Can I Fix It Myself?

What to do. Slide it out of the slot on the furnace or air handler (or the return grille in the wall or ceiling) and hold it up to a light.

What you are looking for. If you cannot see light through it, it is done. Note the size printed on the frame and the arrow on the edge — the arrow points toward the furnace, in the direction the air travels.

What it means. A blocked filter is the single most common cause of weak airflow, a furnace that overheats and shuts off, and an AC coil that freezes into a block of ice. If the filter was badly clogged and the AC had been running, switch cooling off and leave the fan on for a few hours so the coil can thaw before you run it again. One-inch filters last one to three months; the thick four- or five-inch ones, six to twelve.

What to do. Check the screen is on, the mode is set to Heat or Cool (not Off), and the set temperature is a few degrees past the room temperature. If the screen is blank, change the batteries — most take two AA or AAA behind the faceplate.

What you are looking for. A schedule or Hold that has quietly moved the setpoint. The fan set to On instead of Auto, which blows unheated air between cycles and feels like a fault. On a heat pump, Emergency Heat left switched on, which keeps you warm and doubles the bill.

What it means. A surprising share of no-heat and no-cool calls end here. If the thermostat is fine and the system still does nothing, carry on down the list.

What to do. Look on the side of the furnace or on a wall or joist nearby for a switch that looks exactly like an ordinary light switch. Make sure it is on. Then check the blower door on the front of the furnace is pushed fully home.

What you are looking for. That switch gets knocked off by someone carrying a box through the garage more often than anyone would believe. The blower door has a safety switch behind it, and a door that is off or not quite seated makes the whole furnace play dead.

What it means. If the furnace comes back to life, you are done. If it starts and then stops again within a minute or two, something real is wrong and it is time to book.

What to do. At the electrical panel, find the breakers for the furnace or air handler and for the outdoor unit — the outdoor one is usually a double-width breaker. Push a tripped breaker fully to Off, then back to On. Outside, check the grey box on the wall near the outdoor unit has its pull-out block pushed in the right way up.

What you are looking for. A breaker sitting in the middle position, which means it has tripped. A pull-out disconnect that a painter or landscaper removed and put back upside down, or not at all.

What it means. Reset once. If it trips again straight away, stop — that is the red section below, not a nuisance to be reset until it holds.

What to do. Switch the system off at the thermostat first. Clear leaves, grass clippings, cobwebs and anything stacked against the unit, so there is about two feet of open space all round and nothing on top. If the fins are matted with fluff, rinse them gently from the outside with a garden hose on a soft setting.

What you are looking for. Fins packed with cottonwood or dryer lint. A cover still on from winter. Shrubs grown in tight. In winter, a heat pump buried in snow or sitting in a solid shell of ice.

What it means. An outdoor unit that cannot breathe cannot shed heat, and the system either loses capacity or trips itself off on high pressure. Never use a pressure washer on it, never bend the fins, and never chip ice off a heat pump coil — let it defrost on its own.

What to do. If the air conditioner or high-efficiency furnace has shut down and there is water near the indoor unit, find the white PVC drain line leaving it. Many have a small box or float switch on the line or in the drain pan, and a capped T fitting near the top. Pour a cup of distilled white vinegar into the T, and outside, hold a wet-dry vacuum tightly over the end of the drain line for a minute.

What you are looking for. Standing water in the pan, a float switch that has lifted, or a slimy blockage at the outlet. If the drain line runs to a small pump with a reservoir, check the pump has power and the reservoir is not full.

What it means. A blocked drain trips the float switch, which is designed to shut the system down before the pan overflows. Clear the blockage and the system usually restarts on its own. If it keeps happening, the line needs a proper flush and a look at the slope.

What to do. Walk the house and make sure every supply vent is open and every return grille is clear of furniture, rugs, curtains and pet beds.

What you are looking for. Vents closed in spare rooms to "save money", a couch pushed over the return, a dresser sitting on a floor register.

What it means. Closing vents does not save anything on a modern system — it raises the pressure in the ducts, strains the blower, and can freeze the coil. Open everything up and give it a day.

What to do. Set the thermostat to Off, wait five minutes, and set it back. If a furnace still does nothing, flip the furnace power switch off for thirty seconds and back on.

What you are looking for. Furnaces lock themselves out after three failed ignition attempts and will sit silent for an hour or more. Cycling the power clears the lockout once.

What it means. If it starts and stays running, watch it for a day. If it goes straight back into lockout, the fault is real — usually a dirty flame sensor, an igniter or a pressure switch — and repeated restarts will not fix it.

Can I replace the capacitor myself? It is fifteen dollars online.

We would not. A capacitor stores a charge after the power is off and has to be discharged properly before it is touched; the replacement has to match the microfarad and voltage rating exactly, and a hum-but-no-spin symptom is not always the capacitor in the first place. When we replace one, we measure the old one first, so you are told the reading and the rating — that is the part you are paying for.

Can I clean the outdoor coil with a hose?

Yes, gently. Switch the system off at the thermostat, use an ordinary garden hose from the outside of the unit, and let it dry before switching back on. Never a pressure washer, never a stiff brush, and no harsh chemicals — the fins are thin aluminum and bend if you look at them wrong. A coil that is greasy or packed solid with cottonwood needs a proper coil cleaner and a technician.

Can I clear the condensate drain myself?

Yes. A cup of distilled white vinegar in the T fitting and a wet-dry vacuum on the outside end of the line clears most blockages. If it clogs again within a few weeks, the line has a low spot or a growth problem that needs a proper flush and a look at how it is routed.

Can I replace my own thermostat?

A like-for-like swap on a simple furnace or AC is within reach of most people: photograph the wiring before you disconnect anything and label each wire. Where it goes wrong is a heat pump, a two-stage system, or a smart thermostat on a system with no common wire — a mis-wired thermostat can blow the fuse on the control board and turn a twenty-minute job into a service call. If any of those describe your system, let us do it.

How long do I wait after the easy checks before I book?

If the system runs, give it a day and see whether it holds. If it did nothing, or it ran and then stopped again, book straight away — especially in a cold snap or a heat wave, when the schedule fills. Tell us which checks you did; it saves the technician repeating them and gets you to the actual fault faster.