Home A/C problem

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Feb 17, 2007
Clermont, Florida
The AC in my girlfriend's house doesn't work right. It blows out cool air from the vents in the ceiling but it hardly moves any air at all. When the thermostat is on about 75 degrees the AC will cycle on and off at normal intervals but the house is still excessively warm and it takes several hours to move the thermometer down even 3 or 4 degrees. Even then it never gets past 78 degrees or so. It will run for hours nonstop if you set the thermostat at 70 or below.

This is about an 1800 square foot house, and is laid out like a traditional single story home with a long hall, living room and kitchen and 3 bedrooms and 2 baths. I already removed the fan from the air handler and cleaned off the dust and dirt from the blades, and I cleaned the dust and dirt off the screen covering the condenser tubes. The air filter is new and clean and you can feel a great deal of velocity of air moving through the air handler. Just not much comes out from the vents. You can't even feel any air from the vent with your hand about 15 inches from it. She says it has been like that for a long time and it always takes a while to cool the house down.

Next weekend I plan to get into the attic to see if the duct work is damaged. If anyone here has any HVAC experience, I am all ears for what else to check. It can't be normal for it to work so hard yet produce so little cool air flow. Do animals get into attics and chew up the duct tubing material?

Any info is appreciated here.
you didnt mention what the outdoor temp was, but most a/c's will lower the air temperature in the house 15 to 20 degrees lower than the outdoor temperature.

setting the thermostat at 70 degrees is a huge was of money.

it sounds like you have checked all the simple stuff, although looking over the duct work is a great idea.

make sure that it is insulated well and all the joints are sealed with foil tape or mastic.

Should your duct work be ok, i would find a good a/c repair company, not the cheapest. it sort of sounds like your low on refridgerant or you have an air flow issue. You could also have a weak valve in the compressor, which wouldnt allow the compressor to do its job.
He's cycling routinely without reaching setpoint. This tells me that the cooling end of it is working okay (first impression). This sounds like an air delivery/volume issue. Otherwise, he would be @ 100% duty cycle without reaching setpoint. His evap is reaching temp.

He should set the thermostat at whatever temp is comfortable and leave the system alone. It takes too long to cycle the entire house volume once you feel that the thing needs to be turned on.
Dirty evaporator? Dampers misadjusted? Exteme humidity? Outdoor condensing unit fan running?
What kind of filter do you have? If it's one of those 3M filters with lots of pleats, get rid of it, too restrictive. I would pull the filter out and run the fan, see if air flow increases without it.

What speed is the fan in? Should be in high speed on the board, usually a Black or Blue wire is high.

Also, the coil will only freeze if there isn't enough airflow across it.
A big help are the booster fans that mount inside your ductwork for faraway outlets-I need 2 or 3 myself-they help with airflow A LOT. If your coil iced up you'd have no cooling at all-if your system is undersized it'll take as much as a day to get all the way cooled down-try setting your thermostat fan switch to FAN ON, that should force the blower into high speed all the time, that may help somewhat.
Thanks for the replies. She lives near Orlando, Florida. Outside temp has been in high 80's to low 90's for the last couple weeks. No rain and low humidity for now.

I think the evap and compressor are OK. The PCB for the fan speed has instructions on it to change the wire connections to increase fan speed, I did it and it made no difference. The air in the air handler sounds like a tornado but at the vent it's like a whisper.

I replaced the fan capacitor last week when it failed and the AC didn't work at all. But even before that happened the air flow in her house has always been very weak, in my own opinion. In comparison, in my own house if I set the thermostat on 72, in 15 minutes the house is freezing and I have to get up and change the setting. And I have enough air flow out my vents that it will move the blades on my ceiling fans and interfere with the motion of the ceiling fan blades unless I direct the flow down and away from the fan. My house is about 2/3 the square footage of hers though, and I have a tar and gravel roof which keeps it really cool in summer.

I hope I am able to find something simple like a torn duct hose causing her AC to be so weak. I agree it is a huge waste of money for it to run incessantly and still have the house at 78 or 80 degrees. Good AC is a way of life here in Florida.

I appreciate the help here. If anyone else has any ideas, please post. Thanks.
Dang, you are describing my house to a "T". The landlord will have a repair man over sometime this week (he better...shakes fist).
The coil looked OK when I looked at it last week. There are no return registers in the house, the only air filter/intake I can find is the one in the bottom of the air handler. All the posts I can find on the internet seem to point to a problem with the ducts or possibly a closed damper. I'm going to get in the attic Saturday and see what I can find. As hard as the fan and air handler are working, I think it has to be something to do with the ducts.

Are there returns hidden in houses that I don't know about? My gf said it has always taken a long time to cool the house, but I have only been dating her for 4 months and she has been in the house for 2 years.

Any other info is really appreciated. Thanks.
Most houses have the registers on the outside walls and returns near the center. You won't get good circulation unless the warm, humid air can get back to the air handler.

And yes, who knows what critters may have done.
If it's cycling on and off but not reaching the set point, something is wrong. Too little air flow will make the evap too cold, and trip the freeze limit, shutting off the compressor.

Is there a good flow of air getting sucked in the return hole? ducts can come loose, so the attic might be getting nice and cool.
completely missed the returns part of the post, agrees totally with brett!

as to it not reaching the set point on the thermostat, the thermostat could just be out level or inaccurate, depending on the model of the thermostat.

Take some pictures if you can of the duct system and have some measurements of the duct, and the size of the a/c.

if you dont know the size post the model numbers of the outdoor unit and the indoor air handler
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It's a Trane AC unit. I guess it's about 10 years old. It had the compressor replaced last summer. The thermostat is original also, the type with a slide on the end to adjust the temperature and an analog thermometer. How can I tell for sure if the thermostat is bad? I know I can get one for a reasonable price at Home Depot if it needs one.

I went all through the house and the only return I can find is the one at the air handler. The air handler is in a closet in the hall with the slatted type sliding doors. Are there filters or returns in the attic? This is an AC in Florida so it has a furnace, not a heat pump. We don't use the heat very often down here. The unit outside is strictly an AC. I just wonder if there is some kind of duct problem going on in the attic. There is plenty of air flowing up into the bottom of the air handler. It will suck the air filter out of your hands and take it away from you.

Thanks for all the help on this.
He says that there is no air coming from the ducts. This is an air flow problem. There seem to be other problems too ( no returns, etc) but that is not going to cause low air flow, just inefficiency.

The problem is that something is plugged or severely leaking in the ductwork or a damper is not opening causing low air flow. Have you tried taking out the filter to test?
I think some smaller houses are set up that way and will work as long the air is free to flow from the supply registers to the closet. Does it do any better with the closet doors open? Bedroom doors?

It doesn't sound like the thermostat, but you can check it by shorting the red, yellow, and green wires together at the furnace or thermostat. Red is hot, yellow is the compressor, and green is the furnace fan.
Can you close off your vents? If so, close them all off except for one and see how the flow is. If it's good, maybe your fan isn't blowing enough volume for the entire house. If it's not good, sounds like a blocked vent. Has she ever had the ducts cleaned?
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