A heat pump that runs but blows cool or lukewarm air in heating mode is almost always dealing with one of five things: a thermostat stuck in cooling or emergency heat, a defrost cycle that never completes, a reversing valve that failed, low refrigerant from a leak, or an airflow restriction inside the ductwork. In McLean, the most common single cause we find between October and March is a defrost board or sensor failure on an outdoor unit that has been running continuously through a cold snap. This post walks through the checks you can do safely in ten minutes, then explains the repairs that need a licensed technician and what those repairs cost in this area.
Start with the thermostat and emergency heat settings
Before you touch the outdoor unit, verify the thermostat is actually calling for heat. A heat pump not heating in McLean VA often traces back to a thermostat that was bumped into cool mode, set below room temperature, or locked into emergency heat after a power flicker. Check the mode, the setpoint, and whether the display shows "aux" or "em heat" when it shouldn't.
If the thermostat is a smart model, pull it off the wall plate and check for a tripped breaker on the air handler circuit. We see this constantly in McLean homes built in the 1960s and 1970s along the Chain Bridge Road corridor, where a single 15-amp circuit feeds both the thermostat transformer and the condensate pump. A $4 fuse or a $180 thermostat replacement is a much better outcome than a compressor diagnosis.
If you have confirmed the thermostat is calling for heat and the indoor blower is running but the air is cool, the problem is downstream - either the outdoor unit is not engaging or the refrigerant circuit is not moving heat. That is when you move outside.
Why is my heat pump not heating in McLean VA?
Heat pump not heating troubleshooting McLean homeowners can do themselves covers about 20% of the calls we run; the other 80% need gauges, a multimeter, and a refrigerant scale. The short answer: in heating mode, a heat pump reverses the refrigeration cycle to pull heat out of outdoor air and push it inside. Anything that blocks that reversal - a stuck reversing valve, a frozen coil, a failed defrost sensor, low charge - leaves you with a fan blowing room-temperature air.
Here is the order we work through on a diagnostic call, and the order you should think about it too:
- Thermostat and low-voltage control - mode, setpoint, batteries, fuse, transformer.
- Outdoor unit power and contactor - breaker, disconnect, contactor pull-in, capacitor bulge.
- Defrost cycle - board, sensor, and whether the coil is iced over.
- Reversing valve - is the solenoid energized in heat mode, and does the suction line get hot?
- Refrigerant charge - superheat and subcooling readings, leak search if low.
- Airflow - filter, blower wheel, evaporator coil, duct static pressure.
Most McLean systems fail at step two or three. Capacitors and contactors are the two most-replaced parts on outdoor units in this climate, and both are inexpensive. A contactor replacement runs $180-$320 in 2026 pricing; a dual-run capacitor runs $220-$380 installed.
Outdoor unit problems: defrost, reversing valve, and refrigerant
Heat pump not blowing hot air McLean calls in January and February almost always land on one of three outdoor-unit failures. Here is how to tell them apart without opening the refrigerant circuit.
Defrost cycle not working
In heating mode, the outdoor coil gets cold - often 15°F to 25°F below outdoor air temperature - and frost builds on the fins. A working defrost cycle reverses the unit for 2 to 10 minutes every 30 to 90 minutes of runtime to melt that frost. If the defrost board or the outdoor coil sensor fails, ice accumulates and airflow across the coil drops to near zero. You will see a solid block of ice on the coil and the outdoor fan may be running with the compressor off.
If you catch it early, you can switch the thermostat to cool mode for 20 minutes to melt the ice, then back to heat. If the coil ices over again within a day, the defrost board or sensor needs replacement - typically $320-$560 installed, and it is a repair our team handles on the Heat Pump Defrost Cycle Repair McLean VA side of the business almost daily in winter.
Reversing valve stuck
The reversing valve is the part that lets one machine both heat and cool. When it sticks, the unit runs but moves heat the wrong direction - you get cool air from the vents in heat mode, or hot air in cool mode. A stuck reversing valve is usually a solenoid or pilot-pressure problem, not a refrigerant problem. Replacement runs $650-$1,100 installed depending on access, and it is not a DIY job because the valve sits in the refrigerant circuit.
Low refrigerant from a leak
A slow refrigerant leak is the most expensive of the three because the fix is not just recharging - it is finding and sealing the leak. Systems low on charge lose heating capacity gradually, run longer cycles, and eventually freeze the indoor coil. A leak search and repair with recharge runs $600-$1,400 in this area, and we always pressure-test before adding refrigerant. Topping off a leaking system without repairing it is a waste of money and, under EPA Section 608 rules, not something a licensed technician will do.
