Ducted A/C Unit - Bypass Ductwork for Better Cooling??

[otrfun]

Bought a new 5th wheel with 2 Dometic Brisk II 13.5k a/c units a few months ago. As expected the front bedroom non-ducted a/c unit works extremely well due to the small area it has to cool. Not surprisingly, the ducted 2nd a/c unit, which is cooling a much larger area, doesn't cool as well. We've been on two camping trips with 100-112 degree ambient temps. Cooling, although not ideal in the main living area, has been "adequate". We've been using a squirrel cage fan to move the colder air from the front bedroom to the main living area to assist the ducted 2nd a/c unit when temps got above 100.

We're getting ready to go on an extended cross-country trip in a few weeks, so I figured I'd clean the air filters, remove the shrouds to make sure the a/c pan gaskets were torqued down sufficiently, and check the internal "partition" to make sure no hot intake air was leaking into the cold exhaust area. When I removed the shroud on the ducted unit (while it was cooling), I couldn't help but notice the DRAMATIC increase in air movement when I removed the shroud (which was adjusted for a maximum "direct flow" of approx. 50 percent).

This is my first ducted a/c unit, so I'm not sure how other ducted units work. The shroud on this Brisk II unit has a slider that enables one to adjust the amount of air that gets forced into the ducted outlets vs. direct flow downward out of the shroud. I'm estimating the adjustment goes from 100 percent ductwork (baffle completely closed), to 50 percent ductwork/50 percent direct flow. The baffle won't allow 100 percent of the air to flow directly out of the shroud.

Long story short, with the shroud removed, which allows 100 percent of the a/c unit's air to exhaust directly out of the a/c unit (little to no air into the ductwork), cooling increased dramatically. My estimate? It was almost like adding a 2nd 13.5K a/c unit in the main living area. With ambient temps around 103 degrees today, temps in the main living area dropped down to 76 degrees within 1-2 hours (IMO a pretty significant drop for a 33 ft. TH with very high ceilings; 13' 4" outside height). It never dropped below 80 degrees in the main living area with the shroud adjusted to 50/50 (ductwork/direct flow) when ambient temps were above 100 degrees.

Although I have nothing to compare it with, I don't believe the ductwork is obstructed---a "decent" amount of air was exiting each duct prior to removing the shroud.

Anyhow, the Brisk II shroud costs about $55. I decided to cut out the adjustable baffle to allow 100 percent air flow out of the shroud with the shroud installed (not a big risk if my gamble didn't pan out). So far, so good--simply amazed with the increase in cooling.

For those with a ducted a/c unit that won't allow 100 percent direct flow out of the unit (or minimal flow to ductwork) and aren't getting the cooling they desire, temporarily removing the shroud to see if cooling improves is something they may want to try. If it works better, hey great; if not, at least it gave you an excuse to check things out underneath your shroud--lol!

[otrfun]

joebedford wrote:

otrfun wrote:
We also have a Voltage. I assume your a/c unit is located, overhead in the main living/kitchen area. If you're not happy with the way it's currently cooling, I would highly suggest temporarily removing the ducted a/c ceiling assembly to see if your overall cooling improves.

Remove the thingy with the hanging lamps? I don't think the ceiling is open - the a/c vents into the ducts only.

Wow, very nice!

Yeah, I see what you mean. It doesn't like you can do much if the a/c unit is mounted directly above the hanging overhead lights. Even if you gained access, the lights will probably obstruct any attempt to get any air-flow directly out of the a/c unit 😞

[joebedford]

otrfun wrote:
We also have a Voltage. I assume your a/c unit is located, overhead in the main living/kitchen area. If you're not happy with the way it's currently cooling, I would highly suggest temporarily removing the ducted a/c ceiling assembly to see if your overall cooling improves.

Remove the thingy with the hanging lamps? I don't think the ceiling is open - the a/c vents into the ducts only. This isn't my rig - this is a stock photo.

[westend]

otrfun wrote:

westend wrote:
I would bet that the heat transfer between ceiling cavity and duct is horrendous. Are your AC ducts insulated ducts or are they akin to a clothes dryer vent?

The ductwork appears to be about 2x8 in size and made out of a grey-ish colored, pliable kind of hard foam about 1/4" thick.

Thank you. 2" is really going to limit air circulation, IME.
Glad you found a good work-around.

[otrfun]

westend wrote:
I would bet that the heat transfer between ceiling cavity and duct is horrendous. Are your AC ducts insulated ducts or are they akin to a clothes dryer vent?

The ductwork appears to be about 2x8 in size and made out of a grey-ish colored, pliable kind of hard foam about 1/4" thick.

[otrfun]

joebedford wrote:
Ductwork sealing was terrible in our Voltage. I had to retape every opening and retape the unit on the roof.

Helped a lot but it's still not very effective in the LR and totally useless at cooling the garage.

I would open up the ceiling to bypass the ducting altogether too, but there is a big valence? with light fixtures on the ceiling blocking access to the a/c.

We also have a Voltage. I assume your a/c unit is located, overhead in the main living/kitchen area. If you're not happy with the way it's currently cooling, I would highly suggest temporarily removing the ducted a/c ceiling assembly to see if your overall cooling improves.

Doubt you'll be able to do much to improve the cooling in the garage, but at least you might be able to make things really comfortable in the main living/kitchen area.

