Why does one RV air conditioner blow warm air while driving?

By Dave Solberg
Dear Dave, 
I’ve just returned from Talladega, marking our second successful long distance road trip. On both trips we have run the generator and both air conditioner units to keep us cool. Both times while at road speed (55-65 mph) the rear air unit shows it’s cooling but warm air just barely blows out. Once we slow down or stop, it immediately returns to normal cold air flow. The front air cools normally. Have you experienced this? Is the wind on top of the coach combating the efforts of the fan and A/C? —Matt, 2018 Coachmen Sportscoach 404RB

Dear Matt,
You did not specify the make or model of the bedroom roof air conditioner. However, from what I can find on the used RV market, it looks like they used Coleman Mach 3. I would assume the front is the larger 15,000 BTU and the bedroom is 13,000 BTU. Both look to be ducted.

Since it blows normal cold air when you are stopped, it doesn’t seem to be the compressor or a coolant issue. The first thing I would look at is to make sure the evaporator cover is sealed properly and not allowing exterior air to penetrate.

Here is how the unit is designed to work

Warm, moist air is drawn from the inside by the fan that draws it across the evaporator coil. The coolant goes through cycles from liquid to gas and back which flows through coils in the evaporator fins and “flashes” the air. This draws out heat and moisture and sends it back to the ductwork inside the rig. All this should be sealed by an evaporator cover, typically a metal box that is insulated.

Here is a photo of one that we disassembled on a unit that had a clogged evaporator.

If your evaporator cover is not sealed properly, or has a few loose or missing fasteners, it could be allowing exterior air to penetrate and create a whirlwind effect that would just blow exterior air inside the rig. You would need to remove the exterior plastic cover or shroud to inspect this.

Condenser coil

Another issue could be what is happening at the condenser coil in the back. As the coolant flashes through the evaporator, it flows back to the condenser fins and the back fan pulls in exterior air to cool the coils and the coolant before it goes back to the compressor.

The coolant can reach temperatures as high as 140 degrees. It needs to be cooled down to complete the “cycle” of changing from liquid to vapor and back again. If there is a blockage in the airflow or the fan cannot pull in exterior air, the coolant will not cycle and not be able to run efficiently.

Condenser fins

Check the condenser fins to make sure they are not clogged and are straight. Hail or branches can dent and bend the fins so they cannot draw air efficiently. These can be straightened with a knife or a fin comb available on Amazon here.

Newer air conditioners have either the slanted condenser you see in the photo above, or redesigned covers or shrouds that protect the fins and draw air from the side and back.

If the fins are clear and the evaporator cover is sealed, it must be that the high air volume is creating a vacuum or void that is not cooling the lines. Check to make sure there is nothing installed around the air conditioner that might impede airflow.

An update from RV Products (Coleman)

I actually got a call from a technician at RV Products. He stated that all their air conditioners are tested in a wind tunnel to verify windflow/resistance. If there is something installed close to the shroud, it will create turbulence and slow the fan down. This in turn doesn’t cool the coils in the compressor head and allow the unit to work correctly.

MORE FROM DAVE ON RV AIR CONDITIONERS

• Help! My RV air conditioner sounds like a ‘dying cow’
• What to use to power RV roof air conditioner when dry camping
• Why does air conditioner ‘cycle’ several times before staying on?
• When using generator, why does RV roof air conditioner drain house batteries?
• Can we test RV roof air conditioner with 110-volt outlet?

Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

Read more from Dave here. 

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