When I took delivery of my 2007 MSRS3 I noted that the plastic heat duct from the furnace was poorly routed from the furnace to its connection point at the metal infloor ductwork. The duct was mashed severely between the bottom of the central vac and the wooden deck over the gray tank. It was also crushed right over the top of the brake acutator. This duct had more than 1/2 of its diameter restricted in two places. I made a mental note to change that.
Now 3 years later I have finally done it. I discovered the problem was worse than I thought. The duct closest to the sliding doors had large tears in them dumping most of the heat into the basement and the other main heat duct was crushed to half of its diameter under the black tank dump valve. The problem was aggrevated by seemingly haphazard placement of hoses and extraneous wiring which seemed to be going everywhere.
I decided to lower the wood deck, where the brake actuator and water pump mount, down and out so that I could get space to route the duct behind the actuator - not over it. That worked out well and the added bonus is that it is much easier to service the brake actuator fluid levels now. I also shortened the deck (and tapered it) at the furnace end to provide a clear path for the ducts under the central vac unit.
I couldn't find flexible duct in 6" diameter so I repaired the old one. Pictures tell it better than I can so check out these pictures to see how that was done:
Furnace Duct Repairs
Had a chance to check out the results this past week and we are getting a lot more air from the furnace into the living space now. If you are experiencing low air flow you might want to check this out as a possible reason.