Mobile homes have smaller furnaces than stick built homes for two reasons.
Space is limited.
Output is greatly higher in mobile home furnaces to make up for smaller size of furnace.Fan speeds can be greatly different than conventional furnaces. Also Ducting is matched to the original furnace
If you insist on buying one yourselves, Make sure it is for Mobile Home use. Buy it from a mobile home dealer. The price will be higher there, but safety has no price.
One other thing to check !! Check ALL ductwork under or over the ceilings to make sure none have collapsed or become unsafe due to deterioration.
