RV expert Bruce Bernhart on the Care and Maintenance of Your Rooftop Air Conditioner

Bernhart says: Clean the filters at least every two weeks when the air conditioner is in operation

. Running the unit with dirty or clogged filters will decrease the flow of air across the cooling (evaporator) coil, potentially causing the coil to freeze up. If the air conditioner is running for long periods of time with dirty/clogged filters, a potentially expensive cleaning of the evaporator coil by qualified service personnel may be necessary.

Bernhart says: If your motor has oil ports, we recommend using SAE 30 weight non-detergent oil. The motor requires only 1-2 drops of oil per year. DO NOT OVER OIL. The oil ports will be visible on the fan motor with the unit's plastic cover (shroud) removed.

Bernhart says: Clean the outside (condenser) coil at least once a year. Blow out any debris which may have collected on the coil. Since the A/C fan pulls air in through the back of the coil and exhausts it from the sides of the shroud, it is best to blow compressed air through the coil from the inside out. You may also wash this coil with water if you choose to do so. Remember to disconnect all power to the vehicle/air conditioner first.

Understanding how the relay works is fundamental to troubleshooting electrical problems.

So what is a relay? A relay is defined as an electromagnetic mechanism moved by a small

electrical current in a control circuit (12VDC in this case). How does this relay work? As this

mechanism moves back and forth in the relay it will open or close a set of contacts capable of

carrying high voltage (115VAC in this case). All of the control circuit relays are normally

open and the contacts close as power from the thermostat is applied. Bernhart explains that

the 115VAC power to the compressor is interrupted by a set of normally open contacts on the

relay. In order for these contacts to close 12VDC must be applied by the wall thermostat to

the relay coil. When the thermostat switch is placed in the cool position, 12VDC(+) travels

from the thermostat red wire to the thermostat yellow (Y) wire and then to the relay coil. When the coil is activated an electromagnet inside the relay will pull the contacts closed. The

115VAC will now operate the compressor until the thermostat opens or the system switch is

turned to the off position. The entire control circuit for the A/C / heating system would include the rest of the thermostat functions and possibly 2 or 3 more relays. In addition to the

compressor relay you would need a separate relay for Low Fan, one for High Fan, and possibly one for Heat, if a heat pump or electric heating element is used.

Some notes on thermostat location:

All of the air conditioning functions are controlled by the wall mounted thermostat. Bernhat

says these thermostats utilize a 12 VDC electrical circuit which is supplied by the vehicle

manufacturer or the installer of the A/C unit. Most of the thermostats provided by RecreationVehicle Products are combination (Heat / Cool) thermostats. These thermostats are capable of operating both the roof top air conditioner and any furnace with a 12 VDC control circuit. Thermostats are really nothing more than temperature controlled switches. When the need for cooling or heating exists the thermostat sends a 12VDC(+) signal to the control relays or a P.C. board which in turn energizes the air conditioner components or the furnace.

Bernhart points out that thermostats are very sensitive instruments. For accurate temperature control and comfort the following considerations for thermostat locations should be taken into account:

1. Locate the thermostat on an inside wall about five foot above the floor. Pick a dry area

where air circulation is good. The thermostat should be mounted within a reasonable distance

from the appliance the thermostat will control. This will assure a more accurate temperature

relationship between the thermostat and the appliance the thermostat will control.

2. Do not install the thermostat where there are unusual heating conditions; such as direct

sunlight, heat producing appliances (television, radio, wall lamp, etc.); or a furnace or air

conditioner supply register.

3. Note: When installing or servicing these thermostats the technician should take all

necessary precautions not to short any positive wire to ground. Permanent damage to the

thermostat may occur. Make sure all connections are good and tight. Loose connections may

cause relay chattering which leads to welded relay contacts on air conditioner printed circuit

boards.

RV expert Bruce Bernhart on the Care and Maintenance of Your Rooftop Air Conditioner

By: Bruce Bernhart

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