Central AC issues and questions

[Graybeard]

This is a 2 zone system and the AC control is only on the bedroom thermostat which also controls heat in that zone. The other zone is the living room and that thermostat is for heat only no AC control out there. This is a Trane central air system with the unit outside on the east side by the bedroom window and the air handler is in the attic. This was installed in the winter of 1999 during the construction of the house. I have no trees for shade and I have worked this unit hard for 23 years so I know it may be getting to the point of replacement. In this heatwave, AC guys must be working 24 X 7 so I hope to limp along for a week or 2 and investigate what to buy, I know nothing about AC but I do know this Trane unit has been very good. What explains the following behavior by the unit?

Last night the AC was blowing barely cool air, not warm cuz it was slightly cool but certainly not cold. The bedroom thermostat read 76° in the room and was set to 73° for the AC and that room is on the northeast corner of the house so the sun isn't hitting it after about 11 am and the living room which is the southwestern corner of the house has lots of glass and gets hit by the sun all afternoon was 80° in the room. In the past, this warm air issue was due to refrigerant that leaked and refilling it resolved the problem. This was done three times, once in 2005 and again in July 2020 and July 2021.

So here's what I stumbled upon last night and don't understand. Starting a little before 9 pm, I shut the AC off to give it a break. I waited about 20 minutes and turned it back on setting the AC thermostat to 70°. The air was cooler than when I shut it off. In about 15 minutes the air felt a bit warmer again so I shut it off again. I did that cycle for 1 1/2 hours. The odd thing is shutting the AC off and letting it rest resulted in air that was a little cooler and it was a little cooler in the house. When I went to bed, I reset the AC thermostat to 74°. When I woke up 4 hours later it was cool in the bedroom and then I lowered the AC thermostat down to 72° hoping the bedroom would be 72 in the morning and much to my surprise it was. Outside all night it was in the low 70's and humid so I don't think the night temperatures did anything, the cooling was by the AC.

So what does this mean? Why did the shutting it off, letting it rest allow it to blow cooler air. Also this morning, right now, I reset the thermostat to 71° and it is working fine and it is comfortable in here. If I lost refrigerant then how could it be cooling, right now the air blowing out of the vents is very cool to cold. Any thoughts as to what this indicates? I just don't want to assume all is well just to have the unit stop blowing cool air and be stuck without AC for days or weeks.

 

[38Chevy454]

If the system is under charged it can run colder (in the evaporator) and cause freezing up. Do you notice airflow reduction when the temps were not as cold? By shutting it off, you allow the evaporator to thaw. You basically are doing a manual defrost if you think about the process. Normal operating conditions the condensation drips off the evaporator, and ultimately drains to somewhere away. But if too cold, the condensation freezes on the evaporator instead of dripping off.

Since you needed topping off refrigerant last couple years, that indicates your system has a leak. Which also supports theory of being under charged as current problem.

I think you need to get an HVAC person out and check the refrigerant charge level, and potentially topping it off. Ultimately your system has a leak that will only get worse. So either find and repair, or look at new system upgrade.

 

[NW-Bound]

If the system is under charged it can run colder (in the evaporator) and cause freezing up. Do you notice airflow reduction when the temps were not as cold? By shutting it off, you allow the evaporator to thaw...

Excellent explanation and theory.

To speed up the "defrost", it's best to turn the compressor off, but keep the ventilation on. On many thermostats, this is done by selecting "Off" instead of "Cool" for the compressor, while the ventilation switch is put on "On" instead of "Auto". This keeps the compressor off, while the internal fan is kept on to melt the ice on the evaporator.

How does low freon cause frosting on the evaporator? It is counter-intuitive, because low freon should not cause increased cooling. No, the cooling is not increased overall, but rather it causes overcooling of one spot of the evaporator, while the rest of the evaporator is not cooled.

When freon pressure is low, but not too low that the system provides no cooling at all, the liquid freon evaporates immediately at the point where it enters the evaporator. This spot gets awfully cold and causes local frosting, but the rest of the evaporator is not cold. Overall cooling capacity is reduced.

