The compressor is humming, air is coming out of the vents, and the house keeps climbing anyway. It is one of the most common calls we take in July, and a fair share of them end without a repair at all — the system was working and something upstream of it was not.
Here is the order a technician checks things in, and how far you can safely get on your own before calling.
Start where the air comes in, not where it comes out
A clogged filter is the single most common cause, and it is also the one people rule out incorrectly, because a filter that looks grey still passes a flashlight test badly. Pull it and hold it up to a light. If the light does not come through evenly, the system is being suffocated, and everything downstream — weak airflow, warm air, a frozen coil — follows from that.
Then check the returns. Furniture pushed against a return grille, or half the supply registers closed in unused rooms, starves the system just as effectively as a dirty filter. Closing vents does not save money; it raises static pressure and shortens the life of the blower.
Ice is a symptom, and it has exactly two parents
If you find frost on the copper line at the indoor unit, or ice on the coil, turn the cooling off and leave the fan running. Trying to run through it is how a compressor gets damaged.
A frozen coil comes from restricted airflow or from low refrigerant, and those two have very different consequences. Airflow you can often fix yourself with a filter and an open return. Low refrigerant means a leak — refrigerant is not consumed, so a system that is low has lost some — and that is a repair, not a top-up.
What to look at outside
- Is the outdoor fan actually spinning? A humming unit with a still fan is usually a failed capacitor, and running it that way cooks the compressor.
- Is the coil packed with cottonwood, grass clippings, or dryer lint? A blanketed condenser cannot reject heat, so the house never cools no matter how long it runs.
- Is there a tripped breaker or a pulled disconnect? Both are common after a storm.
- Is there at least a couple of feet of clearance around the unit, and nothing growing into it?
Rinsing an outdoor coil gently from the outside in, with the power off at the disconnect, is homeowner-safe. Bending fins, prying at the top, or pressure-washing it is not.
The thermostat traps
Two settings account for a surprising number of “it’s blowing warm” calls. The fan set to ON instead of AUTO runs the blower continuously, including between cooling cycles, so the air out of the vents is room temperature half the time and the house picks up humidity. And on a heat pump, an accidental emergency-heat or heat setting in summer produces exactly the complaint you would expect.
Also worth ruling out: a thermostat on an interior wall that is being read by afternoon sun, and a dead battery in a unit that still shows a display.
When the system is fine and the house still is not
Sometimes everything measures correctly and the house is still uncomfortable. In older Boston housing stock the usual suspects are duct runs through an unconditioned attic losing a large share of what they carry, a system sized for the wrong load, or humidity that never gets removed because the equipment short-cycles. Those are building problems wearing an equipment costume, and no amount of refrigerant fixes them.
If the system is more than a decade old and the answer keeps being “add refrigerant every summer,” it is worth pricing the alternative. A heat pump replacement carries Mass Save money that a repair never will — see the 2026 rebate figures.
When to stop and call
Call once you have replaced the filter, opened the returns, confirmed the outdoor fan spins, and the house still will not hold temperature — or immediately if you see ice, smell something burning, or hear the outdoor unit hum without starting. Diagnosis over the phone is free, and describing what you already checked usually shortens the visit.
We cover central air conditioning repair and window and wall units across Greater Boston, with same-day windows most of the summer. Call (617) 404-9944.
