prevents proper airflow and causes ice damage
Fairfax Mechanical provides heat pump pre-winter tune-ups throughout Northern Virginia. Dominion Energy participating contractor. Schedule at fairfaxmechanical.co/contact.
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