Airflow and ductwork issues that mimic a broken heat pump
Sometimes the heat pump is working perfectly and the house still feels cold. Restricted airflow - a clogged filter, a failing blower motor, a crushed flex duct in the attic - drops the temperature rise across the indoor coil and can trip the high-limit switch, shutting the unit down mid-cycle.
Start with the filter. A 1-inch pleated filter in a McLean home with pets or a recent renovation can load up in 30 days. Swap it and see if the supply air temperature climbs. If it does not, the issue is likely the blower motor or the duct system. A blower motor replacement runs $480-$850 installed; a full duct static-pressure test and rebalance runs $350-$700 and often uncovers a collapsed run that has been choking one side of the house for years. Our Heat Pump Ductwork and Airflow Balancing McLean VA crew handles both in a single visit.
One more thing worth checking: if you have a ductless mini-split in a bonus room or garage conversion, the indoor head has its own filter and its own error codes. A mini-split that stops heating usually flashes a code on the indoor unit - often a communication or coil-sensor fault. Those repairs are different from a central system and start around $280 for a sensor or board swap. If that is your setup, our Ductless Mini-Split Heat Pump Repair McLean VA team can diagnose it the same day.
Common mistakes McLean homeowners make when troubleshooting
The mistakes we see most often on heat pump not heating troubleshooting McLean calls are not dangerous - they just cost money and time. Here is what to avoid.
- Running emergency heat for weeks. Auxiliary heat strips draw 3 to 4 times the electricity of the heat pump itself. A $400 repair becomes a $900 power bill fast.
- Adding refrigerant without a leak search. If the system is low, it leaked. Recharging without repairing just delays the same call.
- Replacing the thermostat first. Thermostats are rarely the failure. Test the low-voltage signal before you buy a new one.
- Ignoring the outdoor unit in a snowstorm. A drift of snow against the coil blocks airflow and mimics a defrost failure. Clear 12 to 18 inches around the unit.
- Resetting the breaker repeatedly. A breaker that trips twice is telling you something. Stop resetting it and call us.
If you are not sure whether you are looking at a control problem or a refrigerant problem, the honest answer is that you cannot tell from the outside. Gauges and a multimeter settle it in 20 minutes.
When to call a professional
Call a licensed technician when the outdoor unit will not start, when the breaker trips more than once, when you see ice on the coil that will not melt, or when the air from the vents is cool after 15 minutes of runtime in heat mode. Those four symptoms cover roughly 90% of the heat pump not heating common causes in McLean VA that need a real diagnosis.
Our technicians carry defrost boards, capacitors, contactors, and reversing valve solenoids on the truck, which is why most repairs are finished on the first visit. We work across McLean, from the Chesterbrook and Langley Forest neighborhoods to the townhomes near Tysons, and we have been inside enough of these systems to know which parts fail first in this climate. If you would rather have someone look at it than guess, that is what we are here for.
Frequently Asked Questions
Why is my heat pump not heating in McLean VA?
In this area, the top three causes are a failed defrost board or sensor, a stuck reversing valve, and low refrigerant from a leak. Thermostat misconfiguration is fourth. If the outdoor unit runs but the air is cool, the problem is almost always in the refrigeration cycle, not the thermostat.
What are common causes of heat pump not heating in McLean?
Defrost failure, reversing valve failure, refrigerant leaks, bad capacitors or contactors, and airflow restrictions from clogged filters or crushed ducts. Capacitor and contactor failures alone account for about a third of the winter repair calls we run in McLean.
Can a heat pump not heating be repaired same-day in McLean Virginia?
Yes, in most cases. Defrost boards, capacitors, contactors, thermostats, and blower motors are common enough that we stock them on the truck. Same-day repair is typical for those. Refrigerant leak searches and reversing valve replacements sometimes need a second visit for parts.
How much does heat pump repair cost in McLean VA?
Typical 2026 pricing in this area: $180-$380 for a capacitor or contactor, $320-$560 for a defrost board or sensor, $480-$850 for a blower motor, $650-$1,100 for a reversing valve, and $600-$1,400 for a leak search with recharge. Diagnostic fees usually run $89-$149 and are often credited toward the repair.
Should I just run emergency heat until spring?
No. Emergency heat uses electric resistance strips that cost three to four times more per hour than the heat pump. Running it for a full McLean winter can add $400-$700 to your power bill and it will not fix the underlying problem.
Get help with Heat Pump Repair in McLean
If your heat pump is running but the house is not warming up, we can diagnose it today and tell you exactly what it needs. Call McLean Heat Pump Pro Masters at (571) 760-4593 and we will get your heat back on.