[Patrick]

Originally Posted by Dragonslayer140

Don, I have seen two different types of ducted units, the one like you have with the slide vents is the more modern style , and the one in my Class C that is just a simple return grill which is the first generation.

These roof top units are really not very efficient at moving air. Most use a fairly small propeller style fan to push the air, because it is small it has to turn quite fast (Which has the nice side effect of increased noise). A small fast moving fan produces a turbulent column of air that is difficult to direct and has a high speed and fairly low volume & static pressure. now you add a rectangular duct system that is only an inch or two in height and a few inches wide, this has a large interior surface area compared to its volume, causing significant friction loss to the air moving through it.

The non ducted units only discharged air in one central location, so you had to deal with no cool air in other rooms of the coach, have multiple roof top units, or leave doors open. Then along came the first ducted units which spread the air around but on hot days really could not move enough air to do a good job of cooling the coach due to duct friction loss. then they came up with the idea of installing a damper/diverter at the unit itself, this way a user can elect to bypass the duct work and have the air blow directly out of the uint like the old days, this allows the unit to work easier and produce a higher volume of cold air due to removing the friction loss of the ducting, cooling down the main portion of the coach faster. Once you have the coach cooled down, or at night when you head to the bedroom, you can close the bypass and direct the air back into the ducting to allow for cool air to reach the whole coach.

You can improve airflow by "balancing" the duct work, adjusting the individual dampers to provide equal airflow from each diffuser. General rule of thumb is air like to travel in a straight line in a path of least resistance, so it will try to come out the farthest down the duct first. so usually you will have that damper closed more than the one before it and so on.

Also there is usually an air filter above the return air grill on the unit. Mine has four screws to remove and then the cover comes off revealing the filter. When I bought my used RV and pulled the filter, I could not believe how much dirt was in the filter. It was enough that it actually made a noticeable difference to my airflow. not to mention made the air considerably fresher when the unit turned on.

Anyway my .02

Dave

This is why I didn't pay extra for ducted air.