Heat pump not heating in cold weather — Washington, DC, Maryland, and Northern Virginia
Heat pumps are increasingly common in DMV homes, but cold-weather performance concerns are the most frequent question homeowners ask. This guide explains why your heat pump may not be heating effectively and what to do.
How heat pumps work in cold weather
Heat pumps extract heat from outdoor air and move it indoors. Even in cold weather, there is heat energy in the air. Modern cold-climate heat pumps can extract usable heat well below 0°F.
However, as outdoor temperature drops, heat pump efficiency and capacity decrease. This is normal and expected. The key question is whether the system is performing within design parameters.
Normal cold-weather behavior
Defrost cycles: In cold, humid conditions, frost builds up on the outdoor coil. The system periodically reverses to defrost mode — briefly blowing cool air indoors while melting frost. This typically lasts 5–15 minutes and occurs every 30–90 minutes depending on conditions.
Auxiliary heat activation: Most ducted heat pumps have electric resistance auxiliary heat (also called supplemental or emergency heat). When the heat pump cannot meet the heating demand alone, auxiliary heat kicks in. You may notice:
- Higher electric bills when auxiliary heat runs frequently
- Warmer air temperature from vents when auxiliary heat is on
- "AUX" or "EM HEAT" indicator on the thermostat
Reduced output at low temperatures: A heat pump's heating output at 17°F is typically 60–70% of its output at 47°F. This is by design and does not indicate a problem if the home maintains temperature.
When to be concerned
The home cannot maintain the thermostat setpoint:
- If the temperature drops steadily despite the system running continuously, there may be an issue.
- Check the thermostat setting and outdoor temperature. If it's below the system's design temperature, auxiliary heat should be supplementing.
The system blows cold air continuously (not just during defrost):
- Brief cool air during defrost is normal
- Continuous cold air indicates a problem
Ice buildup that does not clear:
- Light frost on the outdoor unit is normal
- Heavy ice that persists and grows is not normal
- If ice covers the entire outdoor unit and does not clear during defrost, call a technician
Unusual noises: Grinding, screeching, or loud banging indicates mechanical problems.
Common causes of inadequate heating
Thermostat settings:
- Ensure the system is set to "Heat" not "Cool"
- Check that the temperature setting is reasonable (68–72°F is typical)
- Verify the fan is set to "Auto"
- If the thermostat shows "EM HEAT," the system is running on auxiliary heat only. This is more expensive and indicates the heat pump is not operating.
Dirty air filter: A clogged filter restricts airflow, reducing heating capacity and potentially causing the coil to freeze.
Blocked outdoor unit: Snow, ice, leaves, or debris blocking the outdoor unit reduces airflow and efficiency. Keep the unit clear.
Refrigerant issues: Low refrigerant reduces heating capacity. Signs include:
- Long run times without maintaining temperature
- Ice buildup on refrigerant lines
- Hissing sound
Refrigerant issues require professional repair.
Defrost control failure: If the defrost cycle is not working, ice will build up and block airflow. This requires technician diagnosis.
Reversing valve failure: The reversing valve switches between heating and cooling modes. If stuck, the system may blow cool air in heat mode.
Undersized system: If the heat pump was not properly sized for the home's heating load, it may struggle in cold weather. This is a design/installation issue.
Ductwork problems: Leaky or uninsulated ducts in unconditioned spaces lose heat before it reaches the rooms.
DMV climate-specific considerations
Design temperature: The DMV's winter design temperature is approximately 14–17°F. Properly sized heat pumps are designed to heat the home at this temperature with minimal auxiliary heat use.
Extreme cold events: Occasional temperatures below 10°F (like January 2014 and 2015) may require auxiliary heat. This is normal and does not indicate a system problem.
Humidity: The DMV's humid winters mean more frost accumulation on outdoor coils, increasing defrost frequency. This is normal.
Dual-fuel systems: If you have a dual-fuel system (heat pump with gas furnace backup), the gas furnace should take over below the balance point (typically 30–40°F). If the furnace is not working, the heat pump alone may not maintain comfort.
When to call a professional
Call a qualified HVAC technician if:
- The home temperature drops despite the system running continuously
- Heavy ice persists on the outdoor unit
- The system blows cold air continuously (outside of brief defrost cycles)
- You hear unusual noises
- The thermostat shows error codes
- Energy bills spike unexpectedly
Do not attempt to repair refrigerant or electrical components yourself.
Preventive maintenance
Annual heating tune-up: Have a professional inspect and service the heat pump before each heating season. This includes:
- Refrigerant level check
- Electrical connection inspection
- Defrost control testing
- Air filter replacement
- Outdoor coil cleaning
- Thermostat calibration
Regular homeowner maintenance:
- Change filters every 1–3 months
- Keep outdoor unit clear of snow, ice, and debris
- Monitor performance and note changes
How Renovessa helps
If your heat pump needs repair or you're considering upgrading to a cold-climate heat pump, Renovessa can connect you with reviewed DMV HVAC contractors. Our estimator helps you compare heat pump options designed for our region's winter temperatures.