Why is my home A/C suddenly weak?
May 29, 2012 8:22 AM Subscribe
I bought a new (to us) house; the 2-zoned A/C worked very well for a week of very hot weather. Starting this weekend, it was having trouble cooling. The evaporator is not iced; the thermostat seems okay. What to look for?
* The evaporator is clean and dry; condensate drain is clear.
* Cold air does blow from the vents, and it's got a 20 degree temp drop, but it blows very weakly. It's blowing and can flutter a piece of paper but it's wimpy.
* Last week, it had no trouble cycling to keep the house at 74 in 92 ambient; this weekend, it ran 100% of the time to keep the house at about 80 in 92 ambient.
* The upstairs zone feels like it's blowing more strongly, but there are only 2 vents up there, so it is hard to say.
* At the start of the problem, the system did ice, I think. I didn't check the condenser at that time but the line to the compressor was covered in 1" of ice all around. I shut it off overnight and that cleared but something is definitely still wrong.
* Filters are clean. Even removing the filter doesn't help anything.
* I haven't done a lot of experimenting with the heat side because it was already eleventy million degrees in the house, but turning to "heat" does deliver hot air.
I feel like it's an airflow problem, but then I don't understand why the condenser isn't icing. I would suspect a low refrigerant charge, but that wouldn't explain the airflow. Tonight, I go under the house, but I hoping to get some tips on what to look for. I want to check the dampers to make sure they're not stuck; and my most fervent hope is that a trunk line has come off and we've simply been cooling the crawlspace. My question is, what else should I look for?
I'm an engineer and I know a little but only enough to be dangerous so please feel free to talk to me like a 4 year old!
* The evaporator is clean and dry; condensate drain is clear.
* Cold air does blow from the vents, and it's got a 20 degree temp drop, but it blows very weakly. It's blowing and can flutter a piece of paper but it's wimpy.
* Last week, it had no trouble cycling to keep the house at 74 in 92 ambient; this weekend, it ran 100% of the time to keep the house at about 80 in 92 ambient.
* The upstairs zone feels like it's blowing more strongly, but there are only 2 vents up there, so it is hard to say.
* At the start of the problem, the system did ice, I think. I didn't check the condenser at that time but the line to the compressor was covered in 1" of ice all around. I shut it off overnight and that cleared but something is definitely still wrong.
* Filters are clean. Even removing the filter doesn't help anything.
* I haven't done a lot of experimenting with the heat side because it was already eleventy million degrees in the house, but turning to "heat" does deliver hot air.
I feel like it's an airflow problem, but then I don't understand why the condenser isn't icing. I would suspect a low refrigerant charge, but that wouldn't explain the airflow. Tonight, I go under the house, but I hoping to get some tips on what to look for. I want to check the dampers to make sure they're not stuck; and my most fervent hope is that a trunk line has come off and we've simply been cooling the crawlspace. My question is, what else should I look for?
I'm an engineer and I know a little but only enough to be dangerous so please feel free to talk to me like a 4 year old!
How old is the system? It is entirely possible that the unit is either old and just done, or too small to keep the house cool when it's stupid hot out. Having an improperly sized system was the problem with my first home. The PO was either cheap or the HVAC installer didn't know what he/she was doing. When the hot weather came I had the thermostat set to 65 degrees to get 70 on the first floor and 80 on the top floor. Needless to say I ended up getting an appropriately sized system in short order. I know a few people who have had this problem. Hopefully in your case it is something simpler and less expensive to fix.
posted by chosemerveilleux at 8:47 AM on May 29, 2012
posted by chosemerveilleux at 8:47 AM on May 29, 2012
Most units can handle a 20 degree difference, once you start getting up in to the high nineties or higher, there's only so much it can do.
One thing we do is close all the vents in the basement. Since heat rises, the hot air goes upstairs, and the basement is cool anyway. It actually gets kind of arctic if we DON'T close the vents down there.
Also, change your filters frequently. Especially if you've got super-grade allergenic ones. I do mine about once every other month.
Get a HVAC guy out. I have a contract where I pay $200 per year and they come out twice a year to prep the system for the season change. There's a bunch of stuff that needs to happen and I'm just not into it. Also, I'm nursing an older system, and if $200 per year can save me from spending $7,000, then hey, whatever.
I'd worry about the flow, that means you've got a blockage or the blower/fan isn't working correctly.
Is this happening on just one system or on both? (I'm assuming that if you have 2 zones, that you have 2 compressors.)
posted by Ruthless Bunny at 9:00 AM on May 29, 2012
One thing we do is close all the vents in the basement. Since heat rises, the hot air goes upstairs, and the basement is cool anyway. It actually gets kind of arctic if we DON'T close the vents down there.
