View Full Version : M33 Underbelly Modifications

01-25-2017, 01:38 AM
So last September I bought a 2017 M33. My family and I are full timers. When the cold weather hit, we had water flow problems. Thinking the supply hose froze, that was where we turned first. Minor inspection led us to frozen pipes inside the underbelly. Further inspection was required so I pulled down the coroplast and I discovered a few things:
1. The duct work that leads to the front bedroom had a 4" slice in it 2 feet from where it comes through the floor resulting in the top temperature air reaching a weak 88 degrees.
2. All the duct work is uninsulated even though some runs are in excess of 30 feet long
3. Heartlands idea of a "heated" underbelly is a 2" diameter duct run from the heater to a hole in the floor but does not extend into the underbelly area.
4. The bathroom was always colder than the rest of the rig and utilized a 2" duct as well.
5. The bunk house was WAY to hot when the heat was set comfortable in the rest of the coach.
6. Heartland utilizes only 5 of the 9 available heat ports on the furnace.

Now before I start with the remedies let me start with a statement. Atwood expressly states NOT to use 2" duct on their units. This is stated in the book, in black and white, that came with my rig. Now for the remedies. I replaced ALL duct work with R-4.2 insulated duct rated to 200 degrees available on the internet in 25 foot lengths, for less than $20 each. I used 3-25' sections in my install. This raised the outlet temperature in the master bedroom to 138 degrees. Big difference!!! Save the old ducting, some will be used later. I removed the duct work for the bathroom utilizing the access panel on the left side of the entertainment center in the bunk house. I enlarged the hole from 2" to 4 1/8' and installed 4" ducting from the furnace to the bathroom. I used the Atwood adapters for the furnace I found on the internet for less than $4. I bought 4 and used 3. The 2" duct material from the bathroom was installed in place of the 4" duct leading to the bunkhouse. I then knocked out two more slots on the furnace and installed 2 more adapters. I drilled two 4 1/8" holes in the floor on the right side of the furnace area closest to the intake grate. STAY AWAY FROM THE LEFT REAR CORNER!! That area is directly over the black tank. I then used some of the OLD duct, ran one from the furnace to the space between the black and grey tanks and ran a second one forward to the fresh water tank. Doing this increased the heating from 2" to 8" combined. A 300% increase!!! I reinstalled the chloroplast with tech screws, resealed the plumbing and gaps with closed cell spray foam in a can, and used 3M aluminum cold weather duct tape to cover any exposed foam and I taped the ENTIRE edge where the chloroplast meets the frame to minimize air movement.

When I tested the furnace I can heat the camper 4 degrees in 6 minutes timed from fan on, to fan off. Pretty good!! We have had 3 days of 30 degree highs and no frozen water and much more even heat in the coach. It took me and a friend 2 days to complete this project.

01-25-2017, 08:26 AM
Did you take any pictures? I am interested in what you did to fix your rig.