If the freon pressure is higher, the liquid freon takes longer to evaporate, and gets to flow further down the evaporator while still in liquid form. This provides a more even cooling of the entire evaporator.

In short, this AC needs more freon again due to the ongoing leak.

 

[Aerides]

In short, this AC needs more freon again due to the ongoing leak.

In short, this 23 year old AC is long past its expiry date, and needs to be replaced.

 

[target2019]

I wonder if the lines have been tested? I'm not familiar with the intricacies of this, but a lineset replacement will need to be done with a new system. Just a random thought.

I suppose a competent, honest technician could determine where the leak is. Maybe I'm wrong.

 

[NW-Bound]

I should have elaborated further on the frosting process of the evaporator.

The frosting first forms on the corner of the evaporator, where the liquid freon enters it. Once that corner is iced over, the airflow is blocked there. Liquid freon needs the warmth to evaporate, so it now stays in liquid form and flows further down the evaporator and flash-boils there. The frosting slowly spreads across the entire evaporator, and the airflow is eventually entirely choked off.

Where it is super dry like my location, there's little humidity for this frosting to occur. When my AC ran low in freon, the cooling was gradually reduced down to nothing. The airflow remained high.

What flagged my attention was that I monitored the power draw of the AC from my solar+battery system, and saw that it drew high power, but provided little cooling. The variable-speed AC cranked at max speed while pumping little freon. It eventually displayed an error code about low refrigerant, but it was way late.

 

[NW-Bound]

In short, this 23 year old AC is long past its expiry date, and needs to be replaced.

It definitely needs replacement. In an emergency, a bit of added freon will buy some time. This may be against the law, and still costs money for a visit from a service guy.

I wonder if the lines have been tested? I'm not familiar with the intricacies of this, but a lineset replacement will need to be done with a new system. Just a random thought.

I suppose a competent, honest technician could determine where the leak is. Maybe I'm wrong.

A very slow leak is difficult to detect. Or it may be at a bad spot in the condenser that is impossible to repair. For an old AC that's inefficient compared to modern units, it's time for it to go.

 

[Car-Guy]

In short, this 23 year old AC is long past its expiry date, and needs to be replaced.

+1 Sounds like he is trying to nurse it along too, by turning it off to "give it a break". Been there, done that. Finding and fixing the current problem can probably be done with a little time and money (and sweat).. BUT it sounds like it's time to let it go. Probably would pay for itself with electricity savings with a more efficient unit before long. I know my new one cut my electric usage by ~20% and it stays cool. Matter of fact, as I write this, my AC just cycled off and it's set for 72. (it's almost 90 outside at this time). YMMV

 

[Lsbcal]

I don't know much at all about HVAC and have our units tested each year.

Our HVAC guy was here just a few days ago. The units are original and the house was built in 1985. The Train unit we have just does the downstairs and has operated flawlessly. We only use it on hot summer days. Generally the drill is to turn on the system at 76F and this happens on 90F+ days at about 3pm. Then the unit is turned off at 9pm and the AC unit upstairs is turns on.

Our HVAC guy who is very experienced told me that the unit was great and is longer lasting then current ones. Just need to replace a capacitor or other component as called for.

 

[OldShooter]

We have been through this. Low freon, frozen evaporator, etc. First time was about 6 or 7 years ago. Much weeping and wailing from the tech; freon is leaking, no way to figure out where, time to buy a new AC. I insisted on topping up the freon, IIRC maybe $300 as the stuff is expensive now.

System went a few years happily on the topped-up freon, then same freezing, low freon again. Same situation, weeping and wailing, recommending new system, me insisting on topping up the freon. IIRC maybe $200 that time.

This is our third year I think since the second top-up and system has been happily keeping us cool even in the current heat wave.

So, you pays your money and you takes your chances. We have been lucky with a system now around 20YO. Others might not be so lucky.

 

[Graybeard]

Thanks everyone for the detailed explanations!

