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So, the first warm-ish day of summer finally showed up in the Black Hills yesterday and I noticed that our central air conditioning was not really keeping up... the thermostat was set at 72, but the interior temperature was 76 and didn't drop a degree after several hours of continuous operation of the A/C.

Troubleshooting, I identified the following issues:

  • only two open/unblocked registers in the entire house. Solution: opened/unblocked all registers and delivered a semi-patronizing lecture to my children about not covering the vents with boxes.
  • An extremely dirty air filter. Solution: changed air filter. Stern lecture to myself about remembering to do this every 60 days or so.

Thinking that this would solve the problem, I sat on the couch and waited to be bathed in the A/C's cool embrace. Yet, after another hour, the temperature had still not dropped even a degree. The next steps in my investigation revealed the following:

  • the air blowing out of the registers didn't seem all that cool (but it did seem slightly cooler than room temp).
  • the insulation covering the line coming in from the outside unit were soaking wet and had visible condensation.
  • the outside unit seemed to be blowing cold air.

I've taken a look at this question:

AC Unit Blow Warm Air Indoor But Colder Air Outdoor

but I am daring to risk being flagged as a duplicate due to the additional details I have provided, hoping that they might help in the further diagnosis of my problem. Is there anything else I can try or do I need to call upon the arcane services of an HVAC professional?

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There is some additional easy trouble-shooting you can do yourself.

Check temperature at the vents:

Get a little thermometer. Put it right on or just inside one of the air vents so that it's getting the full effect of the air temp coming out of the vents. The air should be about 20 degrees cooler than ambient in your house. So, if your house is 75, the air coming out should be 55 or around there. If it's not, then you may be low on coolant.

Check for freeze-up:

If the air filter was very dirty and vents were being blocked, you may have frozen your evaporator coils. In my case these coils sit right above the furnace. I have to take a front panel off to see them. They are literally, copper coils on a condenser / radiator looking thing.

  • By "freeze-up" do you mean literally frozen (e.g, encased in ice)? What would the solution be in that case? – seanicus Jun 10 '15 at 20:23
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    More like encased in a thick white frost - yes. The solution is wait it out. Turn it off and wait for it to thaw. It just means it could not expel enough cold air and so it froze itself. It happens... All will be fine in the end. – maplemale Jun 10 '15 at 20:34
  • When I got home, I didn't find any frost on the coils, but I did find a bit of water pooled beneath them... I'd left the AC off for the previous 18 hours... wonder if that was the melt water of the frost? Anyway, its been tough to see if the issue has been resolved due to recent cool weather (temps in the 60's) but today it is supposed to be close 80, so we'll see if the AC can hold its own. If it does, I'll consider my question answered and give you the check mark, since I think you were the first to respond. – seanicus Jun 12 '15 at 16:07
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If you have a set of gauges, you should connect them at the condensing unit and measure the pressures in the refrigerant lines.

If you don't have a set of gauges, you should contact somebody who does.

Troubleshooting and repairing A/C units requires special tools, without them you can only speculate as to what the problem is.

If I had to speculate, I'd say you're refrigerant is low.

If you really want to poke around yourself, you could open up the air handler and visually inspect the evaporator coil. Make sure it's not iced up, or covered in dust, dirt, or debris. If it is dirty, you'll likely want to have a professional clean it, as the fins are easily damaged.

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One day several years ago it was 118° here, and our A/C couldn't get the house any cooler than 87°. Eventually I discovered there wasn't enough aluminum tape around the return air duct, so I was just blowing a lot of cool air into the attic. Taping it up solved the problem. I don't know if that even applies in your case, but if so, it's worth a look.

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