How can I determine why my A/C runs all day?

Question

I have a Bryant AC/Furnace unit. My house is 10 years old. A heat wave hit this weekend & temps got up to 95°F outside. I had my thermostat set to 75°, and it ran all day. The thermostat never dropped below 80° during the day & eventually cooled down towards the evening. How can I tell if it's a problem with my AC or with the insulation in the house? The attic has insulation "blown" in it, and from what I can tell, was done recently before we bought the house 5 years ago.

Does the "blown" insulation have to be re-blown every so often? Or is it more likely a problem with the AC unit? I've never had it serviced since I bought the house, but I have changed the air filters monthly.

Edit

Something else I remembered: my house is bi-level. I keep the downstairs vents closed because if they're all open, it gets pretty cold down there. With them closed, it's maybe a little warmer than upstairs. We're upstairs most of the time anyway. Would that affect anything?

Answer 1

Your question almost exactly sums up the situation I had at my home last August.

What I found then was that despite a properly sized air conditioning unit and what I believe to be good insulation (the house holds heat well during the winter), the unit still couldn't keep up.

I don't have full proof, but I think a few factors caused my situation and might be worth checking for you:

1. The condenser (outdoor unit) sits in an "inside corner" with the house on two sides of it and a chimney and small storage shed that partially blocks a third side of it. This prevents good airflow across the unit. [I can't do anything about this without relocating the unit, which is not a DIY job for me.]

2. We had very little wind at all during the heat wave, which also added to the lack of airflow across the condenser.

3. The condenser was visibly dirty, which makes heat transfer to the outdoor air less efficient. Lots of dirt, dust, pollen, grass clippings, etc. build up on the fins over time. Normally rain will clean it off somewhat, but we hadn't had rain in ages.

Despite some advice I've seen not to do it, I used a garden hose to wash off the fins of the condenser (it gets wet from rain, right?) very gently, taking care not to make it full of mud or to bend the fins.

I could see water evaporating immediately on contact, and I have an infrared thermometer I used that showed a 30F or so drop in temperature in short order. The AC performed much better after that, but we also started to get a gentle breeze that helped things along.

Answer 2

The easy answer is because your thermostat is telling it to run all day. So, should it or should it not be? Is the thermostat in a good position in the house? If it's directly in the path of a sunbeam, then it will get hot and run more than necessary to cool the rest of the the house down. Is there enough cool air coming out each of the vents? If you close most of the vents, do the remaining ones have a stronger air force? If not, you might be leaking some air out of the ducts. If your A/C unit is 10 years old and has not been serviced in at least the past 5 years, it's probably worth the money for an inspection/service. This shouldn't be terribly expensive and is a good starting point. Monthly air filter changes also sounds like a little excessive to me. Also: Check for a second air filter if it's an older system. I was surprised to find a second one in my unit and promptly changed it.

Answer 3

Another likely problem is that your system needs the coolant recharged. If the coolant is low, there is not enough to cool the air as it passes through the coil. You may have a leak somewhere, but the house is cooling somewhat, so the system is not empty. Regardless, call a qualified A/C person to service the unit.