My A/C hasn't been able to keep up with the heat lately (it's been in the high 90's with high humidity, and today the house was up to about 82 inside even though it's set on 70). I went up into the attic a few minutes ago to see if my untrained eye could see anything wrong with the unit, and I noticed two things:

  1. There is a hissing noise coming from the unit where the refrigerant lines go into the evaporator. I know that I have a TEV, and I've read online that these can sometimes hiss, but I wanted to see if that's likely what the cause is or if it's possibly something else.
  2. Cold air (quite cold) is blowing out of the hole where the refrigerant return line comes out of this box (this is also where I'm hearing the hissing), and the attic was cooler than the rest of my house!

I just had a popular local A/C company out here to investigate, and he said that the unit appeared to have been undercharged from the time it was installed (the house was built in 2006), so he added two pounds of refrigerant. He said that he didn't find any leaks. Are either of these things issues?

2 Answers 2


Have you lived there long enough to compare the performance of the system to previous summers? (Sometimes units are undersized, or paired with poor duct design.)

Is the air coming out of the vents cold? Is it as cold as you remember before problems started? Is the airflow at the vents good?

After he added coolant, is the system working better? If yes, the system was low on charge. If not, something else is wrong.

It is possible for the system to be low on charge, but still be able to keep up on days that are moderate (albeit at a lower efficiency than a properly charged system).

It is also possible for there to be a leak, but the HVAC guy missed it. He probably only did a quick leak check (soap bubbles). There are more involved (expensive) tests he can do. However, they may not be necessary. If adding coolant made the system work well, wait and see. If the performance degrades again in a few weeks/months, you have a leak.

If there is a leak, it is usually in the outdoor unit, which is exposed to the elements, and takes a beating.

The good news is that the unit is probably still under parts warranty (typically 5 years for a cheap unit, 10 years for a good one), so if there is something expensive wrong, the parts should be covered.

(When was the last time your air filter was replaced? If it's been more than 6mo, look at it and see if it is OK. Replace if necessary. If you aren't up for replacing your air filter at least once a year, you should consider a service contract where the techs do it for you.)

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    Actually, another tech came out today and discovered a leak in the evaporator. It is still under warranty, but I'm still looking at paying labor for them to install a new one. As for the filter, I replace the return filters every three months; are there more filters than that? Commented Jul 23, 2010 at 2:30
  • Should be just the one filter in the base of the air handler, unless you have a fancy system with anti-allergen stuff. (Count yourself lucky they found the leak so quickly. It took 5 different service calls before we found the leak in my heat pump!) Commented Jul 23, 2010 at 12:27
  • Oh, there's a filter in the base of the air handler? I doubt that's ever been replaced. Commented Jul 23, 2010 at 14:55
  • Depends on the unit (I am mostly familiar with basement installs), but there is usually one main filter right where the main return attaches to the air handler. In basement systems, it is usually at the base of the air handler. Commented Jul 23, 2010 at 15:03
  • So there are filters on the returns in the house as well as there? Commented Jul 23, 2010 at 17:08

I doubt there are filters on the individual returns, just at the main air handler return. I change filters every month (if I remember) because we live in the country and have dogs and cats. Sometimes there's a small drift of hair at the main return in the great room, which is a sign we need to sweep and reminds me to change the AC/furnace filter.

What happens if you don't change your filter often enough is the fan motor burns out much sooner than it should, which will cost a 4 digit number to replace. Last time I replaced the whole furnace/air handler because it only cost $200 more than the fan motor parts.

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