AC Not Keeping Up: Why It Cannot Get Below 75

By Ray Bennett · Updated 2 October 2026 · 7 min read · Checked against the sources at the end

AC Not Keeping Up: Why It Cannot Get Below 75

Short answer

A house system is built to hold roughly twenty degrees below the outdoor temperature, so on a hundred degree afternoon a house sitting at 78 is a system doing its job rather than a system failing. The question that separates a fault from physics is whether it used to manage it. If it did, something has changed, and the free checks find most of it.

An air conditioner that will not keep up is a different complaint from one that is not working. This one runs, the air at the vents is cold, and the house still sits at 78 with the thermostat set to 72.

Some of those houses have a fault. Quite a lot of them have an air conditioner doing exactly what it was built to do, on a day it was never built for, and it is worth knowing which you have before you spend anything.

Before you start

What to carry: two thermometers, a torch, a new filter and a garden hose. An hour, spread over a day.

Gas or electric? Neither matters here, because this is the cooling side. What does matter is whether you have a split system with an outdoor unit, or a heat pump running in cooling, because they behave identically in summer and everything below applies to both.

Ask one question first. Did it keep up last summer? A system that managed this house for eight years and cannot now has a fault. A system that has never quite managed has a different problem, and no repair will fix it.

Two things worth refusing. An attic in the afternoon. It can be over 130 degrees up there and people are hospitalised every summer for going up at the wrong time of day. Go early in the morning, tell somebody you are going, take water, and come straight down if you feel light headed.

A refrigerant top up with no leak found. Refrigerant is not fuel and it is not used up. A system that is low has a leak, and paying to refill it without finding that leak buys you the same visit next year at the same price.

First, the arithmetic

This is the part nobody explains. A house system is not built to deliver any temperature you ask for, only a gap below the one outside.

Check 1

Two thermometers and one subtraction

Put one thermometer in the shade outside and read the one in the house, in the middle of the afternoon.

A house system is sized to hold roughly twenty degrees below the outdoor temperature when it is hot. At 95 outside, about 75 inside is the machine working. At 105 outside, about 85 is working. Setting the thermostat lower does not add any capacity, it only guarantees the system never stops running.

So if the gap is around twenty degrees, nothing is broken and the rest of this page is about clawing back a few more.

If the gap is much smaller than that, or it has shrunk compared to previous summers, keep going. And if the air at the vents is not actually cold, this is the wrong page: measure the drop across the coil first.

The free things that give capacity back

Check 2

Airflow, outside then inside

The outdoor coil. This is the big one. The fins all the way round that unit are how the heat leaves your house, and grass clippings, dust and cottonwood mat into them until it cannot. Switch it off at the breaker and rinse gently from the inside outwards, never with a pressure washer.

What is growing round it. Shrubs, a new fence, a deck skirt, a pile of firewood. Air has to get in at the sides and straight out of the top, and a foot of clearance all round is the usual minimum.

The filter. Airflow is capacity. A filter that has collapsed against its frame costs you more than a dirty one, so look at its shape as well as its colour.

Registers closed in spare rooms. This is advice that has been passed around for decades and it makes things worse. Closing registers raises the pressure in the ducts, which pushes more air out of every leak, and starves the coil at the same time.

The sun through the glass. On a west facing room in the afternoon, the glass can be most of the heat coming in. Blinds and curtains closed on the sunny side is free, and it is not a small effect.

If all of that is clean and clear and it still will not manage, the indoor coil is the next suspect, and that one is behind a panel with a technician’s name on it.

Where the cool air goes on the way

The ducts usually run through the hottest part of the house. Anything they lose up there, they lose twice, because hot attic air gets pulled in to replace it.

Check 3

Follow the air from the coil to the room

Compare two vents. Hold your hand at a vent close to the air handler and then at the furthest one in the house. A big difference in how cold and how strong the air is means it is being lost on the way rather than never made.

Go up and look, early in the morning. Flexible duct that is crushed, kinked, sagging in long loops, or simply pulled off its boot. A branch that has come adrift is dumping the whole of that room’s cooling into the attic, and it is far more common than people expect.

The insulation on the ducts. Thin, torn or missing wrap in an attic that is hotter than the outdoors is a large and permanent loss.

The return side. Not enough return air starves everything, and it is the half of the system nobody looks at. A bedroom with a supply vent, no return, and the door shut cannot be cooled properly no matter what you do at the other end.

In a typical house, ENERGY STAR says about 20 to 30 percent of the air moving through the ducts is lost to leaks, holes and poorly connected ducts before it reaches a room. That is a big reason a perfectly healthy air conditioner cannot keep up.

When it really is too small

Sometimes the answer is that the system was never big enough, or the house has changed around it. A conservatory, a converted garage, a loft bedroom, more glass or simply more people all add load that nobody recalculated.

Two things are worth knowing before anybody quotes you.

Bigger is not better. An oversized system cools the air fast, satisfies the thermostat, and shuts off before it has pulled the moisture out. You end up with a house that is cold and clammy and a compressor that starts and stops all day: short runs also freeze coils. How a new one should be sized: what size AC unit you need.

Sizing is a calculation, not a rule of thumb. A proper one measures the house: the windows, the insulation, the orientation, the air leakage. Anybody who sizes a system by square feet alone, or who suggests going up a size because the old one struggled, is guessing with your money.

Fix the duct losses first. It is cheaper, it is often the whole answer, and a good contractor will want it done before sizing anything anyway.

Questions people ask

Will setting it to 65 cool the house faster?

No. It cools at one rate, and a lower setting only means it reaches its limit and then runs without ever satisfying. All you change is the electricity bill.

It keeps up at night but never in the afternoon.

That is what a system at the edge of its capacity looks like, and it points at the duct and coil losses rather than at a broken part. The night is when the load finally drops below what it can deliver.

Upstairs is always hotter than downstairs.

Heat rises, the roof is above it, and the upstairs ducts are usually the longest run. Balancing the dampers helps a little, sealing the attic ducts helps a lot.

Is it normal for it to run all day?

On the hottest days of the year, yes, and a system that never stops in August is often correctly sized rather than failing. Running constantly in mild weather is a different matter.

How much difference does a dirty outdoor coil really make?

Enough to notice. The whole job of that coil is dumping heat, so when it is matted over, the system works harder for less, and it is the cheapest thing on this page to put right.

The house is cool but damp and unpleasant.

That is usually the opposite problem, a system that is too large and shuts off before it has removed any moisture. The fix is not a bigger one.

Sources: Bryant "What Temperature to Set Air Conditioner in Summer" (about 20°F below the outdoor temperature, 78 to 80 inside at 100 outside), Trane "8 Air Conditioning Myths That Are Costing You Money" (a lower setting does not cool faster, keep vents open, load calculation), ENERGY STAR "Duct Sealing" (20 to 30 percent of duct air lost), Lawrence Berkeley National Laboratory LBNL-54005 "Register Closing Effects on Forced Air Heating System Performance", Carrier 25HBC/25HCC/25HCE installation instructions (outdoor unit clearances), Lennox "How to Clean Air Conditioner Fins" (inside out, no high-pressure water), the DOE Energy Saver Guide (2017) (window coverings, oversized units, disconnected ducts) and Florida Solar Energy Center FSEC-PF-336-98 (peak attic air temperatures)