When I was an HVACR technician, one of the gigs I often got from my clients was that their air conditioner wouldn’t cool below 75 degrees.

Depending on the humidity level or outside temperature, most homeowners prefer to set their thermostats to lower temperatures (75°, for example).

Sometimes, it struggles to get lower than that.

If you have the same problem, you don’t necessarily have to give me a buzz (I’m retired), so I will just tell you how to fix it.

But this is what you MUST know:

Whenever your air conditioner struggles to cool lower than that, 9 out of 10 times, it results from a dirty air filter, inadequate insulation, faulty thermostat, refrigerant leak, or clogged drain line.

Air Conditioner Won’t Cool Below 75 Degrees: Troubleshooting Tips and Solutions

Now you know why your air conditioner is struggling to reach lower temperatures, let’s discuss how we can fix it.

But before we proceed, I want to make this clear:

In the sweltering days, your air conditioner will struggle to cool your house down sooner than expected.

That is because it has to work harder to extract the heat from the inside air and expel it outdoors.

Air conditioners are programmed to cool by 20° below the outdoor temperature.

Therefore, if it is 95° outside, your AC will drop by 20 –that is 75° cooling.

However, in a furnace-like condition above 96°, the AC will maintain a cooling temperature above 75°.

But if this is not the case and it’s still not cooling.

Well, it is time to get our hands dirty!

5 Reasons Why Your Air Conditioner Won’t Cool Below 75 Degrees

air conditioner won't cool below 75

1. The Unit Is Improperly Sized

One reason your air conditioner won’t cool below 75 degrees is when the unit is undersized.

Imagine using a window AC for a 1,000 SQFT. Of course, it will labor to attain your desired room temperature because it doesn’t have enough output power to cool the space efficiently.

And it doesn’t matter if it is set below 75°f because it won’t feel like it.

Oversize AC is another problem. Your air conditioner will turn on and off, not remove all the humidity, which could lead to mold growth, and you will have higher energy bills.

That is why it is SO crucial to determine the proper air conditioner size that fits your space before purchase. An HVAC professional can help you with that.

But if it is a window-mounted or portable home air conditioner, this table will help you figure that out:

Maximum Cooled AreaAC Capacity
150 SQFT5,000 BTU
250 SQFT6,000 BTU
300 SQFT7,000 BTU
350 SQFT8,000 BTU
400 SQFT9,000 BTU
450 SQFT10,000 BTU
550 SQFT12,000 BTU
700 SQFT14,000 BTU
1,000 SQFT18,000 BTU
1,200 SQFT21,000 BTU
1,400 SQFT23,000 BTU
1,500 SQFT24,000 BTU

2. Insufficient Airflow

For starters, a dirty or clogged air filter is the most likely culprit, as it can cause a significant decrease in cooling performance.

The airflow redundancy implies less air is passing over the evaporator coil, making heat absorbing less effective.

It could also lead to a frozen evaporator coil, which reduces cooling capacity since it cannot absorb heat.

Since the AC is less capable of removing heat from the inside, it may struggle to reach lower temperatures.

Before cleaning the air filter, you must determine the type.

Is it a fiberglass, pleated, reusable, or HEPA filter?

Disposable filthy filters should be removed from their slots and replaced rather than cleaned.

However, if the air filter is reusable, vacuum it using a soft brush to remove dust and debris.

You can also well it with water, but allow it to dry thoroughly before inserting it back.

All vents MUST be opened; no furniture or drapes should cover the registers. You also have to inspect your ductwork for any leaks or obstructions.

Sometimes, cracks or holes could let air escape, making it difficult for the air conditioner to push out the air.

3. A Faulty Thermostat wiring Or Dead Batteries

However, if your air filter is debris-free, you may want to use the thermostat.

The thermostat is such a vital piece of your HVAC system.

It must display the current temperature and use that information to control the system on and off.

However, the problem with a thermostat is either weak/ dead batteries or faulty wiring. Sometimes, the thermostat exposed to direct sunlight could misread the temperature.

You have to inspect for the batteries if your thermostat is battery-powered; if it is dead or weak, the voltage will drop –which may significantly impact the unit’s performance even though it still powers the thermostat. 

You will have to change the batteries. If the ray of sunshine always finds a way to contact your thermostat, a set of drapes will have prevented that.

Meanwhile, contact an HVAC expert if you suspect faulty thermostat wiring (any loose, frayed, or disconnected wires) after removing its cover.

4. Clogged Drain Line

Reduced Cooling Efficiency:

Water will back up and accumulate in the drain pan whenever this PVC pipe is clogged.

While this could reduce the heat exchange efficiency but consequently hamper the cooling capacity, the excess water pile on the rain pan is the perfect breeding ground for mold and mildew, affecting the indoor air quality.

 Even worse, the overflow can cause water damage to your floors, walls, or even ceilings –depending on where the unit is mounted.

But how do you address a clogged drain line:

First things first, is to ensure safety. Turn Off the System at the thermostat and the

 circuit breaker.

Identify the Drain Line. The PVC pipe that runs from the indoor unit to the outside for most units.

Next is to check for Clogs. Visually inspect if there is any debris or buildup within the drain line.

If there is any, use a Wet or Dry Vacuum to suction out the clog, positioning the vacuum hose at the drain pan’s opening or at the end of the drain line.

The last is to flush with water.

Most homeowner uses algae tablets or drain pan treatment tablet to prevent future clogs caused by mold and algae in the drain line. You, too, can.

But if the problem persists, do not hesitate to schedule professional maintenance.

5. Refrigerant Leak

The Refrigerant is such an integral part of your air conditioning, as it is in charge of dissipating heat.

The system will start experiencing lower temps whenever there is a possible leak. This can eventually freeze your evaporator or condenser coils.

In many ways, refrigerant leaks also lead to high energy bills.

So check for any ice or frost buildup within the refrigerant line.

If there is any, shut the system down immediately.

But hey, you have to be careful with this!

Refrigerant leaks are detrimental not just to your health but to the environment.

They contain harmful chemicals that, when breathed in, can cripple you from the inside out. And because they are odorless, you are less likely to know when you have dosed yourself with them.

Hence, trying to fix the problem yourself is highly prohibited. Instead, get a licensed HVAC pro.

Do-It-Yourself Or Call A HVAC Professional, Which One Is Best

There are boundaries to DIY projects. Forget about trying to save costs.

You CANNOT proceed with what you have limited knowledge of. It could be dangerous.

So, there is no shame in paying an HVAC contractor a penny to know the problem.

It saves you the risk of hurting yourself or further damaging the appliance.

Particularly for me, I won’t recommend trying anything more complex than changing the filter or swapping out the thermostat battery.

It is not outside your area of expertise. So respect that.

However, if you have a background knowledge about HVAC, maybe you just need a little help on what to look for –please, by all means –you are in for the taking.

Besides that, you need professional help.


Before we wrap this up, I also want to touch on the key to maintaining optimal cooling performance: maintenance.

You can schedule professional tune-ups once in a while and do your winter preparations properly, especially if you live by the sea, where it is extra salty. 

This helps ensure long-term efficiency rather than untimely repairs.

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