Thursday, June 1, 2023


Ask Dave: House air conditioner blows air but it’s not cold. Why?

Dear Dave, 
The house air conditioner is not working. The unit comes on and the air blows through the ducts but it’s not cold. It is a Coleman-Mach 15,000 BTU. Thanks. —Glenn, 2018 Winnebago Sunstar LX

Dear Glenn,
According to the Winnebago 3D parts listing, your Coleman is a Mach 15. There have been a few issues with those, according to owner relations.

The archive brochure is a two-page list of capacities and doesn’t go into any detail about standard and optional features. I assume your unit is controlled by a thermostat on the wall rather than controls on the unit itself, since you indicated air comes out of the ducts. An option on this unit is a heat pump. It runs the operation backwards and draws BTUs from the outside air. Make sure you do not have it in the heat pump mode. That sounds elementary, but I have seen it happen. Since the compressor and fan are running, owners think it should be cooling.

Get an anemometer to check air conditioner performance

Next, I would recommend getting an anemometer, which is an airflow and temperature gauge, so you can have specifics. The roof air conditioner can only condition or cool the air in your coach by 16 degrees. So, if the ambient temperature coming into the air return is hot and humid, such as 95 degrees or higher, the air coming out of the vents would only be 79, which might seem warm.

If the outcoming air is not much different than the ambient air, the next step would be to remove the cover to the air return and make sure the filter is clean. Also, take a look at the evaporator fins. Air is drawn up through the return air vent and pulled through the evaporator. At that time, the compressor has flashed the coolant through the lines to pull out heat and moisture. If the fins are not clean, the air will not draw through and get “conditioned”. Your return air vent should look something like this.

Here is a photo of a unit covered in body powder! This unit actually stopped working as the compressor got hotter and eventually ruined it. You might need to take the outside shroud off and the evaporator metal shield to get a better look.

Next, make sure the diverter in the outgoing air return is in place and sealed. Coleman sends a generic diverter for ducted air units to the manufacturer. They cut it to the right specification and insert it to divert the airflow in the direction of the ductwork. They are all different. I have seen several units that had very minimal tape holding the diverter and it broke loose. It was pulling warm air in and it just circulated in the open cavity and blew back out the ducts to the vents. There is a company called RV Airflow that has developed an aftermarket diverter that is installed in the cavity and improves the airflow.

Inspect compressor and cooling lines

If all this is working well and you are still getting warm air, I would inspect the compressor and cooling lines. The Coleman unit has had several issues with vibration causing a crack in the fittings or copper lines and the coolant escapes. Visually inspect all the lines and compressor and verify it is actually working.

This is not a rechargeable system, so it would need to be worked on by a qualified technician. Winnebago has a great exploded view for reference.


 You might also enjoy this from Dave 

RV ‘Gremlins’, Part 4: RV roof air conditioners: Sometimes they work, sometimes they don’t

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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Dave Solberg
Dave Solberg
Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club. He has been in the RV Industry since 1983 and conducts over 15 seminars at RV shows throughout the country.


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8 months ago

Unless obvious things are the cause, it’s common for a/c’s of all types that blow warm air have low/no refrigerant left. For rv a/c units you can buy a pinch valve that permanently installs on the copper low pressure side. After that it’s easy to replenish the refrigerant.

8 months ago

A quick check online shows an anemometer/thermometer costs on the order of $200 or more. Rather expensive for a once in a rare time use.

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