AC Compressor shut off - start up time

[JHZR2]

Hi,

My father alwyas goes by the rule that if you shut off an AC compressor, it should stay off for at least 3 minutes before you turn it back on again... It can cycle how it desires, but main shut off/start up should wait at least that time.

He read this long ago, but was more or less unsure why... he just knows that having 10+ year old cars that still have the original charge, cold air and operate great (he also ALWAYS has the AC on, even in the winter) is indication that something is right about what he is doing.

Im curious if anyone has heard of this 'rule' and can speculate why it is so.

THanks,

JMH

 

[kenw]

unfortunately, on most vehicles shutting it off with the "power" button is no different that allowing to to cycle. Both merely activate the compressor clutch.

His rule would hold true for building/residential AC, as these compressors do not cycle like automotive units. And yes, for those types of units, the 3 minute rule is a good one. In fact, many building t-stats or compressor relays have timers built in to prevent rapid cycling.

The reason his AC charge lasted so long is that it is used often. Infrequent use is the leading cause of leaks as seals dry.

 

[JHZR2]

Thanks!

Sop what is the reasoning behind leaving it off? Is it so the refrigerant can get heat soaked, pressures reduced, compressor cooled down, or something else?

I appreciate the info... I was not aware that building and residential AC systems dont cycle... Here is a dumb question: why dont they cycle, and auto units do?

JMH

 

[XS650]

quote:

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Originally posted by JHZR2:
Thanks!

Sop what is the reasoning behind leaving it off? Is it so the refrigerant can get heat soaked, pressures reduced, compressor cooled down, or something else?

I appreciate the info... I was not aware that building and residential AC systems dont cycle... Here is a dumb question: why dont they cycle, and auto units do?

JMH

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Actually, they do, just at a very different rate.

A car AC may cycle more often than once minute. A house AC typically cycles a very few times an hour, or not at all if running at full capcity.

Car ACs have large over capacity so they can cool a heat soaked car down to a habitable temperature in a few minutes. A house AC is much more closely matched to steady state conditions and may only be able to lower the temperature inside a house by a few degrees an hour,

Houses tend to be well insulated and kept at relatively constant inside tmperatures, so they don't need the over capacity that automotive AC units have.

 

[kenw]

Allowing the home unit to sit for a few minutes ensures that the pressures are equalized, which means there is no liquid in the system, which therefore means it can't slug the compressor.

It is why you NEVER add liquid refrigerant to the suction line fitting in either the house or car AC. This is called "slugging". As liquids don't compress, feeding liquid into the inlet side will almost guarantee damage to the compressor.

 

[JHZR2]

quote:

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Originally posted by kenw:
It is why you NEVER add liquid refrigerant to the suction line fitting in either the house or car AC. This is called "slugging". As liquids don't compress, feeding liquid into the inlet side will almost guarantee damage to the compressor.

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Thanks, ken!

Just to be sure... what youre explaining, regarding adding liquid refrigerant on the suction line fitting is NOT the same as adding a can of pressurized refrigerant to the low-pressure adapter in a car unit, right???

I had to add some r134a to my GF's integra, and I bought one of the interdynamics kits, and some DuPont SUVA r134a. Added it and all has been perfect since - over a year now and very cold air. But I dont want to do it again if it is a bad way to add refrigerant.

Thanks again for all the info!

JMH

 

[JHZR2]

aha, good to know... glad I did it right!

Thanks again,

JMH

 

[kenw]

actually it is exactly the same.

when you add a can of freon, always hold it so the top of the can is UP, that ensures that GAS is leaving the can and entering the system.

Pointing the top of the can DOWN will put liquid into the system and is bad.

 

[tom slick]

the best rule of thumb with automotive a/c is do what you want with it. turn it on, off, high, low whenever you want. you are not going to hurt it.

 

[bmwtechguy]

You can add liquid (top of can down) to the low pressure side, but you have to know what you are doing so you do not damage your compressor. First, if you add gas and not liquid (top of can up), you will probably have to place that can in a hot water "bath" to empty out the can and keep it from freezing up solid. You are right, this is the safest way to charge your system back up if you are new to this sort of thing. Remember to bleed some refrigerant gas through the hose before connecting it to the low pressure (suction) side valve so as not to introduce any ambient air into the system. It is possible to charge faster and without the hot water bath by flipping the can over, and just limiting your charge pressure by using the valve on the can to regulate how much you are letting go in. The rule of thumb I use is to not increase the pressure by more than 5 psi as it is charging. This of couse would require using one of the charging hoses that has a gauge built into it. Charging at this rate will allow the liquid refrigerant entering the valve/ac line to vaporize fully way before it could slug the compressor. Usually if a system has lost some charge over a period of time, one twelve ounce can of R134a will bring the system pressure back up enough to cool well and stop a constantly cycling compressor due to the low pressure cutout switch. Always charge with fan on high and recirc off (normal mode)on a warm to hot day. If low pressure line at compressor is cold and "sweating", you probably have sufficient charge. Hope this helps someone. I do a lot of ac work.