38Chevy454 - Yes I thought the air flow in the master bathroom was less than normal last night but I wasn't sure. It was so darned warm in the house I was not sure if it was reduced air flow or just that the air coming out of the vent was basically the same temperature as the air in the room and I didn't feel the lower air flow from the ceiling vent. I agree that since I needed coolant in 2005, I think in 2020 (that was a capacitor replacement) and it did need coolant for sure in 2021, I know there is a leak. I hate to pay for a service call and the VERY expensive coolant every year because it seems now that the leak is enough to require attention each season. On a system 23 years old that is run hard from May through September, it probably is throwing good money after bad to just be adding coolant.

NW-Bound - "To speed up the "defrost", it's best to turn the compressor off, but keep the ventilation on. On many thermostats, this is done by selecting "Off" instead of "Cool" for the compressor, while the ventilation switch is put on "On" instead of "Auto". This keeps the compressor off, while the internal fan is kept on to melt the ice on the evaporator." I have a dial thermostat, 1 side has 3 manual switch settings Cool Off Heat, the other side has 2 manual settings Fan Only and Auto. The Fan Only blows air into the house so that is either air from inside the house or outside maybe but it runs through the air handler in the attic where temperatures are easily 120° and with it in the mid 90's maybe 140°. It seems that would be blowing HOT air into the house which is definitely not something to do as it'll require more AC to remove that heat unless I am missing something. You also mentioned super low humidity where you are, here the dew points this week are in the upper 60's to mid 70's with low to upper 90's air temperaturs so it is as humid as Florida.

 

[latexman]

I hope to limp along for a week or 2 and investigate what to buy, I know nothing about AC but I do know this Trane unit has been very good.

We can probably help. Trane is a good, reputable brand. Take a look on the specification tag on the outside compressor/condenser unit. What refrigerant does it use? What is the SEER (Seasonal Energy Efficiency Ratio)? How big is the unit, i.e. how many tons (12,000 BTU/hr = 1 ton) of refrigeration?

Here's a good history lesson I came across.
The Refrigerant Story: From R-22 to R-410A

 

[NW-Bound]

I have a dial thermostat, 1 side has 3 manual switch settings Cool Off Heat, the other side has 2 manual settings Fan Only and Auto. The Fan Only blows air into the house so that is either air from inside the house or outside maybe but it runs through the air handler in the attic where temperatures are easily 120° and with it in the mid 90's maybe 140°. It seems that would be blowing HOT air into the house which is definitely not something to do as it'll require more AC to remove that heat unless I am missing something. You also mentioned super low humidity where you are, here the dew points this week are in the upper 60's to mid 70's with low to upper 90's air temperaturs so it is as humid as Florida.

No outside air! Central AC ductwork is never vented to the outside air.

Usually, the fan is wired to turn on in conjunction with the compressor. Some thermostats have a feature that keeps the fan running for a minute or two after the compressor has cycled off. This is to squeeze the last amount of "cool" left in the evaporator (air handler).

When the fan is running without the compressor on, the air discharged at the outlet feels a bit warmer. That's just the heat gain from the duct work being in the superhot attic.

Again, the central AC never sucks in air from the outside.

PS. Frost-free or rather auto-defrost refrigerators work by periodically turning on a heater inside the freezer compartment. Yes, a HEATER! How else do you melt ice?

If you do not like that, then you have to defrost your fridge the old way: take all the food out, turn the fridge off with the door open for the ice to melt. Impatient people pry the ice off with a knife. Ah, the good ole days.

 

[latexman]

Google is your friend. How Does an Air Conditioner Work?

 

[jazz4cash]

I read this quickly so maybe I missed it….low airflow is also a result of dirty/ plugged filters.

 

[Time2]

PS. Frost-free or rather auto-defrost refrigerators work by periodically turning on a heater inside the freezer compartment. Yes, a HEATER! How else do you melt ice?

Yep, my 27 year old Kenmore freezer has had ice build up covering the evaporator coils in the freezer, it then doesn't cool well. I have had to replace the heater coil twice. Both times it went open.

 

[NW-Bound]

^^^ It sounds like anathema to have a friggin' heater inside the freezer compartment of your refrigerator, but that's how the auto-defrost works.

Any modern fancy stuff still has to conform to the immutable laws of physics. Cold freezes moisture into ice. Heat melts ice. That's it.

 

[Time2]

^^^ It sounds like anathema to have a friggin' heater inside the freezer compartment of your refrigerator, but that's how the auto-defrost works.

