I bought a portable air conditioner from B&Q. The instructions say the end of the flexible vent tube should be placed outside of an opening window. But all of my opening windows are too high for the tube to reach, and the unit is too heavy to place on a high pedestal. I am told that I cannot increase the length of the tube because this will cause too much resistance to the flow and cause the unit to fail. But I do have a fully functioning open fireplace and chimney. Can I mount the end of the air con vent tube to vent up the chimney?