A/C question

[Sawdust]

Can frost build-up on the evaporator coil cause my A/C to short cycle?

The system works fine during the day, but late at night it will run for a while then shut off, but then two minutes later it will kick on again.

Rinse and repeat. Shut it off at the tstat for the rest of the night, and it runs fine again the next day until the late evening.

It's been unusually humid, too.

So, could this behavior be caused by frost/ice build-up on the evaporator coil?

Thanks,

Sawdust

[Brad Johnson]

Yes.  Have you noticed reduced airflow during the times when the air handler is short cycling?

Call a tech and have the system serviced.  Something is amiss.

Brad

[280plus]

Is the air filter clean? Is it running continuously because it cannot reach set temp?

[Sawdust]

It reaches set temp.

I'm just wanting to know if there is some sort of sensor that detects either reduced airflow across the evap coil, or senses frost/ice and then shuts the system down.

I will be checking the filter today; my guess is that it needs cleaning.

Sawdust

[Brad Johnson]

Is the air filter clean? Is it running continuously because it cannot reach set temp?

Forgot about that.  In that line of thought, make sure you know where ALL your air filters are.  Cousin of mine was having problems with her system that looked like airflow problems, but all the filters were fresh media.  Turns out a previous owner had put in garage shelves by opening up a section of wall that he thought was just unused, wasted space between the framing.  Turns out that unused space was the main return.  Oops. 

I guess the 12x24 opening in the bottom of the return space and the big hole at the top with the ducting attached wasn't obvious enough to merit a pause for consideration, so he walled over it.   

Brad

[K Frame]

The important thing is, what KIND of A/C do you have?

Is this a large outdoor unit with an air handler?

A indoor console unit?

A windowshaker?

The answer may not be the same for all types.

[Sawdust]

I dunno what the term is.

Compressor unit outside; combo gas furnace / A/C unit inside...

Sawdust

[Nick1911]

That would be a split system.  Is this a heat pump?

The unit might be cycling off on low pressure due to ice/low airflow.  If the evaporator case comes apart easily, take a look when you're having the problem.

How old is the indoor thermostat?  It seems to rarely be the problem, but a consideration.  When the outdoor unit short cycles, can you hear the CLICK of the contactor? 

[Sawdust]

No, not a heat pump.

The tstat is about five years old.

Yes, everything sounds normal when it kicks-on.

So, there *is* some sort of sensor that detects either low airflow across the evaporator or ice and will shut the unit down?

Sawdust

[Nick1911]

So, there *is* some sort of sensor that detects either low airflow across the evaporator or ice and will shut the unit down?

Not that I've ever seen in a residential split.  But, an iced over coil causes the temperature of the refrigerant to drop, and with it the pressure [It's a saturated mixture].  Some systems have low pressure switches on the suction side which might trip in this condition.

[Sawdust]

Ah!

Thanks - I'm betting that this is what is happening.

The system runs fine throughout the day and early evening.

I'm expecting a clogged air filter as I haven't been on top of the maintenance since starting my kitchen remodel 10 weeks ago...

Sawdust

[Jim147]

A freeze stat on a resi unit is rare but some installers put them on.

Is everything shutting off or just the inside blower? Could be tripping on internal limit due to lack of airflow or weak cap.

Change the filter and keep an eye on when it is doing what.

jim

[AZRedhawk44]

Ah!

Thanks - I'm betting that this is what is happening.

The system runs fine throughout the day and early evening.

I'm expecting a clogged air filter as I haven't been on top of the maintenance since starting my kitchen remodel 10 weeks ago...

Sawdust

Perhaps linked?  Extra-cloggy filter?

[280plus]

Right, if it's freezing it could go off on a low pressure switch.

[Cliffh]

Might it possibly be low on freon?

Earlier this year ours was acting similar; run OK during the day & froze over at night.  Turned out to be a couple pounds low on freon.  Filled 'er up and been running fine since.

[Sawdust]

Cleaned the filter, but it started doing the cycling thing again at about 9:00 tonight.

The internal unit shuts off, then restarts after 2-3 minutes. The compressor fan outside never stops.

Tstat says that the room temp is at the set temp.

Could be low on Freon, I guess.

