Miller mobile home furnaces get a bad rap. A lot of service tech's hate them and they have a reputation for being prone to having cracked heat exchangers. Here's just such an example. This unit was recently removed from a mobile home due to a cracked heat exchanger. Upon further examination the cause of the crack is pretty obvious. This particular model pulls the exhaust from under the rear baffle, at the bottom of the combustion chamber. The six inches of scale built up around the base caused the unit to not vent properly which leads to excessive heat build up at the upper regions of the heat exchanger which leads to cracks. Had this unit been properly serviced it would still be in good operating service today.