Question: How to Identify Ducted AC

[bushpilot]

Can someone tell me how i can identify if i have ducted ac ?

Now before you laugh - i know what a duct is, and yes i have them in my ceiling (cant say i have ever felt much flow out of these ducts though).

Im wondering if there is a difference in the overhead units.

Do NON ducted ac's have controls on the overhead unit themselves (instead of a conventional wall thermostat) ?

Specifically WHAT is the difference between a DUCTED & NON DUCTED AC/Roof unit ?

[volvoman]

a non ducted ac unit blows out vents right on ac unit. a ducted one would blow down ceiling or floor through registers.

[bushpilot]

As I would expect - however I never got much flow out of the ducts in mu ceiling
Most of my air flow came from the a/c unit itself.

I wonder if the previous owner changed the unit out ? The a/c in the "master suite" workes the same way (not much out the ducted vents)

[bushpilot]

wow 50 views & only one comment ??

So at the risk of feeling a bit STUPID - i went over to the coach last night and tested the duct work...we have (for god knows what reason) been running the A/C w/ the (large panel a/c side vents open)...im trying to figure out WHY...i think its because it seems to cool the coach down faster (texas heat ?).

anyway - closing the vents does force more air thru the duct work (duh) as expected.

im wondering - is everyone elses ceiling mounted a/c look like mine ?
do you too have the vents (identified by the arrows) ?

anyone know why a manufacture would build a grill like this ? and what the best operation is ? (when should or shouldnt you open these "grill" vents ?

[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

[bushpilot]

thanks dave - all very logical, a great explanation (and pretty much as i suspected.
I've been in both of mine, cleaning everything up & we do keep the intake/return air filter clean - speaking of which...anyone found a great way to clean 'em other than rinsing w/ water ?

Final question - i see retailers/manufactures building units for ducted and non ducted set ups...is there really any difference in the units ? if so what ?

[hot rod]

From when I bought my Carrier heat pump about 2 years ago, I seem to remember that the a/c unit itself is the same, but the difference is in the lower/ceiling assembly they supply with it. Also, it seemed like the ducted was the only one I could find that worked with a remote thermostat. I am set up for a non-ducted a/c, but it would be nice to mount a thermostat or control by the bed so we don't have to crawl out of the bunk and reach up to turn the damn thing off after it cools down at night.

[Dragonslayer140]

If i remember correctly, the non-ducted (at least the one I had on my old RV) roof top units run line voltage(120v) through their controls, While the remote stats on the ducted units are running on low voltage (12V).

I rewired my roof top to bypass the internal controls on my old camper. I removed the shroud from the ac unit and added some relays in a small junction box inside the housing. Its been a long time, but I think it took 3 relays. One for the compressor and two to control the two speed fan. you then remove the wires from the back of the stock controls and connect them to the relay contacts. Then you have to find a way to run some thermostat wire up to the ac unit from the stat location. you can then use an RV thermostat or a programmable household one as long as it powered by batteries (the hard wired ones use 24 volts AC). connect the stat wires to the relay coil terminals.

All in all quite a bit of effort, but the best part was the fan would come on and off with the compressor. with the stock configuration as long as the ac unit was on the fan would run continuous, just the compressor would cycle on and off.

I am betting now there is probably some wireless remote thermostat that could be adapted...

Problem is also when you work in engineering, there is a internal need to see if everything can be improved, even if it already works...lol

Dave

[hot rod]

That all sounds complicated to us non-engineer types. I'm thinking maybe just mount a 110v wall switch by the bed, inline between the breaker and the a/c unit. At least that way I could shut it off at night without getting out of bed. During the day it's not a big deal to reach up and shut it off. And it is exactly your point about the fan running full time whether the compressor is or not. The thermostat on the ceiling unit shuts the compressor off when the temperature drops, but the noisy fan stays running.

So, with your setup you mentioned 2 relays for high and low fan speed, does that mean your wall thermostat had a hi/lo switch for the fan that controlled the relays? Or did you just end up with one fan speed? Is the entire factory control bypassed, or do you still use the main on/off switch on the unit, and just the original thermostat dial function is bypassed?

[Dragonslayer140]

Using a wall mounted switch to kill the power to the entire unit would be a simple way to get the job done if you are just looking to turn off the unit from bed. Plus you do not have to run any extra wires. Just be sure you use a 20 amp motor rated switch, a standard light switch is only rated at 15 amps and the startup surge from the motor will destroy the contacts rapidly.

It should also be possible to add a relay up in the ac unit (line voltage coil) to control the fan from the built in compressor thermostat. Connect the relay coil to common and the output from the thermostat, and break the hot wire feeding the fan switch, and run it through the relay contacts. The the fan will shut off with the compressor.

As far as controlling fan speed with my wall mount stat conversion, if you use an RV stat it has a built in switch for manually selecting fan speed. With a standard battery operated thermostat you will only have one speed. you could install a single pole double throw switch adjacent to the stat, and that would let you select the speed the same as the rv stat, but still us a programmable stat. There are also programmable stats (Honeywell makes some) that have up to three fan speeds built in. The thermostat uses the difference in temperature from actual temp to programmed temp to decide what speed to run the fan, high speed when it is much warmer than what you want, and slows it down once it has caught up.

When I modified mine I removed the wiring from the internal controls disabling them. It might be possible to leave them in place and wire a remote stat in parallel, but i did not see the need.

[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.

[bushpilot]

Originally Posted by Patrick

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

now that i know how BEST to use ours & how it was designed i like it....
its the best of both worlds imo - run the cover OPEN and and cool the coach,
leave it open on hot days (100+ degrees)....shut it down and get better coverage by using the ducts.