Why is my AC not cooling?

The four usual causes, and what each costs

This Old House names them:

low refrigerant levels, clogged air filters, dirty evaporator coils, or thermostat malfunctions. Generally, these are simple issues you can get fixed for a relatively low cost.

Fixr puts the prices on:

The average AC repair, across everything, is $375, and Fixr's minimum service call fee is $75.

Our reading, marked as ours: the table divides cleanly in two. Everything down to the air ducts is a few hundred dollars. The last three rows are system work, and our AC compressor page and evaporator coil page cover those in full. This Old House says the same thing in words: the common causes are cheap, but a lack of cooling could also indicate problems with the outdoor condenser unit, requiring more expensive repairs.

The one sentence worth reading before you call

Fixr prints this, and it is the most useful consumer protection in the whole guide:

An AC unit should never need a refrigerant refill. The only time it's necessary is after a leak is fixed or when a major repair, such as replacing the compressor, requires the system to be opened.

Our reading. Refrigerant is not consumed. A system that is low has lost it somewhere. So a recharge at $100 to $600 on a system that has not just been opened is treating a symptom, and the leak repair at $200 to $1,500 is the actual job. If you are offered a recharge and nothing else, the question to ask is where it went.

Our arithmetic, marked as ours. Two recharges at the middle of that band, about $350 each, come to $700, which is more than the middle of the leak repair range. A recharge is only cheaper once.

Start with the free checks

Before any of the above, both This Old House pages point at things that cost nothing.

The filter. A blocked return-air filter restricts airflow, and This Old House says that with one, there is a good chance the evaporator coil will start to freeze. A frozen coil stops the system cooling entirely, which our frozen AC page covers.

The thermostat setbacks. This Old House's heating expert checks these second, because setbacks that are too far apart during the day make the system work too hard to catch up.

The outdoor coil. The same expert stresses that a dirty outdoor condenser coil reduces the system's ability to remove humidity, because leaves, pollen and debris block the fine aluminum fins and restrict airflow.

Our note: none of those three needs a technician to identify, and all three appear above the paid repairs in the professionals' own order of checking.

What a proper diagnosis actually involves

This is the part most pages skip, and it is why a $75 service call and a full diagnosis are not the same purchase. This Old House sets out what its experts do:

Their expert's analogy for the static pressure test: it is "kind of analogous to, like, blood pressure, but for ductwork".

One finding from that process is worth repeating on its own. On a This Old House project, technicians measured supply airflow in a bedroom and found a dramatic difference depending on whether the door was open or closed.

Our reading, marked as ours: if one room is warm and the rest of the house is fine, the problem may not be the air conditioner at all. That is a different and much cheaper conversation than the one you were about to have.

When it cools but feels clammy

This is a distinct fault and it gets misdiagnosed as weak cooling. This Old House's expert checks, in order: equipment sizing, then thermostat setbacks, then the specifics. In the case documented on that page, a UV light was requiring the fan to run continuously, which kept re-evaporating moisture off the coil; the fix was to remove the light and set the fan to run only in cooling mode.

The page also records a homeowner in New Orleans for whom humidity was not a problem in summer, when the AC ran constantly, but became severe in spring and autumn.

Our reading: an oversized system is the common thread. It cools the air fast, shuts off before it has removed the moisture, and does it worst in mild weather when the run times are shortest. That is the same mechanism our sizing page warns about, and sizing is the first thing both of these experts check.

The order we would work in

1. Change the filter. Free, immediate, and the single most common cause on both publishers' lists.
2. Check the thermostat, including its setbacks and its batteries, before assuming the machine is at fault.
3. Look at the outdoor unit. Clear leaves and debris from the fins.
4. Check whether it is the whole house or one room. One room points at ducts and doors, not at the air conditioner.
5. Then call, and ask for the fault to be named before the price. The table above turns a named fault into an expected range.
6. If the answer is "it needs refrigerant", ask where it went. On Fixr's own words, the recharge is not the repair, and if the system runs on R-22 our Freon page explains why the answer matters more than it used to.
7. If the complaint is noise rather than temperature, our noise page covers it, and most of it is not a fault at all.