forced air duct length and register location?

J.D. Hiatt

We are adding dormers and finishing the attic. The furnace is in the cellar so the duct runs will be fairly lengthy. Is this of great concern? Also regarding register location, is under the window still the best location? I wondered if the advancements in window efficiency had made window location less of a prominent factor in register location. Thank you for any help you can provide!

Brad White_143

Some thoughts and questions

How much further are the new registers from the "formerly furthest registers"?

If significant, a small in-duct booster fan may be preferable to boosting the pressure for the entire system. If you do that, you increase the pressure for all flow, not just the defining longest branch. Tail wagging the dog here...

Downsides of boosting the pressure (speeding up the fan) is that the registers closest to the fan have to be dampered down (to have equal resistance to the most distant branch). An increase in electrical consumption is also to be expected.

Upside of what you are doing is that warm air has buoyancy and if the vertical risers are straight, they act as chimneys compensating for distance. This will not compensate for 100% of the additional duct resistance but it helps. Seal the bejeezus out of the ductwork too, both new and existing. Hard to make up for unplanned leakage. Horsepower goes up by the cube of the volume in a given system and you do not get that back.

Register location: Yes, if in the floor or overhead, in front of the glass. Sure, glass has become quite efficient these days, but then, so have walls. You want a film of warm air between any surface cooler than your body. That may be the wall too, but more likely, the glass, no matter how good it is.

My $0.02

Brad

Over-sizing is a good idea and forming a loop is also a neat way to get very even pressure.