RV AC Turns On With The Furnace? Two Fixes (One Easy, One Hard)

You wanted to warm up the RV, but you got blasted with cold air instead. You may be wondering, why does my AC come on with my furnace in my RV? Whether you are living off the grid or taking a trip in extreme temperatures, it is important that you understand how your HVAC system works in order to run it properly. If you’re camping in sub-zero weather, the last thing you need is to turn on the heating and for it to fill the RV with freezing cold air.

Why Does My Air Conditioner Come On When the Heat Is On?

It may seem like the AC in your RV comes on with the furnace, but it is more likely that the fan on the furnace is turned on. If you reset your furnace to auto, then it won’t blow cold air. Another possibility is that the HVAC is blowing cold air because something is broken and it can’t generate enough heat.

If your furnace or heater won’t turn on or won’t blow warm air, that is a sign that something needs repair. In some cases, it may be necessary to replace the unit. For larger RVs, dual-zone thermostats are your best option for keeping your RV cool in the summer and warm in the winter.

Sources include Learn To RV, The RVGeeks, RV Share, Dometic, and Life on Route.

How To Fix An RV AC Turning On With The Furnace

To answer this question, you have to know what kind of climate control system you have. Some RVs have a combined HVAC unit, which both heats and cools the inside of the RV. Other RVs have the AC and the heating separated into 2 different units: the furnace and an overhead air conditioning unit. 

If you have a combined HVAC system, there are 2 possibilities for why the AC comes on when you turn up the heat.

The Easy Fix: The Fan Is On

The first possibility is that the AC seems to come on with the heat because the fan on the unit is turned on. If the fan is on, then the unit will blow cold air no matter what the thermostat temperature is set for.

This one is an easy fix. All you have to do is turn the fan off, and the unit will no longer blow cold air on you when you turn on the heat.

Depending on the model of your HVAC system, this can be done by selecting the fan function and hitting the On/Off button to turn it off. It can also be done by changing the thermostat mode from Fan to Auto.

The Harder Fix: Something Is Broken

If turning the fan off doesn’t solve the issue, the next possibility is that something is broken. Sometimes a furnace will only blow cold air, which feels like AC, because it is unable to generate warm air. 

The most common issue is that there is an issue with the fuel source. Check to see if you need to replace your propane tank if the propane valve is open and if the propane regulator is clogged.

If your propane tank is full, the valves are open, and the regulator is functional, then the problem may be with your sail switch or low battery voltage. 

A furnace may be unable to generate heat if there is not enough power for it to operate the blower that moves air through the furnace itself. If the blower is inoperational, that can be a dangerous safety hazard for your RV. The sail switch is a safety mechanism that is designed to shut down the blower before the furnace ignites if the blower does not have enough power.

To get your furnace working again, check your RV’s battery levels and plug into shore power if necessary. If you have enough power to run the furnace, it is possible that the sail switch is stuck. It can be repaired with a good cleaning and the installation of an insect screen to prevent insects from clogging up the switch.

The last thing that could be preventing the furnace from generating heat is if the ignitor isn’t working. If the combustion chamber in the furnace cannot ignite, then the furnace has no way to produce heat. 

The best way to test the ignitor is by removing it from the combustion chamber while keeping it hooked up to the furnace, and then turning on the furnace and see if the ignitor produces any sparks. If you do not see sparks, then the ignitor may need to be replaced.

Does the Furnace Run When the AC Is On?

In an RV, like in a home, the furnace and the AC are separate systems. Depending on the model of your RV, you may have a combined HVAC unit or a separate furnace and rooftop air conditioner. For both types of climate control systems, the answer is no.

In a combined system, the AC function uses the same fans and filters as the furnace to distribute cold air. But the air conditioner itself is taking hot air and cooling it, whereas a furnace takes cold air and warms it with the combustion chamber. If a unit was trying to heat air and cool air all at once, it would overload the unit and not make the inside of the RV very pleasant. 

In a separate system, the furnace should be off when the AC is on. If you want your RV to be cooler on a 100-degree day, it doesn’t make sense to be blasting hot air through the inside of the RV. 

The thing that complicates this is the heat pump. Some rooftop AC units have a heat pump, which allows the unit to act like a small-scale furnace to help warm up the interior. 

Unlike a furnace, a heat pump uses electricity to warm up the air, rather than propane. If your RV AC unit has a heat pump, then you will need to run your AC in order to work the heat pump.

How Do I Know If My Central Air Thermostat Is Bad?

There are 5 signs that your thermostat is in need of replacement:

  1. The screen stays blank.
  2. It does not respond to temperature changes.
  3. The thermostat is showing error codes that can’t be dismissed.
  4. A multimeter or Ohmmeter shows that there is no output from the thermostat.
  5. Troubleshooting like changing the batteries and resetting the thermostat do not fix the problem.

How Do You Use the Dometic Dual Zone Thermostat?

A Dometic dual zone thermostat can be used to replace a broken thermostat. This model works with a rooftop air conditioning unit that has a heat pump. After the thermostat is installed, you will need to press the power button twice to turn it on. 

The Auto setting will keep the inside of the RV at a set temperature. If the thermostat is on High or Low then it will blast hot or cold air continuously. 

What makes the model unique is that you can set two zones for your RV. This is ideal for larger RVs where the back bedroom area can get chilly, and the front area can get too hot. After you set the zones and the desired temperature, the thermostat will turn the AC unit and heat pump on and off to keep those areas at a comfortable temperature.

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