4-inch heat duct worth the effort?

[rossow (mn)]

I'm putting a half bath in a 5x9 well-insulated room over an inaccessible (from below) crawl space. I plan on a 220-volt in-the-wall electric heater and possibly a limited area of under-tile heat in the floor. But I'd also like to add a little heat from my forced-air furnace system to help reduce the cost of all-electric heat. For reasons I won't belabor here, I only have 4-3/4 inches under the floor to run a forced-air duct, so nothing bigger than a round 4-inch will work (including 6-inch oval). The duct would come directly off the furnace plenum and involve a total of about 12 feet (2 feet plus 6 feet plus 4 feet) of straight run and three 90-degree turns to get from the furnace, around a stair stringer and up through the floor. I know this is not an ideal setup, but is it worth even trying? Maybe even with a duct-boost fan? (Return air is not an issue, nor are codes.) I appreciate any insight you may offer (unless it's to consult an HVAC engineer -- unfortunately, that's not in the budget). Thanks.

[Lightwave]

Without taking duct losses into account, the best case for a properly balanced 4" duct is about 900 BTU/hr, or 250 watts equivalent. Actual heat delivered will be lower because of heat loss through the duct into the crawl space and friction due to all the elbows in your route. It's probably not worth it unless the space is insulated well enough that you can heat it with a 500W heater or smaller.

You could make up some custom rectangular ducting that's only 4" tall but 12" wide (etc) to get more air.

[nukeman]

Why won't the 6" oval work? Oval duct is small enough to fit in a 2x4 cavity (3.5"). You could also use wallstack duct (typically ~3.25"x10") or could do something custom. However, quite a bit is lost in the corners with rectangular duct and oval will get you the same equivalent flow in a smaller space.

[rossow (mn)]

Why won't the 6" oval work? Oval duct is small enough to fit in a 2x4 cavity (3.5"). You could also use wallstack duct (typically ~3.25"x10") or could do something custom. However, quite a bit is lost in the corners with rectangular duct and oval will get you the same equivalent flow in a smaller space.

I may buy some 6-inch oval and try to make it fit. My problem is in getting around a stair stringer and accompanying framing, plus existing plumbing (supply lines and DWV) that I really don't want to try to move. It's a really tight, crowded situation to try to sneak a duct through. I'm sure you know how it is trying to do work on existing structures -- always something where you wish it wasn't. I may end up reframing some of the stair-stringer support to clear a better path for ductwork. I know I'll be happier with 6-inch ductwork if I can get it to fit.

[Wally Hays]

Without taking duct losses into account, the best case for a properly balanced 4" duct is about 900 BTU/hr, or 250 watts equivalent. Actual heat delivered will be lower because of heat loss through the duct into the crawl space and friction due to all the elbows in your route. It's probably not worth it unless the space is insulated well enough that you can heat it with a 500W heater or smaller.

You could make up some custom rectangular ducting that's only 4" tall but 12" wide (etc) to get more air.

How do you come up with that without knowing the blower cfm and the static?

[chas22]

Put a Heat, Vent, Light in the ceiling and call it good!