In the Katy, Cinco Ranch, Cypress areas of Texas we are finding gas furnaces installed vertically in attics. This type of installation should be avoided and here is why: Gas furnaces now days are more compact then ever. The air flow goes in the furnace through the side when installed vertically and in the bottom when installed horizontally. This is the best way to install it,and here is why: When you install a furnace vertically because of it’s size you are limited to the opening. The manufacturer’s installation instrutions show that they want you to use a square opening not less then the recommended size. If you use sheet metal duct work with a liner you will loose a total of three inches on the four sides of the inside of the duct. Sheet metal wrapped with insullation on the outside does not reduce the inside deminsions. But, the problem we are finding is that the contractors are using flex duct and it is round and it reduces the air flow…Big Time. Most of the furnaces we have run across have static pressure drops of .75 and higher. The most it should be is .50. When you install it horizontally you can add a return air plenum and then you can take as many return air flex ducts as needed. Also, in a vertical installation,the evaporator coil is on top of the furnace. I have not seen one condensation over flow switch at the evaporator coil only. I have only seen it at the over flow pan that the furnace is sitting in. This allows the condensation to run down the furnace damaging the heat exchanger and the printed circuit board. Also, if there is not enough air flow across the furnace it raises the temperature on the evaporator coil, expanding the tubing in the heating cycle and contracting the tubing on the cooling cycle ,causing premature failure to the evaporator coil resulting in freon leaks. In a horizontal installation, if the condensate drain stops up the condensation leaks into the overflow drain pan under the evaporator not causing damage to the furnace.