Basement A/C ducting

tcolley · January 27, 2016, 03:26:59 PM

I'm about to acquire a new project this weekend, a 1973 MC7 combo. Both roof airs need replacing and I'll most likely be installing basement A/C and I was wondering if it would make any difference if it was ducted through the floor or overhead. Seems like most examples I've looked at were overhead but it seems so much simpler to duct underneath. Opinions?

MB LeMirage · January 27, 2016, 03:37:48 PM

cold air falls. Floor ducting generally requires larger fans to force the air to the ceiling, otherwise you have nice cool socks but your head is kinda hot. Extra fan pressure means more noise, vibration and electrons. Now ducting it to the ceiling first requires far less fan speed because it is in a confined air space (4x10 or 4x12), not trying to push the heat and humidity out of the way in the bus cabin.
Ryan.

tcolley · January 27, 2016, 03:52:16 PM

I figured that was part of it. But I should have mentioned that I was actually planning on not only basement A/C but also heat, in the form of a heat pump. I'm sure the preference there would be to have the heat ducted through the floor. But can't have it both ways. Still better to duct up top?

buswarrior · January 27, 2016, 07:50:13 PM

You may also consider a high mounted air return, with floor feed?

happy coaching!
buswarrior

TomC · January 27, 2016, 07:57:11 PM

You'll be happiest just replacing your roof airs. You'll notice even high end motorhomes and 5th wheels are using 2 or 3 roof airs. I have three Coleman 13,500btu A/C only and typically only need two to cool the bus. Good Luck, TomC

MB LeMirage · January 28, 2016, 01:29:22 AM

If you are doing a heatpump as well then you usually will already have a higher volume fan, so floor is ok. You generally want heat starting low inside the cabin, and cold starting up high. Dilemma dilemma.
Ryan.

oltrunt · January 28, 2016, 05:27:36 AM

Why does it have to be one or the other? Duct the cold to the top and the hot to the bottom with a servo operated door between. A HVAC man could show you how to set it up and you'd be able to eat your cake and have it too. Jack

MB LeMirage · January 28, 2016, 06:24:35 AM

That is very true. If you have the space, go for it.
Ryan.

Geom · January 28, 2016, 07:58:30 AM

One thing to keep in mind with ducted heat through your basement is, at least in our case, the bays get pretty chilly in winter. So you'll be spending a good deal of energy bleeding heat through the ducts before it actually makes it inside. While you may do this intentionally to keep your bays warm in winter, you'll probably need to insulate the bays well to keep that heat in. You could of course insulate the ducting, which will increase the overall diameter needed for your duct runs.

Also, not sure if it's part of your design, but keep in mind that heat pumps become really inefficient to useless below about 34 degrees and some aux heat source will be needed.

On the flip side with bay AC, the bays once again might be an issue, because they'll get pretty toasty warm in the summer and you'll have to make sure the coils are quite well vented to dump the exchanged heat outside. But that's likely already a part of your plan.
With cold air tending to sink, you'll have to really strategize on where to place floor vents, in order to ensure good convective airflow. A piece of furniture or similar in the way (not necessarily on top) of a vent (or return) will reduce smooth air flow and make for a very chilly piece of furniture and little else cold air getting around. With a bus, space is pretty tight as is, and it might be difficult to find the sweet spot to ensure obstruction free airflow.

Good luck,
George

kyle4501 · January 28, 2016, 02:33:03 PM

My Newell has factory installed basement air and 2 more recently added roof airs. Inside the coach, the basement air is less noisy inside than the roof airs, but not by much. Outside, the basement air condenser fans are loud - as much or more noise than the generator.

The roof airs can be connected to a single plenum along the center of the roof. A heat pump in the center works well to chase the chill away. Toe kick heaters work well, so do the propane heaters.

Basement air requires ducting and room under the floor. That ducting takes up a huge amount of space. Space I would rather have for other uses.

In summary, I plan on eliminating the basement air and putting 3 low profile roof airs connected with a single ceiling plenum.

Your coach, your choice

viking1 · January 29, 2016, 09:16:22 AM

I have a 2 ton basement ac and have never been very happy with the performance. I'm considering replacing it with 2 or 3 roof ac's.

Emcemv · January 30, 2016, 06:24:29 AM

Tcolley

We also have a 73 7 combo. Basement airs. If you want to trade some pics and info, email me at emcemv at charter.net

Bruce

PRZNBUS · January 30, 2016, 11:46:59 AM

Right now, I'm researching a Thermo King Transit AC. I should be able to mount the guts in the back above the radiator fans in their own compartment with the other component on the roof. Not sure how to make it all work but anything is possible. Haven't received a price yet but I'm sure it's not cheap however it's VERY heavy duty which equals reliability.

Bruce

TomC · January 30, 2016, 12:28:45 PM

Very Heavy-duty also equals VERY expensive when it does break. There's a real good reason most all motorhomes and trailers have gone back to rood top airs-they are cheap, easy to replace and do a good job of cooling. Don't try to reinvent the wheel-just stick with the roof tops. Good Luck, TomC

kyle4501 · January 30, 2016, 02:23:00 PM

I used to hate roof tops - then I experienced how easy it was on the pocket book & time wise to replace it. Very low impact on the interior too.
After that, roof tops look lots better to me.
The storage space taken by the basement air system reinforces the positives of roof air for me.

The vanity of the clean roof top was replaced with the practicality of use & service.