Furnace orientation and air flow question?
kwik
| Posted in Energy, Heating & Insulation on
When installing a furnace that has a rectangular plenum (22 x 14) does it matter which direction it faces in relation to the direction of air flow into the main duct? In other words, does it matter if the main ducts (rectangular) attach to the long (22″) or short (14″) side of the plenum?
Replies
in practical tems, it is more
In practical terms, it is more likely that branch takeoffs from the 22" side will increase airflow--not because it's impossible to achieve the same airflow from the 14" side--it's just that headroom is usually a limiting factor in the size of the branch ducts.
This assumes that the branch takeoff fittings will be made of sheet metal, but if the branch ducts will be flex duct, say, 12" diameter that just tap into the side of the plenum, then there will not be an appreciable difference.
OTOH, the trunk is most likely to be oriented horizontally, so as to not consume so much space. So you'd most likely see the takeoffs on the 14" side (which is plenty wide for a duct).
But, in any event, it's best if the takeoffs are on the side with holes in it.
In any event,
DanH wrote:
But, in any event, it's best if the takeoffs are on the side with holes in it.
Actually, the takeoffs will only work if they are where the holes are. :)
Smilies, we need smilies.
The reason I’m asking is because if I orient the furnace with the long side facing the horizontal flow, my cold air return drop into the furnace would be inline and directly under one of those horizontal truck lines. I would then have to somehow turn the return drop so it would head toward the back of the furnace and go up instead of going straight up.
Has anyone done this? Can the cold air drop exit the side toward the back of the furnace than 90 up to the ceiling to avoid the horizontal trunk lines?
The return air plenum can be located on the back side of the furnace cabinet, or you could connect it to the side of the furnace cabinet, and then offset the main return duct to the side of the supply duct above it. If the offset is done using 45 degree fittings, there will be much less pressure loss than by using 90's.
Also, any 90-degree turns that you make in either the supply or the return main ducts would greatly benefit by installing turning vanes in the fitting. This will reduce the turbulence and the resulting pressure loss by about 10 times compared to the loss from a square throat 90.
System Effect
There is a phenomenon called "system effect", where downstream of a centrifugal blower the air flow is most efficient if any turns are in the same orientation as the the fan rotates. In most (upflow) residential furnaces, the fan is mounted with discharge at the back and the scroll/wheel closer to the front. Looking at the unit from the right side (the front being the side with the removable service panel) the blower wheel rotates counter clockwise.
Based on the system effect, the optimal side to put a take-off is the front side of the plenum. In practice, once the air has left the blower, passed through the heat exchanger and then through the cooling coil and another 2 to 3 feet of a plenum, it doesn't matter. The rotational energy of the air stream has been straightened out.
What does matter is the type and location of the take-off. Those with a 45 degree inlet leg on the upstream side will be the most efficient. Take-offs in the top of the plenum can be square cut (i.e. a tap collar) but not recommended. The top of side take-offs should be no closer than 2" from the top of the plenum, if possible.