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Thread: Help! AC Stops Blowing Thru Vents?

  1. #1
    DIY Member BDP's Avatar
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    Default Help! AC Stops Blowing Thru Vents?

    I've got a problem with my home's air conditioning unit that I was hoping I could get some suggestions on, regarding where to start. I'm a rookie at this but did manage to replace my inducer blower when the heat died over the winter.

    What's happening now is I turned on the AC for the first time this season. Both outdoor blower and indoor unit fan seem to be running fine. I got cool air out of the vents, but in a few hours, the airflow through the house vents just stopped. Downstairs, I could still feel cooled air seeping out through the edges of the ducts at the unit itself, it's just as if the fan is being blocked from passing air from the main unit up into the house somehow.

    The next day, I turned just the fan on without any heat or ac, and the fan blew air through the vents all day. I also tried heat with similar results. The ducts only eventually seem to stop blowing air when I turn on the AC (after a few hours).

    The cooling in general, when the air WAS blowing, seemed a bit inefficient, slower than past years.

    Does anyone have any ideas where I can start here? I am a rookie DIY'er but a good student. Of course, if this is an advanced issue that I will need to call in a repair for, let me know that as well. I am worried there may be a freon leak that is somehow blocking an air vent/line as the AC cools down.

    Thanks for any assistance.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    I think you have a refrigerant leak and the system is low. This can cause the evaporator to ice up, which restricts air flow. When this happens, take off the access panel and check. I think the fan motor will be happily churning along...my guess the evaporator is covered in ice, blocking the air. The evaporator coils are the part that is in the air duct - usually on top of the furnace; if you have an air handler, it is inside of that. In my limited experience, when they start to leak, unless you can find a fitting that has a problem where you can fix it easily, it may end up being time to think about replacement, since it is probably the compressor or the evaporator coil.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Default a/c

    Have you done any preventive maint. yet: Clean off any debris from outside of unit that could be blocking airflow. Replace air filter, and clean coils.

    If your air filter is covered with ice, that indicates a large freon leak, and you need to fix it right away.

    You probably need a freon recharge anyway, if the air itself isn't as cool as in prior years. That would indicate a smaller freon leak, which they should find as well.

    Note: unless you are certified, it is illegal for you to do anything involving freon.

  4. #4
    DIY Member George R's Avatar
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    A lower VOLUME of air flowing over the evaporator could also cause it to ice up. You said you replaced the (inducer?) blower this past winter. Could you be more specific? Is it possible you have a lower RPM on the blower, and hence less volume of air flow over the evap coil, than you did last year?
    Last edited by George R; 05-16-2007 at 06:06 AM.

  5. #5
    Moderator & Master Plumber hj's Avatar
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    Default air flow

    When the flow stops, remove the filter and check the AC coil to see if it is iced over, which is what your description seems to imply.

  6. #6
    DIY Member BDP's Avatar
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    If by air filter you guys just mean the removable sheet filter that goes inside the HVAC unit, I just took it out right now to eliminate it as a factor. I replaced it pretty recently, anyway.

    It got weirder yesterday -- I had the AC on all day and it took about 7 hours to cool the house down by about 5 degrees from 77 to 72 -- The air feels cold coming out of the vents but airflow just generally seems a bit less. Last weekend when I ran the AC overnight the flow had stopped by the morning -- This morning, go figure, I still am getting airflow from the vents.

    But -- Is it normal to feel so much cold air escaping along the ductwork that runs up alongside the indoor unit? I don't recall that much air there and think this may be a result of a partial blockage somewhere (perhaps this icing over you guys are talking about). Which may explain why it's taking so much longer to cool the house and why the AC continues to have to work so hard to KEEP it cool. It's not that warm here in PA yet.

    I placed a photo here:

    AC Photo

    That's my indoor unit. The black plastic part near the bottom is the inducer blower. I'm not sure if this runs with AC or only heat.

    Sorry for all the novice questions -- Where do I find the AC coil that you guys are suggesting might be freezing over?

    Now that it's working again, I'm less inclined to call in an expert to look at it, though it's been 8 years since I bought the house (new) and it's never been looked at or serviced, so maybe it's just time?

  7. #7
    Retired Defense Industry Engineer jadnashua's Avatar
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    Follow the refrigerant lines from the compressor. They connect to the coils. Probably above the furnace in the duct, but could be elsewhere.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  8. #8

    Default

    The ice should be obvious if you removed the air filter. The coil that usually ices is the inside evaporator coil (that the air filter rests against.) By now, most of your freon has escaped I think, so you may not see any icing.

    If you have an air thermometer, test the temperature of the air coming out of the vents. It should be 15-20 degrees cooler than the room temp, if everything is working properly. Also, the copper pipes coming out of the compressor should be cold and sweating (like a cold beer can at the beech).

    One other thing. Look for the return air vent. It should be the biggest and lowest vent near your unit. If it is blocked by furniture or covered with dust, that can restrict air flow as well. As can the little fuzzys that collect on the outside coil of the unit. You must hose those off.
    Last edited by Verdeboy; 05-16-2007 at 09:59 AM.

  9. #9
    DIY Senior Member SteveW's Avatar
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    Depending on the layout of your AC/furnace, the ice may or may not be visible even with the filter out. It wasn't on mine when it started to ice up.

    Another way to test for icing: Next time the airflow goes down, turn off the AC, but keep the fan running. Over the next hour or so, monitor the water coming out of the AC condensate drain (it should increase dramatically), and monitor the airflow coming out of the vents (it should also dramatically increase, as the ice melts).

  10. #10
    DIY Member BDP's Avatar
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    Thanks for taking the time to respond, everyone.

