Roof Air Conditioner question...........

Kirk W · April 9, 2017

I have been around RVs for a very long time and I've worked on most of the appliances found in one at some time or another, including roof air conditioners. I have even fixed several of the a/c units but it crossed my mind that there is a basic question about them which I'm not so sure about the answer to.

What electrically is the difference with a typical roof air conditioner between the setting for "low cool" and the one for "high cool?" Ours don't have the wall thermostat and the previous RV which did have a thermostat on the wall also only had a high fan and low fan setting. In fooling with the unit I suspect that the fan speed is all that changes on this one as well, but have never traced the thing out with a meter and really don't want to crawl around the roof to do so. Anyone know for sure?

I prefer the thing on low when in our RV with this one mostly because it isn't as loud.

Ed ke6bnl · April 9, 2017

Well speaking from the old days, the Compressor has one speed you are usually only change the fan speed. BUT there are residential 2 speed Compressors. Not sure if they have made there way to the RV market. Some window unit had a mode to let outside air in.

Daveh · April 9, 2017

What are the odds. I was thinking about this question less than two hours ago and had never really wondered about it before. An Airstream, not that big, came in with two AC units and I got to wondering whether that was overkill and then got to wondering whether you run them both on low and what exactly is the difference between low and high.

Dutch_12078 · April 9, 2017

The fan speed is the only difference. Running at the low fan speed in high humidity areas can sometimes lead to excessive coil icing and diminished cooling on some units.

TXiceman · April 10, 2017

Generally I do not recommend the use of LOW speed of the A/C fans. All that happens is the fan speed is reduced and the unit will rebalance at a lower evaporator temperature and less capacity.

The big problem is with the lower evaporator temperature, there is more chance for the evaporator coil to ice or freeze. Most units have a freeze sensor in the evaporator coil to shut down the unit or at least the compressor.

Ken

Jimalberta · April 10, 2017

Txiceman and Dutch are both correct. The only difference is the fan speed. Generally the low fan speed is purposed to slow the air down over the evaporator thus getting the evap colder and allowing more condensation to facilitate more moisture removal in high humidity situations. Not recommended to run low fan in low humidity areas as you run the risk of freezing up the evaporator.

Kirk W · April 10, 2017

13 hours ago, Daveh said:

What are the odds. I was thinking about this question less than two hours ago and had never really wondered about it before.

Hmmmm....... Great minds, perhaps? :lol:

10 hours ago, Jimalberta said:

The only difference is the fan speed. Generally the low fan speed is purposed to slow the air down over the evaporator thus getting the evap colder and allowing more condensation to facilitate more moisture removal in high humidity situations.

While I too suspect that fan speed is all that changes, I have not verified that with a meter or schematic. Have you?

Your explanation of what the lower speed is for doesn't match at all what my present air conditioner's operator's instructions state. It says to operate at high cool for maximum cooling when first cooling down the RV or when in hot weather. "Low cool may be used when less cooling capacity is needed." When experimenting with mine I find that the only detectable change is air movement and noise level and when we do use "low cool" it is normally to lower the noise level. What got me to thinking about the actual differences was a situation where I'm quite sure the unit had frozen up and it was set to "low cool" at the time.

Quote

Mechanical Cooling Operation:

Set the thermostat at the desired temperature level. Select the fan speed that best satisfies your needs. See FIG. B1.

a. HIGH COOL - Selected when maximum cooling and dehumidification is required.

b. LOW COOL - Selected when room is at desired comfort level and needs to be maintained. Normally this speed is used for nighttime operation.

sundancer268 · April 11, 2017

It is strictly the fan speed from the Coleman Schematics and service manuals.

Dutch_12078 · April 11, 2017

I didn't dig through all of the available Dometic/DuoTherm A/C service manuals, but of the several I did look at, only the fan motors were shown as tapped for dual speeds.

I'mdonewiththis forum · April 11, 2017

Kirk, you can use the low speed after you get the coach/trailer cooled down since the A/C won't (shouldn't) need to run as long. The problem with low speed is what Jimalberta said. The evap will ice up and the shorter run time (not as high of a load) will allow it to thaw between cycles (at least that's the was it is supposed to work).

oldjohnt · April 11, 2017

Good question, here's my take: As a past used RV dealer and 40+ year user I saw way more AC units then the average Joe and as I recall on ALL those the low medium high cool switch ONLY switched fan speed and NOT any of the compressor circuitry or functioning. I always intuitively (didn't have manuals usually) ran it on HIGH cool at first to quickly as possible bring the temp down followed by medium or low later on to lessen fan noise. Of course, the CFM of air past the coils is critical for the best efficiency and correct heat transfer and potential icing etc., which is why a manual will describe best methods of operation which can depend on temperatures and humidity levels etc.

John T