A/C Short Cycling

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HudsonDIY

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Its hotter than.. well pretty much anything short of the surface of the sun in Florida right now and of course our A/C is working overtime. However, not working as hard as it should. It is cutting off short of the temperature set on the thermostat. While its running it puts out very cold air and when the outside temps cool off at night the house cools to the set temp. In the heat of the afternoon the A/C is short cycling. Thermostat set on 76, A/C shuts off at 81. I know this is a very general question and I haven't provided any specs on the system but I'm just trying to get an idea of what I might be up against.

By the way, yes the drip pan has a micro shut off switch. That's not it. The pan is pretty much dry when the unit shuts down with the exception of a small amount of condensation.

We just moved into the house in March, the unit was serviced and a Freon check was done. I can't really speak to the reliability of the company or service guy doing the service he was provided by the realty company.
 

WorthFlorida

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You're right, it been a hot sucker this summer. Everyday it been over 90 since about the beginning of July.

You are low on refrigerant. When the refrigerant is low, the dew point drops below freezing and ice forms on the coils in the air handler. When it is iced over it blocks air flow. This places a bigtime load on the compressor and you may notice that it make a much louder noise when running. Finally the low pressure switch cut out the compressor. As the fan in the air handler is still running, the warm air start to melt the ice and when it melts enough to allow air flow, the ice melts and you get a nice blast of cold air. As it melts the pressure built=ds a little and the low pressure switch closes and the compressor starts up again and the cycle repeats.

Since it may have been recently topped off it may mean that it was low when you bought the home. Slows leaks are on the low pressure side, fast leaks are on the high pressure side. This has nothing to do with the service company you used.

The only thing to do is you need a tech to put gauges on it. Depending on the age and type of refrigerant will depend if it needs to be replaced or not. Usually, when the refrigerant gets low it is because of a leak on the low pressure side and that is usually at the coil inside the air handler. If you have an old unit and it is R22, do not waste any time or money to get it fixed, replace it. If it is a newer unit, R134a refrigerant, cost to replace the coil could be nearly 1/2 the cost of an entire new unit. Some techs are good and if they can find the leak, some can braze it to patch the hole, but most will no do it. New units have all aluminum coils so repair is usually not possible since most techs do have the tools to braze or weld aluminum.

New AC units are now quite efficient. If you go with a new one do get upto SEER 16, SEER 14 is old tech, over SEER 16 it gets expensive. If you are new to Florida, depending on the size of the unit (tonnage), replacement cost should be between $4,000-$5,000 dollars. If you want more information when asking for bids on what you should be asking, please reply to this post.
 

Dana

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"If it is a newer unit, R134a refrigerant, cost to replace the coil could be nearly 1/2 the cost of an entire new unit. "

I thought most recent-vintage and current AC units were designed to run R410A, not R134A ??? Was there an era when R134A was being used in split AC? (Seems relegated to automotive AC now.)
 

bgard

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unless this a high end condensing unit it most likely does not have a low pressure cut out switch on it, most construction models don't. if it was low on refrigerant the evaporator would become a block of ice and there would also be icing on the suction line near the evaporator and condenser, the air handler overflow pan would also most likely be flooded with water. there is a higher probability that the system is over charged with refrigerant and dirty condenser coils causing the pressures to increase which in turn causes the evaporator temperature increase and capacity to reduce which makes the system run but not lower the temperature or humidity until the ambient temperature outside lowers enough to allow the condenser to actually condense the refrigerant and actually transfer some heat to the atmosphere. if the system thermostat is actually turning off at 81 and the thermostat is set to 76 that sounds like a thermostat calibration issue.
 

HudsonDIY

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Well the mystery is solved.

I pretty much ruled out low refrigerant right off the bat since there was no signs of freezing. I knew there wasn't a problem with the thermostat since when it was not hot as blazes the unit performed flawlessly and shut down at the proper temp. That also led me to put any electrical problem low on the list.

Shining a flashlight down into the condenser I discovered it was clogged with garbage and leaves. The unit sits under a very large tree and it seems through the years the leaves have found their way through the grill and fan and piled up in the bottom of the unit creating a nice insulating blanket keeping the compressor nice and hot. Additionally the condenser coil was full of garbage. That combined caused the high pressure switch to shut the unit down before it reached the thermostat setting but only when the ambient temps outside were high enough to affect the compressor and condenser cooling. You'd think at some point in the last 14 years some service guy might have been conscientious enough to check, guess not. At least the last guy should have.

Thanks for all the replies!
 

Dana

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I would think that a homeowner would be checking the crud build-up in the outdoor coil at least annually, if not more frequently. I'm not sure if that would be on the checklist with service guys when checking a system that's nominally working without symptoms.
 

HudsonDIY

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I would think that a homeowner would be checking the crud build-up in the outdoor coil at least annually, if not more frequently. I'm not sure if that would be on the checklist with service guys when checking a system that's nominally working without symptoms.

There was supposed to have been a full system check done. If you ask me the job was half-a**ed done. I'm sure he wouldn't have been happy with being paid for only the half he did.
 

Reach4

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I would think that a homeowner would be checking the crud build-up in the outdoor coil at least annually, if not more frequently. I'm not sure if that would be on the checklist with service guys when checking a system that's nominally working without symptoms.
What would be on that PM checklist? Checking for build-up on the outside coils would seem to me to be important.
 

jacobsond

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A full system check requires a clean condenser coil. if your tech does not check that you need a new tech. Pressures on the gauges will not be correct and a tech that does not verify a clean coil could add refrigerant or take out some because of the pressure readings. If you paid someone to check your system and found out the condenser was plugged after they left you need to get your money back.
 

Dana

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What would be on that PM checklist? Checking for build-up on the outside coils would seem to me to be important.

Cruddy evaporator coils and filters are a more common coil problem, when there IS a problem.

HudsonDIY said the real estate folks' contractor checked the refrigerant levels, and I suspect they took a few temperature measurements. Since the thing was reportedly working almost to spec there probably wasn't an alarm bells going off in that type of testing.

We have no idea what the contractor was actually asked to do, and it may even have been OK six months ago when the contractor was out there.

But as a homeowner even though my AC runs at a tiny fraction of the duty cycle seen in FL, I always check the outdoor coils for crud & damage when I pull the covers off in spring (it stays covered to avoid being filled with snow- we get 10+ feet some years) and always eyeball the evaporator coil when changing the filters (had to remove a dead mouse and a few leaves from the indoor plants out of the evaporator coil once) Maybe once in five years is it worth vacuuming out the crap in the condenser despite nearby overhanging oaks & pines. In the 20+ years since it was installed there has never been a "full system check" or any service/maintenance call of any type. YMMV.
 
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