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.
HAVE A QUESTION FOR DAVE?
We have started a new forum link for Ask Dave. Please be as brief as possible. Attach a photo or two if it might help Dave with his response. Click to visit Dave’s forum. Or send your inquiries to him using the form below.
##RVDT1960
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.
Type airflow meter on Amazon and you’ll find dozens for $14.95
Here is an example: https://amzn.to/3USsMDd
I’ve got one of those .. works well.
Now $16.95 just checked
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.
This is interesting. Today Dave’s post says, “Probably the best answer I got from one technician is the pressure in the line is too high for any port to be safely installed” https://www.rvtravel.com/why-cant-i-recharge-my-rvs-roof-air-conditioner-like-truck-or-house-a/c-rvdt-2169/
After looking at piercing valves on Amazon, I asked ChatGpt 3.5 to compare the pressures. It responded with this: “The refrigerant pressure in a typical home refrigerator’s cooling system usually ranges between 2 to 15 psi” vs. “The refrigerant pressure in an RV air conditioner typically ranges from 100 to 450 psi.”
If this be true, the pressure differences range from perhaps 7x to 30x.
(Continued from below) So a home fridge piercing valve would be significantly overloaded by pressure, vibration, heat, and cold on the roof of an RV. I wonder how the neoprene (?) seal would hold up.
And, as Dave’s article points out, we are dealing with probably Soft copper tubing (bendable); either way it is real soft compared even to hard 1/2-inch copper water line and nothing like steel. How much does copper deform over time under clamping pressure?
I would love to see some long term (months, years) reports on how this holds up under actual RV’ing conditions.
Not sure if this helps answer your question or not, we had a 15 year old Mountain Aire 38’ gas MH that still had the original bedroom A/C unit working. The front A/C had seized up but the rear still worked. Six months later the plastic condensation tray developed a crack which I fixed with waterproofed epoxy after a RV dealer couldn’t in 2 tries at a cost of $179 labor and parts. That should give you an idea how long it lasts, by the way these were Dometic units. I have always had great luck with Dometic everything. Maybe I’m lucky with the name, but I’ve had those products since 1978.
Thank you but a plastic tray holding water is not a copper line at 100 to 450 psi.
Your repair is a perfect example of what we can all do with epoxy in situations where there is no pressure or very little.
“The roof air conditioner can only condition or cool the air in your coach by 16 degrees.” I thought this was curiously specific. In AZ cool late spring (100F outside, 93F inside), I got 27F with a 2006 MACH 3 PLUS 8333B.
Today I asked ChatGpt 3.5 the following: “what is the maximum temperature reduction of a rooftop rv air conditioner?” The response was, “As of my last update in September 2021, rooftop RV air conditioners typically have a maximum temperature reduction capability of around 20 to 30 degrees Fahrenheit (11 to 17 degrees Celsius) from the outside ambient temperature.”
If this be true (ITBT), it helps to explain the discrepancy. Still curious where that 16F came from.
Dave has posted a very interesting update here:
https://www.rvtravel.com/rv-roof-air-conditioner-efficiency-rvdt-2173/