Air Conditioning Mystery

[bateau beau]

One of the Cruisair units on my 1985 43' Motor Yacht occasionally has fits. It cycles on and off in 5 second intervals. It might be my imagination, but when it shuts down after the 5 second "on" cycle, it sounds different than a normal shut down. If I shut it off for a couple of hours, it seems to recover and works fine.

I'm still using the old Honeywell "brick" thermostats, but I plan to change them as soon as I can find the time.

Does this cycling problem sound like a compressor problem, or a thermostat problem?

[krush]

The quick cycling thing is bad for the compressor. Traditionally it is called "short cycling" and is harmful because the compressor isn't designed to have to start when the pressures in the system have not equalized....try starting your boat in gear as an analogy.


As for compressor vs thermostat problem, it's hard to tell from your info, but easy to figure out. Turn the 'stats all the way down to 50. See if it cycles then.

[Trojan]

Check the position of your temperature bulb. It may be directly against the evap unit giving it a false reading. It thinks it's colder than it is. Try repositioning it. It also could be positioned on the low side of the evap unit where the temp is the coldest. Just a thought.

BILL

[bateau beau]

I tried cranking the thermostat all the way down, and the problem continued...............

[Boatsb]

Short cycling can be caused by low freon levels. Check the gas to see if there is an issue. It will shut off on low pressure when the gas is low and remember they are cooled by the returning freon so it is necessary to have enough in there. Also they will shut down on high pressure if they are over filled.

[krush]

As mentioned, there are many things that can cause the short cycling. Pretty sure you ruled out the thermostat.

Like boatsb said, low refrigerant charge can cause short cycling, but other things can too.

Depending on the unit:

A)low pressure cut off will shut the compressor off

B)high pressure cut off (but the simple systems usually don't have this, instead there is a pressure relief built into the compressor)

C)internal temperature cut out in the compressor cuts out due to high temp OR sometimes they can cut out due to low voltage (acts as an auto reset circuit breaker...could be external too).



A quick way to check if you have a super duper low charge is to turn the unit on full cold and high fan, go down to the compressor and put your hand on the suction (return line)....it will be the cold one. Careful, the other one may be really hot!. Simply put, the suction line should have some condensation on it or at least be cold.

This isn't the preferred method for charging or checking levels, but it will serve as a decent diagnostic tool if you don't have gauges and thermocouples to measure line temps.

[SKYCHENEY]

Check the waterflow through that unit. It sounds like a high temp cutout to me.