Is there a reason some HVAC supply and returns are low to the floor in the wall ( or in the floor ) vs in the ceiling other than keeping the ceiling clean looking? I've heard different opinions.
Home heating and a/c duct systems have changed over the years. The best systems and the most versatile were installed with both high and low returns and supplies. For heating the low registers were used to supply the heat at the floor and the low returns were to take the cold air off the floor. In summer the high supply and returns were used for the same reason, take the hot air off the ceiling and supply the cold air from above. Today it too expensive to install duct systems this way. Today the installation practice is to install high return grills and low or high supply registers. I prefer low supply registers and both high and low return grills; use high returns in the summer and low returns in the winter. Ask an old timer how to do this.