A client had a new double wide set (27x56) in1997 a couple years later had an addition (27x20) to the west side with 27x20 attached garage to that addition, the general contractor tells the client a bigger furnace will be needed. He (general contractor) brings in his HVAC guy who installs a furnace clearly stamped "Installation for site built only", the front of the return box was hacked out to provide for a 16x20 filter plus the back has a run coming off of it into the master bed closet with a filter slot (no filter in place)down into the crawl and up into the wall cavities of the addition all metal return ducting uninsulated in crawl. The supply to addition comes off the original duct board below the furnace through the crawl to four insulated flex runs in addition and for some reason they insulated ,or tried to,  the metal ducting in addition crawl but not under manufacture home crawl and to make sure the owner got a good job the jumper duct was cut about four out from the furnace and left that way. The only thing I can come up with is they were going to tie into to the new heat run and forgot. So forget the general contractor, HVAC guy and last but not least the county inspector, so now what. As far as I'm concered that furnace has got to go and "Manufacture Home" approved installed, period. For the supply runs remove all that ducting, seal duct board back up and tie into bottom of original ducting at each west end run with insulated flex to existing boots, and naturally new jumper. I'm thinking the return in addition can go, the passage way to addition is over 8'. What are your thoughts, am I on the right track or in line with the three fore mentioned proffesionals. "The things that make your head go," BOOM ! " 

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The manufacture home has higher SP ( rated Static Pressure) supply and no return  so some are rated to .8 to 1" in water vs most res are .5 in water.  Most Manufacture homes have a little room off the hall way  so the fan is a foot from door with wimpy filter.  Manufacture home have no branch duct just a 5" stream off the main trunk.     That's 2597 SF and Mob home air handler have higher efficiency  with smaller area,  With the transfer main trunk the efficiency just drops 10-15%

  Manufacture homes furnaces also take outside air on outside the flue pipe and have to buy the chimney from them.  When test system Efficiency (BTU in Vs BTU out) I get mid 40% on new and low 30% on old duct work.    To me it’s just temporally housing with high power bills.



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