The unit in question is Goodman GMV9509050XBA gas furnace (95% eff.,
90,000 BTU.)
It says in the Installation Instructions (page 7, Location Requirements
& Considerations) that the following must be observed:

"The temperature of the return air entering the furnace is between 55F
and 100F when the furnace is heating."

I am curious why the 55F requirement. I mean, when I'm not in the
house, I would like to set the temp as low as possible in order to save
on my heating bill. I think I could otherwise set it as low as 45-50F
and still keep the water pipes from freezing. But I wonder why I'm not
supposed to go below 55F. What could happen? Could the unit get damaged
and why?

Thanks

P.S. I'm in the Denver, CO area - 5,300 ft altitude, if that matters.