No heat, no cold air, or a system that won't stop running. Start by finding out what that pattern actually means.

Cold air from the vents does not always mean the furnace has failed: sometimes it means it is doing exactly what it was built to do, at the wrong moment.

A furnace that runs a normal cycle, or close to it, does not short cycle. This is about a furnace that keeps shutting off long before the house has warmed up, then starting again minutes later.

A furnace that does nothing (no hum, no click, no blower) usually has a simple explanation. The trick is checking things in the right order instead of guessing.

Ice on an air conditioner's coil or lines feels like a strange kind of malfunction. Too much cold, from a machine that is supposed to make things cold. It is almost always a sign of restricted airflow or low refrigerant, not a system running at full strength.

There is a real difference between an AC that is off, one that is running weakly. And one that is running normally but has stopped cooling: and the fixes are different for each.

Because a heat pump uses the same equipment to heat and cool, a heat pump that will not heat can fail in ways a furnace never does. And some of what looks broken is the system deliberately working around outdoor temperature.

Long run times in extreme weather can be normal for a heat pump. A system that never shuts off even in mild conditions, or that keeps running well past the setpoint, is a different situation.

A thermostat that says it cannot connect, a thermostat with a dead screen. And a thermostat that looks perfectly normal while the furnace ignores it are three separate problems: and the fastest way to separate them is to stop looking at the thermostat.

A blank screen, a screen that shows the wrong temperature. And a screen that works but does nothing when you change it are three different problems: and only one of them is really about the thermostat.

A mini-split has no ducts to check, which removes one whole category of possible causes. But it still shares the same core parts as central air, and most of the same failure points.

Duct sizing is the least visible part of a heating and cooling system and one of the most consequential: a correctly sized system attached to undersized ductwork will still heat and cool badly, and no thermostat setting fixes it.

Both high-efficiency furnaces and air conditioners produce condensate water that has to drain somewhere. And a clog in that drain is one of the more common reasons a system simply shuts itself off.

A single thermostat controls the whole system based on the temperature at one location, which means every other room depends on ductwork, insulation. And airflow to actually get its fair share of conditioned air.

A boiler heats water that circulates through radiators or in-floor piping rather than heating air directly, which means several of the usual furnace checks do not apply: and a few boiler-specific ones do.

A radiator is a simple object, so a cold one narrows things down quickly. As long as you notice which part of it is cold, and whether it is the only one in the house behaving that way.

“Air purifier” and “air cleaner” are marketing words, not categories. And they get attached to five pieces of equipment that work in completely different ways and are not interchangeable.

A humidifier mounted on the furnace's ductwork is easy to forget about entirely until the house feels noticeably dry. And by then several different simple things could be the cause.

A heating and cooling system is never silent, and some of the sounds that alarm homeowners most (especially on a heat pump) are the system working exactly as designed. The useful skill is telling those apart from the handful that mean shut it down.

“Licensed and insured” is a phrase, not a credential. Behind it sit three separate things issued by three different authorities, and only one of them is federal.

Twice-a-year maintenance is the single most effective thing most homeowners can do to avoid the failures covered elsewhere on this site: short cycling, icing. And sudden no-heat calls in the middle of winter.

Both systems can heat a house well. Which one makes sense in a given home has less to do with which is generally “better” and more to do with climate, existing equipment. And what fuel is already available at the property.