Heat Pump Not Cooling in Sunnyvale, CA

Heat Pump Not Cooling in Sunnyvale

Sunnyvale summers are mild most weeks, but a few hot stretches each year are what trigger these calls. When a heat pump runs and the air coming out is not cold, the homeowner usually notices it on the first real heat of the season, when the unit cycles all afternoon and the house keeps climbing. A heat pump cools using the exact same refrigerant cycle as a straight AC. The difference is a reversing valve that flips the flow between heating and cooling. So when one stops cooling, we diagnose it the way we would any air conditioner, plus one extra check on that valve.

Almost every time, this traces back to a single component. It might be a reversing valve stuck in heat mode. It might be a capacitor that no longer starts the compressor. It can be a refrigerant charge that has dropped from a slow leak, or an outdoor coil packed with dust and pollen. A finding like that is a repair, not a reason to replace the system.

Most Sunnyvale homes are mid-century ranch and tract stock, and a fair number have had square footage added over the decades without the HVAC being resized to match. When that happens, the heat pump is already working near its limit before anything fails. A weak capacitor or a slightly low charge that another house might tolerate will tip a hard-running unit into not keeping up at all. We size the diagnosis to what the system is actually being asked to do.

Common causes

Reversing valve stuck in heat mode. If the heat pump is heating fine but blowing warm air on a cooling call, the reversing valve may not be shifting. We check the solenoid coil for voltage and resistance and listen for the valve to slide when the thermostat calls for cool. A failed solenoid is a part swap. A valve hung up internally sometimes frees with a controlled pressure pulse, otherwise it gets replaced. The finding and the price go on the written estimate before we touch it.

Low refrigerant from a leak. A heat pump does not consume refrigerant, so low charge means a leak somewhere in the coils or line set. We measure superheat and subcooling, then trace the leak with an electronic detector or dye instead of topping it off. Recharging without finding the leak is a refund you pay for twice. On a hard-cycling system a small leak shows up as poor cooling earlier than it would on a lightly loaded house.

Failed run capacitor or contactor. The capacitor starts the compressor and outdoor fan. When it weakens, the fan may spin slowly or the compressor hums and trips. The contactor is the relay that sends power to both. Both are common, inexpensive failures, and both fail faster on equipment that runs long hours during a heat spike. We test capacitance against the rating on the part and check the contactor for pitted contacts.

Dirty outdoor coil. Dust and pollen collect on the outdoor coil, and a unit that runs constantly through a hot stretch cakes up fast. A blocked coil cannot reject heat, head pressure climbs, and cooling output falls even though everything is technically running. We check the coil condition early because a wash is often the cheapest real fix on the list.

Frozen indoor coil from low airflow. If the air handler airflow is choked by a clogged filter, a dirty blower wheel, or a failing blower motor, the indoor coil drops below freezing and ices over. Once it is iced, almost no cool air reaches the rooms. We look for ice on the coil and at the line set, thaw it, then fix the airflow cause instead of clearing the ice and leaving.

Thermostat in the wrong mode. Heat pumps confuse some thermostats, especially after a battery change or a smart-thermostat reconfiguration. If the reversing-valve output, the O/B terminal, is set wrong, the thermostat calls for cool but the system stays in heat. This is a five-minute check and an honest one. If that is all it is, we tell you and you pay the diagnostic, not a repair.

How we diagnose it

Confirm the thermostat is calling for cooling and the O/B reversing-valve setting matches the equipment, since this rules out the cheapest cause first.
Verify the outdoor unit is energized: capacitor capacitance against the rating, contactor condition, and that the compressor and fan actually start.
Read superheat and subcooling to judge the refrigerant charge, and if it reads low, trace the leak rather than top off.
Inspect the indoor coil and filter for ice and the outdoor coil for blockage, since either kills cooling while the system appears to run.
Test the reversing valve solenoid for voltage and proper shift on a cooling call to confirm the valve is moving.

$75 diagnostic, credited toward any repair over $200. You get a written quote before any work begins.

Heat Pump Not Cooling in Sunnyvale: common questions

How fast can you get to Sunnyvale, and do you cover the rest of the South Bay?

We are based in San Ramon and run across the Bay Area including Sunnyvale, Mountain View, Santa Clara, and Cupertino. Same-day is our best effort and depends on where we are when you call, not a guarantee. Call (925) 999-4095 and we will give you an honest window rather than a vague promise.

It only fails on the hottest afternoons. Is the whole system shot?

Usually not. Failing only at peak heat is the classic sign of a marginal part, a weak capacitor or a slightly low charge, that holds up under light load and gives out when the system is pushed. A few hot stretches a year are enough to expose those marginal parts. We find the specific weak point and price the fix, not a replacement you may not need.

The heat pump heats fine in winter but won't cool. What does that point to?

When heating works and cooling does not, the reversing valve is the prime suspect, since the only thing that changes between the two modes is that valve flipping the refrigerant flow. We test the solenoid and confirm whether the valve is shifting. It can also be a thermostat mode setting, which we check first because it costs you nothing to rule out.

Nearby and related

Heat Pump Not Cooling near Sunnyvale: Mountain View · Santa Clara · Cupertino .

This is usually a heat pump installation & service in Sunnyvale job. See our heat pump installation & service overview or the Sunnyvale service area.