Air Conditioner function question?

[lhemrick]

I have a 2015 Salem Cruise Lite with a Dometic roof air conditioner. It functions just fine until the outside temp and the thermostat setting get close together. When that happens the unit starts cycling on and off. Thats the best way I can explain it. It will completely stop fan and all then come back on. Sometimes it will do it several times before it settles in to run. I'm guessing this is not normal?

 

[Gary RV_Wizard]

Well, it may be "normal", i.e. "working as designed", for certain situations. One possible cause of this is an a/c duct that blows onto the thermostat. The a/c comes on and the chilled air on the thermostat convinces it the desired "set" temperature has been reached, so it cycles off. As soon as the cold air ceases, the thermostat senses it is warm and gain signal for more cooling. This continues until the interior air temperature stabilizes. You may need to put some sort of diverter in the way to stop the cold drought from affecting the thermostat.

 

[Kirk]

If this only recently began to happen, but didn't do it in the past then location of the thermostat isn't likely to be the problem.

There are several conditions that could be causing the problem you are experiencing with the rooftop air conditioner in your RV.

Bad Thermostat: I am assuming that you have your RV's AC Thermostat

set to Automatic. If the thermostat is not properly functioning it could cause the Air Conditioner to cycle on and off. The thermostat when working properly should have about 3-4° from the point it starts the cooling process and stops. A few have an adjustment for that but most are fixed and either one could cause the problem if that should be too close together. I would try monitoring the temperature with a good dial thermometer at the thermostat.

Failing Control Board: a bad Control Board in the air conditioner unit itself can also cause the symptoms that you are experiencing.

There is a chance it is freezing up due to a dirty filter and or dirty cooling coil. The freeze sensor on most will keep it off for longer but that depends on the air conditioner that you have. If the humidity is high that would be more likely to happen.

 

[Lou Schneider]

What make and model is your A/C? Is it a ducted unit with a wall thermostat or is it self contained with the air outlets and controls integrated into the roof unit? Some integrated units turn the compressor and fan on and off together when the thermostat reaches it's set temperature.

 

[Henry J Fate]

The AC unit should have a delay for a restart. Delays are designed to prevent a compressor circuit overload when the high side and suction side of the system have a big differential. Typical delays are 3-5 minutes. The delay let's the system balance out before a restart. If the fan is set to auto, the fan will also be delayed when a restart occurs. If the fan is full time, the delay will apply to the compressor only. The delay circuit may be built into the thermostat so if it is shutting on an off within a mi ute or two or less and then trying to come back on, you likely have a thermostat problem.

Another factor is the freeze thermostat. The freeze thermostat is placed into the evaporator coil and will cycle the compressor to prevent freezing. If you can hear the compressor cycling then it could be the freeze thermostat.

If you have time, post the timing of things during one of these events with details of what's shutting off/on and the timing

 

[John From Detroit]

I know for the furnace there is an "Anticipator" setting on the T-stat. (Mechanical stats)
Not sure if such is the case with Air Conditioners.

One thing that can cause short cycling is the frost sensor. if the evap coils get too cold it will kill the compressor (not the blower) YOu need to know which sensor is cycling it. That is IF it has a frost sensor (Look in the evap. it's a probe struck in the coils very obvious)

 

[lhemrick]

I made a trip to the nearest Camping World and purchased a kit that came with a new control board, thermostat and frost sensor. It fixed the cycling off situation. Now I have followed the instructions to set the new Thermostat to also control the furnace. The fan on the air conditioner comes on but I'm getting nothing out of the furnace. It worked the last time I tried and I'm wondering if I put the old Thermostat on the new board to check will it damage the new board?

 

[Henry J Fate]

I made a trip to the nearest Camping World and purchased a kit that came with a new control board, thermostat and frost sensor. It fixed the cycling off situation. Now I have followed the instructions to set the new Thermostat to also control the furnace. The fan on the air conditioner comes on but I'm getting nothing out of the furnace. It worked the last time I tried and I'm wondering if I put the old Thermostat on the new board to check will it damage the new board?

Before switching the thermostat back to old, identify what the furnace is doing. Ex: Is it only calling for the fan or is it trying to light but failing to produce a flame or possibly lighting and then goes out.

Switching back to the old thermostat probably will not hurt the new board but the question cannot be answered with 100% certainty.

 

[Lou Schneider]

The only signal the thermostat sends to the furnace is a contact closure to start the heating cycle. Opening the contacts tells the furnace to shut down. Everything else is handled internally by the furnace.

If the fan is running the furnace is receiving the contact closure from the thermostat. You can confirm this by directly connecting the furnace wires together at the thermostat.

 

[DonTom]

Isn't that issue usually solved by using a thermostat that has a much different turn-on than turn-off temp?

Something like set for 68F, heats up to 70, then shuts off until the temp goes down to 66F and back on again. An even wider range will be better to solve that problem, if you're not too sensitive to a few degrees difference.

-Don- Ashland, OR

 

[Henry J Fate]

The only signal the thermostat sends to the furnace is a contact closure to start the heating cycle. Opening the contacts tells the furnace to shut down. Everything else is handled internally by the furnace.

If the fan is running the furnace is receiving the contact closure from the thermostat. You can confirm this by directly connecting the furnace wires together at the thermostat.

Lou is correct.

The fan will only come on if the thermostat is calling for heat so if the heater fan is running, the thermostat is working and calling for heat. It is not possible to mis-wire the thermostat or a bad thermostat and create a fan only condition.

Check closely to see what the heater is doing when heat is called for.

 

[lhemrick]

The fan on the roof unit comes on to move the heated air throughout the interior but there is no a sound coming from the furnace itself. No clicking like its trying to light its fan does not come on nothing.

 

[Henry J Fate]

Sounds like the thermostat has been wired incorrectly.

When you have a moment, please post your system details. Models etc.

 

[lhemrick]

Thanks for all the info. I found the issue and its working correctly now. There was a loose connection inside the furnace. I have no idea how it came loose but it had. All that matters is it works now.