    I may be in a bit over my head here, as I don't have a clue where to find the AC coils, compressor, condensate drain, etc. I know absolutely nothing about the makeup of the unit itself. I learned the bare minimum to replace the inducer blower fan which you see in that photo below, but haven't taken any other access panels off and have no idea where to look for the other stuff. I'm sure you guys are right, and that I'd be able to find the issue, but even so, if it winds up being something more complicated that just disconnecting and rewiring a new replacement part, I may need to call a pro in anyway...

    I'll see what pieces I can get apart in the meantime, though this one might be a little much for a DIYer like me...

  11. #11

    Default A/c

    I don't think you're understanding what's been said here.

    There are a few simple things you can do to help troubleshoot the problem. You can hose off the coils on the outside of the unit where all those fuzzy tree seeds collect and block your air flow. You can clean off your intake air vent that is near your unit in the inside of the house. You can use an air thermometer to measure the temp of the air coming out of the vents. You can remove and replace the air filter and look for signs of icing. You can look at and feel the 1/2" copper pipes inside the unit to see if they are sweating and cold, or if they are warm. You can listen to hear if the compressor goes on. and wait to see if it goes off again before your house is cooled.

    Once you've done this, you can report the info back to us so we can further advise you. Keep in mind, only a certified A/C tech is allowed to remove/add freon to the system.

    You can also try posting a photo on this site, because your photo is on a "forbidden" server.

    What is the make of your A/C unit? Is it a furnace and A/C combo?

  12. #12
    DIY Member BDP's Avatar
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    Verdeboy - Thanks.

    The outside unit is clean, I do clean it regularly. Also, remember that for around the first 4-6 hours I turn on the AC, it does cool the house and the vents blow normally. I also feel the warm air being blown outside from the unit at this time as well. It is only after around 6 hours or so of constant running (trying to cool the house down around 10 degrees or so) that it stops blowing through the vents. So this does seem to indicate something freezing up. Otherwise, I would think it would not blow cool air at all, even when I first turn it on...Or am I mistaken?

    I was unaware the photo was on a forbidden server. I'll post a better pic as soon as I get home from work. The indoor unit is by Ducane and it is one tall unit that handles both the heating and cooling (combo). The unit was brand new when I bought my house (new build) 8 years ago. I have not done any special cleanings or service on the unit since, though.

    I removed the air filter I placed inside the indoor unit (which I change regularly) and there was no ice buildup.

    I don't own an air thermometer, is there a good spot I can quick grab one? What is the air temperature from the vents supposed to be, typically?

    Thanks, I'll post a better photo in a few hours.

  13. #13

    Default A/c

    You can buy a thermometer at any drug store, hardware store, etc..
    The air coming out of the vents should be 15-20 degrees cooler than the ambient room temp. And it should be coming out at a decent velocity.

    As I mentioned, the pipes coming out of the compressor should be cold to the touch and "sweating", if all is working properly. It sounds like the compressor may be overheating and shutting off prematurely. This could be due to a number of factors including icing.

    There should be a little drain pipe coming out of the bottom of your A/C. This is your condensate drain. It may be tied into another drain or it might just go through the wall and drain outside. Look for a pipe sticking out of the wall near the outside part of your A/C. Do the test that Steve W. mentioned: After the cold air stops coming out, go to the thermostat and turn the A/C off but turn the fan on. Watch for an increase of water coming out of that pipe.

  14. #14
    Retired Defense Industry Engineer jadnashua's Avatar
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    The cooling coil is often placed on top of the furnace in the ductwork, if it is, then you can't see the coils from where the air filter is. As noted, follow the two lines from the compressor (the part outside) and see where they go. They connect to the cooling coils. My guess is that they are getting blocked by ice. This is a gradual process that could take hours, depending on conditions.

    There will also be a drain line coming from underneath the cooling coils. Depending on setup, this may flow to a pump or directly into a drain, or outside of the house. When the a/c is operating, there should be at least some water coming out of this as well.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  15. #15
    DIY Member BDP's Avatar
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    You guys pretty much got it right. I just had to learn where everything was. I found the lines from the compressor, and both were extremely frosted over in their exposed parts, both inside AND outside of the house. I didn't need to open the top plate to know that the coil was icing over, over a period of enough hours.

    So, I let the tech come over figuring this was not something I was going to be able to do on my own. Sure enough, he hooked up his meter and the freon or R-22 or whatever it's called was reading 15. I've got a leak. He said it SHOULD read 40.

    But here was the sad part -- He said he can't tell where the leak is today, he could only fill it back up with new stuff, also adding in a special dye. Then, over time, it WILL leak again and then they can use the dye to determine where. He tells me depending on the size of the leak, it can be anywhere from a few months to a few years until I am low again.

    Here's the thing though - This all cost me $567! Does that sound right to you guys? Almost $100 per pound of the juice plus the dye was another $100 plus the service, etc. Charges like this are why I hate service calls, though I realize sometimes there is no other option.

    The other bother is he tells me that he guesses it's the inside coil but can't be certain, so can't fix that today. When he DOES fix it he's guessing around $1500 total parts and labor. Does that sound right as well? Seems awfully pricey. Not to mention, I'm going to have to pay AGAIN to refill it as his advice was just to let it leak out until it's low again.

    I'm thinking I want to buy a couple things and monitor this leak myself. First, is there anywhere I can get a meter like his to hook into my outside unit so I can read the levels of coolant in there? I want to see how fast down it's dropping from 40. Also thinking of then getting a black light as he advised so I can determine where the dye is leaking from.

    Anyway, $567 later and it's not even fixed. Just a band-aid. Did I get railroaded or is this just the way it is?

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