Air conditioner cycling

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steveinville

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May 16, 2018
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My roof top A/C unit runs this way:  both the compressor and fan come on and go off together.  The problem is that at night, it's hard to sleep because it alternates from the noise of the unit, and then full silence.  And then when it kicks on again, it wakes us up.  I'd like to modify it so that the fan will run constantly, with the compressor cycling on and off as it normally would.  Any ideas?  Would it work to pigtail a power and neutral line from where the 120 V comes into the A/C, to the fan, running the power line through an on/off switch?  Then I could flip that switch to turn on the fan, and presumably set the temp where we want it to control compressor.
 
It may help if you provide the model number of your AC unit. Service and operator manuals may be available online.
From your description, it would appear that you have a malfunction of the fan/cooling control, but that is not clear in your post. You say you want to modify how it operates and not repair it, so that is the reason I'm asking.

For example, most have a control panel that has settings for "system" that allows selection for Cool, Fan, Off, Heat on the panel and also on the panel is another selection for FAN, with options for Auto, Low, High. When the FAN control is in Auto mode, the fan will come on when the AC compressor is commanded on by the thermostat, so both compressor and fan come on at the same time. If the fan control is set to Low or High, the fan runs continuously and the compressor cycles on as commanded by thermostat.

Provide some info on your make and model and I'm sure someone will provide some help.
Good Luck

 
Ok  I will discuss 3 different A/C/s I have worked with

1: Carrier Air V,  I/R Remote.. one of the buttons (on the right) is "FAN" as I recall w/o loooking and it selects AUTO (Default) HIGH and LOW  I use HIGH constant air circulation

2: My Coleman has "Local" (Ceiling mounted) Controls It does not have an AUTO setting, all settings save OFF the blower blows 24x7.

3: Most wall mount. There is a FAN switch  AUTO or ON or AUTO LOW HIGH depending on the T-Stat.
 
Many units will run the fan full time if you select Fan Speed or Low or High on the thermostat (rather than Auto). What t-stat & a/c do you have and does it have settings like that?

I doubt if your pigtail idea would work well, but have never investigated to find out. The fan and compressor are controlled by relays on the a/c control board, so the only way to bypass that would be to wire direct to the fan. Not sure what that would do to the system.
 
We have the same problem re. trying to sleep with the cycling AC. We just turn the fan select to Low instead of Auto and it's like constant white noise. I was concerned about disturbing neighbors with the fan on but it's a lot louder inside the RV than out.
 
I should have provided a bit more info ... it is a Dometic unit with "local" controls -- no wall mounted thermostat.  The only controls are 1) AC low and high, and 2) temp.  There is no separate control for "fan", or "auto".  That's the problem.  This is a small trailer so I supposed it's a very basic AC unit.  So I assume that in order to get the fan to run constantly, I'd have to modify, if possible.  Thanks for the feedback.
 
steveinville said:
I should have provided a bit more info ... it is a Dometic unit with "local" controls -- no wall mounted thermostat.  The only controls are 1) AC low and high, and 2) temp.  There is no separate control for "fan", or "auto".  That's the problem.  This is a small trailer so I supposed it's a very basic AC unit.  So I assume that in order to get the fan to run constantly, I'd have to modify, if possible.  Thanks for the feedback.
Click to expand...

I have a similar model in my cargo trailer with the same type of control. I had planned to make a modification to the controls to add a toggle switch to run the fan separately but haven't got to that project yet.  The intent was to do exactly what you described except that I also wanted to just run the fan for circulation when A/C is not needed.
If you have access to your exact model number, maybe you can find a service manual with a schematic or wiring diagram and we could proceed from there with offering some suggestions.
Reggie
 
  Technically speaking, it should be fairly easy to install a jumper wire from the incoming 120v supply for the compressor/fan start relay, go thru a toggle switch, then connect to the fan wire coming from the relay. There should be no need to mess with the neutral at all. You probably will have to physically disconnect the wire to the fan from the relay and connect your new jumper wire to the wire to the fan, unless you are lucky enough to have a relay with separate contacts for the fan and compressor. Not trying to give wiring advice here, but it sounds feasible and a licensed electrician could easily do what you are asking if you don?t feel comfortable with it.
 
Boonieman ... I finally got around to trying what you suggest.  The problem is that there are 6 wires going into the fan motor.  I played around with them and have identified that 2 of them are for the low and high fan speeds.  Not sure how I would proceed with the jumper.  Would I pick one of those two, having either a manual control for low fan, or for high fan?  Also, it seems that the fan wires still need to go through the relay in order to have auto fan control.  And would it be a problem "feeding" 120 volts "backward" into the fan speed controller?  Might be too complex to make it happen!
 
steveinville said:
Boonieman ... I finally got around to trying what you suggest.  The problem is that there are 6 wires going into the fan motor.  I played around with them and have identified that 2 of them are for the low and high fan speeds.  Not sure how I would proceed with the jumper.  Would I pick one of those two, having either a manual control for low fan, or for high fan?  Also, it seems that the fan wires still need to go through the relay in order to have auto fan control.  And would it be a problem "feeding" 120 volts "backward" into the fan speed controller?  Might be too complex to make it happen!
Click to expand...

Although your post was addressed to Boonieman, I thought I'd reply with my thoughts on how you could modify the AC unit. If you have access to an actual schematic of your AC unit, a more detailed plan could be made.

I've attached a PDF document that describes a method for bypassing the fan speed control using a SPDT switch. Using an external switch, no problem backfeeding the control board.  As always, be safe when modifying appliances that have lethal voltages.
 

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  • RV Air Conditioner fan speed by-pass.pdf
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