Sawdust

[AZRedhawk44]

Cleaned the filter, but it started doing the cycling thing again at about 9:00 tonight.

The internal unit shuts off, then restarts after 2-3 minutes. The compressor fan outside never stops.

Tstat says that the room temp is at the set temp.

Could be low on Freon, I guess.

Sawdust

Bad contactor switch?  It's just a a 24v DC magnet that pulls a larger contactor into place to close a circuit that starts the condenser.  They get fused shut or blocked open by dead bugs pretty often.

[geronimotwo]

try disconnecting the t-stat wire and see if the compressor (outside) shuts down immediatly.  if not, there is a relay problem at the compressor or on the main board inside the air handler (in house).

Forgot about that.  In that line of thought, make sure you know where ALL your air filters are. 

i did a service call on a gas hot air unit where the unit was short cycling, and the temp would not stay warm enough in the winter.  the owners had just replaced "the" filter, but there was very little air flowing.  turns out that in addition to that filter there was an electrostatic filter pack between the plennum and the furnace that hadn't been cleaned since they had bought the house.

[280plus]

Now,with a clean filter, you can somewhat safely assume it's low on freon. All other possibilities must be explored prior to getting on the low freon bandwagon. Make sure they locate and repair a leak if it is. freon should never escape.

[Brad Johnson]

Make sure they locate and repair a leak if it is. freon should never escape.

I wish every AC tech had that mindset.  A lot will just keep shoving refrigerant into the unit every year rather than fixing the leak once.  That, or the homeowner is perfectly happy spending $200 a year in perpetuity having the unit topped off rather than spending a couple hundred more to have it fixed properly.  During my real estate days it was kinda a recurring theme.

Brad

[Triphammer]

Flip the "Fan" switch on the stat to "ON" from "Auto" & let us know what happens. There's a single contact that opens R & Y in auto. Only opens Y in On. There's no reason the evap fan should shut off from the stat if the compressor is still calling.

[Nick1911]

Flip the "Fan" switch on the stat to "ON" from "Auto" & let us know what happens. There's a single contact that opens R & Y in auto. Only opens Y in On. There's no reason the evap fan should shut off from the stat if the compressor is still calling.

This.  Perhaps the indoor blower motor's internal high limit is opening for some reason?  Could just be a weak run cap for the blower or the bearings could be failing.  Of course, this doesn't explain when it only occurs at night...

[Jim147]

I've had one stat that was dropping G in cool this year but it was an old round model with the gold base. The newer stats seem to drop Y and G if they act up like that.

Does the airflow from the vent seem to be much less at night then during the day before it starts tripping off?

jim

[280plus]

I wish every AC tech had that mindset.  A lot will just keep shoving refrigerant into the unit every year rather than fixing the leak once.  That, or the homeowner is perfectly happy spending $200 a year in perpetuity having the unit topped off rather than spending a couple hundred more to have it fixed properly.  During my real estate days it was kinda a recurring theme.

Brad

In the days when r-22 was $0.50 / lb you could get away with it. Some people (homeowners) even believe(d) you need to top off the freon every year like topping off your tires. Today the same R-22 costs ~ $60 / lb retail, here in CT anyways, so there is much more of an incentive to find those leaks. I won't even put freon into anything that has a low charge until I prove (24 hour pressure test) that it is no longer leaking. Lose all that freon cause you're a dumbass and it comes out of your (or the boss') pocket. Do it a few times and the boss will be inclined to fire your *expletive deleted*ss.

[Sawdust]

Thanks for all of the help, guys.

Sorry I haven't responded; I was refeering soccer all day Saturday, and busy with my remodel yesterday.

Anywho...if I shut-off the tstat, the compressor shuts down.

I have to believe that it is low on freon, as the problem only shows-up when the system has been working (relatively) hard for the bulk of the day. Seems to me a fried electrical component would cause the odd behavior at all times of the day.

Yesterday (Sunday) the heat wave broke and the outside high temp was about ten degrees cooler. The system never short-cycled all day.

I will get a tech to examine the system, but I think I will wait until the season is over; or at least when I am done remodeling and have more time to deal with it.

Thanks again for all of the replies - much appreciated.

Sawdust