AC cannot cool below 78F

57 Replies

James and Shanti Kandasamy

Many good suggestions here...

78F is not good enough.

I hope your AC guy did a comprehensive look at the system.  I would want to know the following and if it was in range:



Air Flow of Evaporator (should be 400 CFM per ton of AC)

Good air flow at condenser unit? Yes/No

Condenser Unit dirty or clean?

Evaporator coils clean or dirty?

Squirrel Cage clean or dirty?

Suction Pressure

Discharge Pressure

Duct leakage?

Attic Infiltration?

Humidity of Supply and Returns (may be expressed as Wet Bulb)

Amp Draw of Compressor (and what it is supposed to be)

Temperature Difference of Supply/Return

There are many more items to check, but this should be the minimum on your report/invoice.  "It's good" or "it's just hot outside" is not a satisfactory answer on how the unit is running.

It is disturbing that the outside unit i.e. the condenser was replaced with a larger size but not the evaporator/air handler.  You may have a mismatch.  It is best to match equipment. If it is mismatched, did the AC company modify the piston size or TXV?  You may be flooding the evaporator coil and that will show up with the above information on your unit.

As stated earlier, insulation and duct leakage (both supply and return) should be checked.  You may be sucking in attic air or blowing nice cold air into the attic.

A Manual J calculation, as also mentioned, is the best way to size a unit.  But a good rule of thumb in the deep South is 400 to 450 square feet per ton of air conditioning.  So, for a 2000 square foot house with moderate insulation and no duct issues, a 5 ton would work. You can check this now yourself if you know the tonnage and square feet.  If you have the AC company replace the equipment, see if they can do the Manual J calculation. 

Also, how is the insulation/weather stripping?

I've read the posts and most of them are bunk.

The size of the AC unit is based not just on the square foot of the house, but its age. An older house may not have as much insulation as a newer one and a larger unit will be needed to keep up with the convection heating during really warm weather.

An AC unit is proper working order can an will cool a property well below a 20 degree exterior/interior difference. The only exception to this is if it is very poorly insulated.

1) Insulate the property, especially the attic. You can use blown insulation or you can get a 2 lbs. closed cell polyurethane froth pack from Foam it Green spray foam insulation and spray the underside of the roof. I've used this on several properties, usually the crawl space for keeping the cold out in KS, and have loved the insulation it gives.

2) Hire a reputable HVAC contractor to get your correct AC size.

Here in Kansas, we can get below zero in the winter and over 100 in the summer. So anyone saying their HVAC doesn't keep up with the heat, need to look at their units. Our hardest time here actually, is the winter.

A lot of investors spent a lot of time on goggle becoming “AC Experts”...

If an air conditioning system could only cool to 78% of the outside temperature, there would be a lot of extremely unhappy people living in Arizona and Nevada. That’s common sense.

Get a reputable HVAC contractor in there to solve your problem.

From what i understand that is normal with central A/C.   It is typical for the house temperature to be 20 degrees lower than the outside temperature.  The bill/receipt for a new central A/C will usually have a statement regarding the inside temperature will have a 20 degree difference from the outside temperature an that is normal..  78F is fine provided the humidity is minimal.

Keep in mind that high ceilings, sunny skies, humidity, skylights, etc..will also have an effect on inside temperature.

Window A/C's, portable A/C's and split units will deliver much lower temperatures then central A/C. 

I'd let them out of their lease but I'd also look into getting one of these. We got one for our primary residence - the air coming out of the vents was 8 degrees cooler. Now the house gets up to 74 on a 100 degree day vs. the 81 it would get to before. Well worth the cost of the water, which is minimal.

Since you put in a new unit last year I'd have a different company come out and inspect your ducts and insulation. It's not uncommon for ducts to tear or come loose, in which case your cold air is blowing into your attic. Apply more blown in insulation as well - it's not expensive and is much more cost effective than upgrading your AC again. Because we rely so heavily on A/C in upper northern CA, it's common for companies to offer annual maintenance pretty cheap. For example, we pay $149/year to have twice yearly inspections and maintenance. They clean the coils, which makes the unit run more efficiently, check and repair the ducts if needed and change our filters twice per year. It's a good investment for comfort and maintenance so you get more performance and longevity out of your equipment.

@James Kandasamy insulate your attic. Austin is really hot and all my rentals are 1980's built and we blew about afoot of loose fiberglass insulation in the attics to save wear and tear on the ac units as well as keep the tenants bills lower. you can really tell the difference. We put solar screens on west windows even with low e glass , and insulate west facing garage doors. keep the heat out and your unit works better.

Not sure why the AC guy says that the HVAC equipment only cools to 78% of the outside temperature.  One thing you could do, blow in more insulation in the attic if that's an option.  Cover windows on the west side of the house with solar shades or screens.  A good rule of thumb here in Texas is 1 ton for every 400 square foot, so determine if the unit is sized right.  

My daughter in Scottsdale AZ had a simile problem. When temps outside hit 115DegF the upper floor would not cool, in fact the 5yr old dedicated 4 ton system just could not get it below 100DegF. My daughter had an excellent a/c guy confirm that a couple of the upper floor cold air returns were NOT connected to the upper floor furnace & blowers. Home was built around the late '90's so previous owners must have suffered this problem for many years. But my daughters' roommate uses the upper floor for her home office setup so it was unbearable.

The a/c guys installed the missing connection(s) plus another strategically placed cold air return vent, with minimal damage to the walls & now the upper floor is very comfortable.