Hot Topics: Air Conditioner Not Cooling the House

A modern thermostat.

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Original Post: A/C not cooling the house during the summer

When a homeowner realizes her air conditioner isn't working as it should, the forum helps her track down the problem. (Hint: You can't upgrade a system without also paying attention to the ductwork.)

alsiplisa Member

Hi, everyone. I am hoping someone can help me! We have a 3-ton Goodman AC for an approximately 1868 sq. ft home in Southwest Florida. We keep our thermostat set at 75. During the "winter" it works perfectly. As soon as the summer heat comes, the temp will rise to 80 degrees at times! If it rains (or is really cloudy) the temperature stays where I keep the thermostat set at. My husband blew about 25 bags of insulation in our attic before summer, which hasn't helped! When the sun goes down, so does the temp. He checked the duct work and couldn't find any leaks. When we bought our home we kept the temp at 78, so this wasn't an issue. When we replaced our unit we previously had a 2-ton, and that did not cool during the summer, either. I have an appointment next Monday for a tech to come out. We could really use some advice and opinions. Thank you in advance for any insight you may have!

Bob14525 Member

You didn't say how old the system is. Has any maintenance ever been done on the system (condenser coils cleaned, evaporator coil cleaned)? Also, is the filter in the air handler clean (new)? How cold is the air coming out of the vents? Is there a sufficient volume of air? These kinds of questions should be answered before the experts here can begin to help you.

roughneck77 Member

When you upsized a ton, what changes were made to the ductwork to handle the extra airflow? With 3 tons, you'll need to move around 1200 CFM of air. What's the sizing on the ductwork that attaches to the air handler? And is it flex, round, or square?

alsiplisa Member

There were no changes made to the ductwork. There was an area that was damaged, so it was replaced. It's round and is insulated. We thought it was possibly the ductwork size, however, we had this issue with the 2-ton and the same ductwork.

roughneck77 Member

The ductwork may have been undersized for 2 tons. You can't toss in a bigger unit with no duct changes and expect it to function. What has happened now is you've paid for 3 tons, but your unit isn't putting out 3 tons capacity. Wasted money buying bigger equipment. Was a load calculation done to determine 3 tons was needed? You need to move around 350-400 CFM per ton of capacity at around .05-.07 static. What size are your ducts now? Round, flex, or rectangular?

alsiplisa Member

To be honest, I can't remember if a load calc was done. All of the ductwork is round. I'm not sure of the size, however, my husband just told me that there are different sizes for different areas of our house. Is that common?

roughneck77 Member

Yes, it's required to reduce size downstream to keep needed velocity and static. Read here for unit sizing information:

For 3 tons worth of air, you should have roughly the following size duct for the main trunk:

Flex: 18"-20" so long as it's stretched tight and not kinked

Round metal: 16"

There are a host of different size rectangular ducts that would support 1200 CFM. You'd have to give me your specific size to see.

roughneck77 Member

You can do your own version of manual J here for heat loss and gain. You need manual D for duct sizing. I don't know of a free version to give you.

HVAC Load Calculation - Maunualj - Whole House Loadcalc

What I'm getting at here is that bigger equipment is NOT better.

clocert Member

You need to answer these three questions first: (1) What is the temperature of the cold air from the register? (2) What is the room temperature at that time? (3) Can you feel the cold air from 10 feet away from any of your registers ?

alsiplisa Member

I don't have anything currently to measure the temp. I will do my best to purchase something so my husband can do so and I will post the results. Thanks!

Our air conditioner actually quit so we purchased the 3-ton because we added a room two years later.

roughneck77 Member

The things you'll want to look for when your appointment comes on Monday are:

-operating pressures
-operating superheat
-operating subcooling
-return and supply air temperature at air handler
-return and supply air temperature at diffusers
-operating CFM
-return and supply static pressure

alsiplisa Member

Well, the tech suggested replacing the return vent. It's 9 inches and he wants to replace it with a 16-inch. He measured the vents and said that the CFM was 800 and should be 1200. Does the remedy he suggested sound appropriate? He also did a dye test on the handler (those results will not be done until Friday), but said that our evaporator coil needs to be replaced (would the ductwork cause this?). The coil is on backorder for 30 days!

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