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So I suppose to many this would be a case of stating the blindingly obvious, but even after 20 years in the automotive AC industry, and many many arguments with OEM engineers that the key to system performance isn't a bigger compressor, it's condenser capacity - still, a few days ago was the first time I've cleaned the condenser coils on this unit since it was installed about 10 years ago.
It's been super hot here, and I noticed that the cooling was not as good as I would expect - for a 75 deg setting, the AC was only able to maintain about 81 deg - a 6 deg difference, with outside temps around 102 and late in the day - say 5:00-6:00 pm. Finally it hit me - I probably ought to go clean the condenser.
Well, I got all the side panels off, and the whole thing was practically encased in a layer of dust and lint and who knows what all. I spent at least half an hour with the shop vac and a brush, cleaning and vacuuming up the crap, then I took a hose nozzle and sprayed outward (since the air flow is inward). Put all the side panels back on, and now I'm seeing about a 2 deg. max differential between setpoint and tstat reading, under pretty much the same conditions. The unit's running a lot quieter, too, which would make sense as I'm sure discharge pressure is quite a bit lower and compressor load is lower.
If you want to make your compressor last longer, keep that condenser clean.
A brief recap for those who don't understand how the vapor-compression refrigeration cycle works:
Without getting into the detailed thermodynamics of the thing and latent heats of vaporization and condensation, etc., basically the system circulates refrigerant in a loop, from the compressor through the condenser (these are both located at the outside unit), up through the evaporator cooling coils (in the attic or that hall closet) and back outside. You put heat INTO the system at the evaporator - that's why your house gets colder, because you're taking heat away from the air inside your house, and putting it into the refrigerant - and you take heat OUT of the system at the condenser (The coils I just cleaned). if you can't get as much heat OUT, because your condenser is covered with what's basically an insulating blanket of dust and lint, then you can't put as much heat IN, and the cooling performance of your system will suffer.
So clean those condensers. But be CAREFUL - don't bend the wee fragile fins. Every year the fins get thinner and more delicate.
Good advice. A small garden sprayer with a mix of dawn dishwashing detergent and water can sometimes be needed. Try water first and take a look, as the less put on the coils the better.
To continue with the ideas, time to put to rest that claim about vents being closed damaging the fan motor. That type of motor is robust and usually lasts longer than a compressor. What the REAL danger is rarely is mentioned:
The cooling coil is optimized to cool a certain amount of air about twenty degrees. Input air 90? Output air right after the coil s/b about 70. What happens when you close too many vents or restrict the flow? The air coming out starts getting colder... and colder... and colder... until the coil itself is not warmed enough by the air flowing over it and shifts from dehumidification into being an icemaker. That ice builds up and then starts to block the air flow between the fins on the coil, eventually creating stress on compressor and fan both. The same can happen with a heavily clogged air filter.
I have the system professionally serviced twice a year. Seems to keep it humming along.
I also change the filters on the air returns in the house on a quarterly basis. This is cheap insurance. Filters are not expensive but repairing your a/c sure is.
So I suppose to many this would be a case of stating the blindingly obvious, but even after 20 years in the automotive AC industry, and many many arguments with OEM engineers that the key to system performance isn't a bigger compressor, it's condenser capacity - still, a few days ago was the first time I've cleaned the condenser coils on this unit since it was installed about 10 years ago...
Good advice but was your system really malfunctioning? If it was 102 degrees outside I would imagine the lowest temp your system could keep your house IS 81 degrees. Most manufacturers say a system can only keep your house cool within a 20 degree difference of the outside temperature and what your system was reading is right in line with that. 102 is massively hot. I think any system will struggle to get lower than 81 on those types of days.
We have a mini split system with the living room/kitchen/dining/hallway/bathroom on the duct system but each bedroom and the basement has its own wall unit that can be set to its own temp. In my bedroom, which is usually the hottest in the house since it's got a large sliding glass door to the deck that is exposed to the afternoon sun most of the day, I have the unit set at 70 during the day, and 66 for sleeping. It keeps its cool no problem and its been averaging 95 and very humid here. These systems are very efficient.
Good advice but was your system really malfunctioning? If it was 102 degrees outside I would imagine the lowest temp your system could keep your house IS 81 degrees. Most manufacturers say a system can only keep your house cool within a 20 degree difference of the outside temperature and what your system was reading is right in line with that. 102 is massively hot. I think any system will struggle to get lower than 81 on those types of days.
That’s not exactly true…
A lot can depend on how well insulated and air tight the house is. And, if it’s in direct sunlight (think windows); plus there’s the advantage of radiant barrier.
Good advice but was your system really malfunctioning? If it was 102 degrees outside I would imagine the lowest temp your system could keep your house IS 81 degrees. Most manufacturers say a system can only keep your house cool within a 20 degree difference of the outside temperature and what your system was reading is right in line with that. 102 is massively hot. I think any system will struggle to get lower than 81 on those types of days.
I wouldn't say "malfunctioning" but after a lifetime of living in Texas houses with central air and built between 1935 and 1955, I've got a pretty good idea by feel when the system isn't performing as well as it could.
No, I didn't drag out the themocouples and measure delta T of inlet vs. outlet air, nor did I go up in the attic and compare attic temps the day before I cleaned the condenser vs. the day after, nor did I do any number of a hundred other things I COULD have done.
What I DID do was to remove a thick layer of lint and dust from the entire outer surface of the condenser fins. I don't need detailed measurements to KNOW that a heat exchanger whose surface is covered with what's basically an insulating blanket, on the side where the air enters, is NOT working as well as a clean one.
And the next day, pretty much the same outside conditions, at about the same time of day, difference between setpoint and actual at the tstat went from 6F to 2F.
That is great advice that I have found to be true in nearly all of the poor-performing AC-unit
that I have come across, dirty evaporator and/or condensor.
IMHO, maximizing airflow to both coils is easy to do and usually gives a significant cooling boost. On the current AC-unit I have, I removed the front-cover/filter to the evaporator and cut-away all of the
sheet-metal on the condenser side...........components stay cooler and more air gets to the condenser.
Sure, my setup may get a little dirtier, but it is also very easy to clean, and has to be done once a
year anyways. The AC-filter, why.........I am not using it to clean the air, all it does is reduce airflow to the evaporator.
Good advice but was your system really malfunctioning? If it was 102 degrees outside I would imagine the lowest temp your system could keep your house IS 81 degrees. Most manufacturers say a system can only keep your house cool within a 20 degree difference of the outside temperature and what your system was reading is right in line with that. 102 is massively hot. I think any system will struggle to get lower than 81 on those types of days.
There are two temperature differentials at play -- the delta between indoors and outdoors, and the difference between the return air temperature and the air delivered by supply vents.
The latter is the 20°F difference usually referenced, and is purely about indoor air temperatures.
Yes, there is a limit to the "heat rejection" capacity of the outdoor condenser unit, however a system properly designed for your climate will continue to maintain comfortable (70-76°F) indoor temperatures even when outdoor air temperatures are well above 100°F., assuming a tight insulated building with modern window glass, etc.
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Misunderstanding the "20°F" rule of thumb