Any modern fancy stuff still has to conform to the immutable laws of physics. Heat melts ice. That's it.

I also had a 8' x 8' x 6' walk in freezer, I had cooling problem, called a repair guy, he came out could not find a problem, he added dye to detect leaks and charged me $50. Later that day, I started looking and got my head between the wall and the Evaporator (tight fit) and saw it was all built up with Ice. I increased the Defrost time, and that took care of that.

 

[Bamaman]

It's getting more difficult getting an objective opinion on HVAC. So many of the contractors are now manufacturer owned. And there is such a wide variance in pricing of systems.

But when a unit gets 20+ years old and there are problems, it's a waste of $ to patch up the old unit. Time to replace the unit--inside and out.

I replaced my old Trane unit a year ago. I was quoted from $5300 to $10,000+ for a 3 ton heat pump with some minor sheet metal repair. I ended up finding a small independent contractor with 2 workers that replaced my unit for just over $4K.

 

[Graybeard]

We can probably help. Trane is a good, reputable brand. Take a look on the specification tag on the outside compressor/condenser unit. What refrigerant does it use? What is the SEER (Seasonal Energy Efficiency Ratio)? How big is the unit, i.e. how many tons (12,000 BTU/hr = 1 ton) of refrigeration?

Here's a good history lesson I came across.
The Refrigerant Story: From R-22 to R-410A

It's been recharged with R-22. When I Googled my unit, Trane High Efficiency XE 1000, it said 5-7 possibly 8.1 but I'm not sure. I just checked the paperwork, I may have said this is a 1 1/2 ton unit that is INCORRECT - it is a 2 1/2 ton unit..

 

[Graybeard]

Usually, the fan is wired to turn on in conjunction with the compressor. Some thermostats have a feature that keeps the fan running for a minute or two after the compressor has cycled off. This is to squeeze the last amount of "cool" left in the evaporator (air handler).

Yes my unit will blow air for about 1 1/2 to 2 minutes after the compressor turns off. I know that only cuz I opened my bedroom window once the air cooled off and heard the fan stop but air keep blowing out of the ceiling fans.

Yeah defrosting a freezer was horrible. In an apartment to speed this up I was chiseling the ice out with a screwdriver and punctured the side of the freezer! The landlord was not too happy.

 

[Graybeard]

But when a unit gets 20+ years old and there are problems, it's a waste of $ to patch up the old unit. Time to replace the unit--inside and out.

I assume the air handler in the attic is OK to not replace right? Is that where the coils are? I thought it was a fiber box that directs air to the plastic hoses and into the house.

 

[iloveyoga]

I would replace the whole system with equipment that is rated to work together and provide high efficiency. I would think you would want to keep the zoning. We really like ours.

 

[JoeWras]

I assume the air handler in the attic is OK to not replace right? Is that where the coils are? I thought it was a fiber box that directs air to the plastic hoses and into the house.

Graybeard, at 23 years, it is also time to replace the air handler. Why? I'll give you a few reasons:

1) The control board has age and attic heat on it. Ditto the blower.
2) There's a good chance some condensate water has splashed and is causing some corrosion in the box. There might even be a little mold. But don't panic, I'm just saying it is a wet environment with a lot of years on it.
3) The coil has seen a lot of life. The coil could be where your leak is. This is a very abusive environment. The coil goes through temperature swings (expansion and contraction) and then is doused in condensation (water), cycle after cycle. Even the best metals and workmanship start to suffer after so many expansion cycles.

If it were 10 years, I'd have a different answer.

 

[CatinKS]

Gray Beard I had the same issue as the coils were an ice ball. I took of the cover and used fans to melt the ice. After 4 hours turned back on and temporary remove the filter before the AC unit. (Before I get scolded by others, this was only temporary) this allowed the unit to keep going and not freeze up again on low Freon. I had to run like this for 7 days was as soon as any AC company would arrive. A temporary fix that kept us comfortable if you need to buy some extra time.

On a side note read where they are going to discontinue 410 freon in the near future...Since the Tech recharged mine going well and hopefully wait a few years and buy a unit that will use the R32.