Also, change your filters frequently. Especially if you've got super-grade allergenic ones. I do mine about once every other month.
Get a HVAC guy out. I have a contract where I pay $200 per year and they come out twice a year to prep the system for the season change. There's a bunch of stuff that needs to happen and I'm just not into it. Also, I'm nursing an older system, and if $200 per year can save me from spending $7,000, then hey, whatever.
I'd worry about the flow, that means you've got a blockage or the blower/fan isn't working correctly.
Is this happening on just one system or on both? (I'm assuming that if you have 2 zones, that you have 2 compressors.)
posted by Ruthless Bunny at 9:00 AM on May 29, 2012
Response by poster: Thanks for the input so far!
* I think it's sized correctly just because it worked well at a normal duty cycle last week in similar ambients. In fact I ran it down to 68 just to check and to annoy my wife.
* One condenser; the zones are just in the air handling.
* Didn't think about the multi-speed blower motor! Will check that out.
posted by ftm at 9:27 AM on May 29, 2012
* I think it's sized correctly just because it worked well at a normal duty cycle last week in similar ambients. In fact I ran it down to 68 just to check and to annoy my wife.
* One condenser; the zones are just in the air handling.
* Didn't think about the multi-speed blower motor! Will check that out.
posted by ftm at 9:27 AM on May 29, 2012
Best answer: Another thing to consider is that the evaporator might be clogged (blocking airflow) and needs to be cleaned. If you can get access to the area after the blower but before the evaporator, there will be high pressure in there.
As to why the evaporator isn't icing, that's hard to say. But if there isn't enough moisture in the air that's flowing across it, then you won't get ice. Just a really cold evaporator with maybe some frost on it.
You are also right to check the zone dampers- it's possible they are both stuck partially closed. What you can do is have a helper turn the thermostat down in one zone, and up in another zone, and watch the servos that open and close the dampers. There should be an arm that's connected right to the damper that will indicate open and closed. Parallel to the flow to indicate open, perpendicular to indicate closed.
It could even be that the damper for the lower zone is stuck closed, and because all that extra heat is rising to the second floor, the second floor zone is simply unable to keep up. As opposed to in normal operation, where the second floor zone simply has to "top off" the mostly already cooled air on the second floor.
posted by gjc at 9:41 AM on May 29, 2012
As to why the evaporator isn't icing, that's hard to say. But if there isn't enough moisture in the air that's flowing across it, then you won't get ice. Just a really cold evaporator with maybe some frost on it.
You are also right to check the zone dampers- it's possible they are both stuck partially closed. What you can do is have a helper turn the thermostat down in one zone, and up in another zone, and watch the servos that open and close the dampers. There should be an arm that's connected right to the damper that will indicate open and closed. Parallel to the flow to indicate open, perpendicular to indicate closed.
It could even be that the damper for the lower zone is stuck closed, and because all that extra heat is rising to the second floor, the second floor zone is simply unable to keep up. As opposed to in normal operation, where the second floor zone simply has to "top off" the mostly already cooled air on the second floor.
posted by gjc at 9:41 AM on May 29, 2012
Response by poster: It could even be that the damper for the lower zone is stuck closed
It was this, exactly.
Everyone's answers helped me think about this methodically but especially gjc's... first I pulled off the panel where the evaporator is, meaning that the blower was free to pull in outside air. No flow improvement. Then I pulled the outdoor switch and removed the blower panel on the other side, then replugged the switch; blower spinning fine, no obvious issues.
Under the house, I had my wife change the zones around and I could clearly hear the upstairs damper opening and closing but could see no motion from the downstairs one (where I was positioned). Long story short, I removed the damper closing mechanism from the downstairs damper, letting it spring open. That was the fix. House cooled to requested temperature, HVAC shut off. Thanks!
posted by ftm at 6:28 AM on May 30, 2012
It was this, exactly.
Everyone's answers helped me think about this methodically but especially gjc's... first I pulled off the panel where the evaporator is, meaning that the blower was free to pull in outside air. No flow improvement. Then I pulled the outdoor switch and removed the blower panel on the other side, then replugged the switch; blower spinning fine, no obvious issues.
Under the house, I had my wife change the zones around and I could clearly hear the upstairs damper opening and closing but could see no motion from the downstairs one (where I was positioned). Long story short, I removed the damper closing mechanism from the downstairs damper, letting it spring open. That was the fix. House cooled to requested temperature, HVAC shut off. Thanks!
posted by ftm at 6:28 AM on May 30, 2012
This thread is closed to new comments.
If the compressor is icing, it seems like that means the refrigerant isn't picking up enough heat in the evaporator, legitimizing the airflow idea.
What about the air return area? Is the return clogged or blocked?
posted by gjc at 8:27 AM on May 29, 2012