Hey all,

My central ac unit stops blowing air out of the ducts after a few hours of running. Once you switch the ac off, and turn the fan on to run the air flow returns. I went upstairs and there is a constant flowing water sound going out of the unit in our attic. Assuming the compressor and outdoor unit is working, what's being frozen?? The evaporator coils? Does that explain the reduced air flow? What can i do?

Thanks in advance!

 

Yes, sounds like frozen coils. make sure your filter is cleaned or replaced. Also check to make sure there isn't an additional filter at the unit, besides the typical wall or ceiling mounted one in the living space. Go to the outside unit while it's running and feel both the small and large copper lines going inside. The large one (probably insulated with foam) should be very cold and the other small one, warm enough that it's uncomfortable to grasp. If they are only mildly cold and warm...you probably are low on refrigerant and will need to call a tech. You can look for signs of a leak around the filler connections. If there's an oily stain, you likely found the issue. It may be somewhere else though and will require a service call no matter what.

What brand/model and how old?

 

I'll check the lines this afternoon and for an additional filter. When I looked yesterday, the larger line was frozen at the unit both inside and out. The smaller one was cold from the air handler. But I don't know about at the outside unit.

The unit is quite old. Circa ~1970s. I'll get specifics later when i check the lines.

Does the decreased airflow over the coils because of an additional dirty filter cause the freezing?

Thank you for your help and explanations!

 

Dirty filter can reduce airflow not allowing heat to be transferred as designed and cause frost on the coils, which reduces flow, which causes more frost...etc etc etc.

If it cools well for the first few hours (temperature difference, measured in the airflow before and after coils should be about 15-20 degrees) it could be reduced flow I believe. Low refrigerant normally happens much faster....I think! No HVAC expert by any means. Most people will use a meat thermometer or similar and put it in the return air grill and then take a similar reading at an output register (I can't remember whether it should be furthest from the unit, or closest)...Pros will actually drill small holes in the sheetmetal and insert probes at or near the coil for a more accurate reading.

That small line being cold is a good indication heat is not being transferred...it should be quite warm.

 

Alright. I checked the model numbers on my units. The furnace is a GE 21LU 120A2M1. I did some (limited) reading and the serial number (740422 209) on the unit which says it's from 1972 9th week? The outside unit is a TempStar ACS048A2B1 or FBA048GB1. Don't know year information on this one (currently).

I also checked the lines by the outside unit. There was a high of 89 today and when I got to the lines, the larger was frozen while the smaller wasn't warm to touch at all. Now, I know where the filter is on the unit, but didn't think there was a second one... I've been scouring the interwebs for about a half hour and haven't been able to find much on this furnace. Any ideas where a second filter could be?

Cooling well is not an accurate description from my experience. I typically run the ac for ~2 hours and it'll drop the room temp 2 degrees at the most, then ill switch it to fan only for ~1 hour because the air flow is weak. Then repeat the cycle.

While I understand what you're saying about the 15 degree difference at those positions, the air temp difference at the vent output can't be anywhere near that...

 

There might not be a second filter...I've just run into it before. Was staying at a friends and he said something about his A/C not being all that great. Checked the return filter in house and it was like new, but almost zero airflow. Went to furnace-A/C and it was last serviced 2 yrs before. Poked around and saw fasteners like for an access panel, not just for a cover over the coil area. Pulled the panel and there was at least 2 inches of dust and pet hair on a typical woven filter. Obviously it hadn't been serviced correctly in the past since these people didn't even own a cat!

So anyway...there may not be another filter in yours, but it definitely seems to be low on refrigerant. You can pull the access cover to look at the coils, but I doubt you will find anything other than frost, which melts off and runs out the drain pan like it's supposed to. Anything that old is very hard to find info on unless you are an old time tech with stacks of dusty manuals in your shop.

You should feel that temp difference w/in minutes of it starting, so if you aren't...but it is at least cool, you probably have a slow leak that has just reached it's critical point. A unit that old will require R-12 which is very expensive. Hopefully a tech can just give you enough of a refill to keep you going, but they may be averse to doing that. I believe there's some sort of law that they have to find the source of a leak and fix it? I don't know how closely that is followed or even if I am correct about it. Even if they do, you may be spending $300-500 that might only last a few weeks, til the end of summer...or it could last 2 or 3 years (but doubtful).

I do know, they are going to want to sell you a whole new system, which based on the age of yours will probably require new duct work as well...$$$$$$.

Looks like you are in the Dallas metro area, so I know prices aren't cheap. Depending on your situation, you may want to struggle by with window units and fans if possible. At least til you have enough for the new system.

 

Yeah, I scoured the system. Only one filter which I replaced, but still having the same issue. I'm going to track down if I have a leak. Not going to leave it upto some random tech who doesn't give 2 s***s. That stuff in there is pretty nasty, so worth the effort.

I'll have a tech come out to top it off. I've had it covered under a home warranty for the past 4 years. So, supposedly they'll replace if they can't fix, which I think they'll do everything they can to fix and not replace.

Either way, gotta get it done.

Once I find a leak, I'm guessing the tech knows how to seal?

 

Unless it's big, you may not find a leak...they can be anywhere. Back side of the coils, very slow at a service port, pinhole in a line set. Tech will prob need to troubleshoot with a sniffer, dye or something else...unless it's quite obvious. There's no magic can of sealer that can be used, it will just make matters worse and ensure you'll need complete replacement in a short time. Brazing pinholes or replacing or bad components is the only real fix.

 

Ah, should have read through before hitting the attic. I didn't find anything blatant. So, will see what the tech says.

I was curious... are the air returns supposed to be high on the wall? Or is it ok for them to be either high or low? I would expect them to be high all the time... that way it's "hot" air being cooled at all times.

 

I've had houses where they were on the hall ceiling upstairs and low on a wall downstairs. My last house had one low in a hallway, this house is one in the ceiling in a hallway.

Kind of a compromise I guess, depending on home design and system installed.

As to "I would expect them to be high all the time... that way it's "hot" air being cooled at all times.", what about in heating season? You'd want them low then, right?

Like I said...no HVAC expert.

 

Had a r22 leak at the valve. Explains the repeated recharge with no success.

read into the air return issue. Some are for cooling the others for heating.

thanks for your help and info!

 

Did he fix the leak? That should have been a relatively simple repair. Simple doesn't mean cheap.

 

They replaced the valve, yes. But, the unit froze up the following morning even after the 3lbs. It is cooling better, but should it still be freezing?? I don't think so.

A friend of mine suggested that there has to be another leak if it's freezing at all.

Thoughts?