the model furnace I have is cg-4-spdn I am using it in a car wash. What I need to know is this is a used unit and I think that it is plumbed up backwards. The orginal owner said the reason for getting rid of the unit is that it used to much gas. The orginal installer has the pump attached to the bottom of the unit and it forces water up and through the boiler unit and then comes out the top. I have never installed this brand before but have always hooked the unit up so that the water was pulled from bottom.