Only have to remove the ceiling assembly. Shouldn't have to block off the ductwork just to do this test. The entrances to the ductwork are at a 90 degree angles to the blower fan. Significant air shouldn't enter the ductwork unless the ducted ceiling assembly is installed.

Good luck!

[otrfun]

TomG2 wrote:
Could it be as simple as replacing the ducted ceiling unit with a un-ducted one?

Good idea. I thought about that, too. I believe the ceiling assembly itself is the only difference between the ducted and non-ducted a/c rooftop units. Could be wrong tho.

I suppose I could install a non-ducted ceiling assembly which should allow more air-flow directly into the living area than the non-modded ducted ceiling assembly (which is purposely designed to always force some air into the ductwork). However, I believe the ducted ceiling assembly with the baffle assembly cut-out allows much better direct air flow into the main living area then the non-ducted unit. Reason being, the non-ducted ceiling assembly forces most of the air to move at 90 degree angles within the ceiling assembly before exiting.

In any event, I was able to make it out to my unit yesterday to do some more work on the main living area a/c. The ducted a/c unit feeds two sets of ductwork. All the ductwork in my unit appears to be about 2x8 in size and made out of a grey-ish colored, pliable kind of hard foam about 1/4" thick. I blocked off the ductwork that feeds the main living area itself. Don't need it since the baffle mod I did does a much, much better job of cooling the main living area. The other set of ductwork feeds the bathroom vent, then continues on to the front bedroom. The front bedroom has its own a/c unit--so this ductwork is not necessary. We do want some cool air in the bathroom. So, I put some foam just inside the bathroom vent, in the ductwork itself (on the exit side leading to the bedroom) to prevent air from being wasted trying to feed the front bedroom ductwork.

Next I installed a small, curved aluminum baffle just below the squirrel cage fan exit to force some air into the ductwork that feeds only the bathroom. I fabricated it so I can bend a portion of the baffle to adjust the amount of air flow into the bathroom. I adjusted it for just a minimal amount of air-flow, just enough to take the edge off the heat in the bathroom. We don't spend enough time in there to sacrifice cooling in the main living area.

Lastly, just wanted to mention these new, re-engineered Dometic Brisk Air II a/c units are advertised as being more "efficient" and lighter. Yes, the insulating, structural material they used around the compressor, condenser, and fan, insulates better and is lighter. However, electrically this a/c is as inefficient as they come. It's only a 13.5k unit, but it has an LRA (locked rotor amps) of 68 amps---equivalent to the most inefficient 15k units. This high LRA is not an issue while using shore power, but most 3000w generators will have a very difficult time starting it, if at all. Two paralleled 2k Hondas would probably have to be run with the Eco mode off to start them. Finally, it consistently draws 15-16a---roughly 2-3a more than 13.5k units I've used in the past. In this day and age where there's a big focus on efficient use of power and electricity---Dometic spared nothing to make this unit exactly the opposite.

[Dr_Quick]

On a previous trailer I had a problem with ducts not being sealed. One time I was at the antenna crank and felt a lot of air blowing out at the crank hole. Checked my ceiling vents, and the CHEAP tape that was used to seal duct to the ceiling had come off. So got some good quality aluminum duct tape and resealed all the vents, then took off the AC ceiling grill and closed up all the visible holes where I thought that air might be leaking. Quite an improvement in system working better.

[otrfun]

fischer wrote:
I think pictures would have been nice to see. Just saying

I hear ya. I read posts like this all the time and sometimes it's difficult to visualize everything. Unfortunately, I didn't have my phone with me when all this went down.

[Stefonius]

In my rig, the ceiling ducts seemed to be made from polyisocyanurate foam boards with the foil facing in. I don't know how thick the foam boards are.

I can tell you that my main problem isn't the temperature of the air coming out, but rather the volume and velocity.

[fischer]

I think pictures would have been nice to see. Just saying

[TomG2]

Could it be as simple as replacing the ducted ceiling unit with a un-ducted one?

[joebedford]

Ductwork sealing was terrible in our Voltage. I had to retape every opening and retape the unit on the roof.

Helped a lot but it's still not very effective in the LR and totally useless at cooling the garage.

I would open up the ceiling to bypass the ducting altogether too, but there is a big valence? with light fixtures on the ceiling blocking access to the a/c.

[westend]

I would bet that the heat transfer between ceiling cavity and duct is horrendous. Are your AC ducts insulated ducts or are they akin to a clothes dryer vent?

[down_home]

otrfun wrote:

down home wrote:
Why? You like a ho bedroom while the living area is cool?
Both our units are front and then one over the kitchen area with connecting ductwork.
Sometimes only one unit, the rear or main one, is needed as the cooled air is spread throughout the coach.
We can close the bedroom door and be cool and no noise at tall, fro the ducts in the bedroom.

What's your definition of "cool"?

At night we/I like it about 68 degrees. A nice warm blanket and sleep like a log.
During day we keep the coach at about 72 even going down the road. I personally like it about 70 but she complains.
Most, including our coach, could use another inch, of insulation, in the ceiling and front cap and rear cap.
A couple more ducts and small thermostat regulated fans, at each duct, would give more even distribution and maybe alter the need for the unit blowers, when starting, sometimes sound like a Chinook spooling up the engines.