Saturday, May 14, 2016

HVAC 2 - Ducts

Having settled on a central forced air furnace and air conditioner, ducts need to be installed to distribute the air throughout the house.  The conventional approach is to place supply registers on the floor under each window.  However, for low heating load homes, articles from NREL the Department of Energy, and Energy Star recommend compact duct system.  In a compact duct system, supply registers are located high on interior walls and blow the air across the ceiling toward the windows.  Attempting to apply this philosophy to our house runs into a problem - we have relatively few interior walls.  As I walked the house with the HVAC contractor to discuss duct placement, I learned that we have even fewer usable walls that I thought.  He explained that walls that are aligned with a floor joist are not accessible from below, so they are not suitable for ducts.  I wish I understood these constraints back when I was designing the floorplan.  For the most part, we ended up with registers under the windows.

We elected to set up three zones: first floor, second floor, and basement.  However, some rooms on the first floor will be part of the second floor zone.  That will make the heating loads of the zones closer to equal.  The office will tend to get solar heat gain from the window at the same times that the master bedroom on the second floor does, so it makes sense to have them in the same zone even though they are on different floors.

The duct system begins and ends at the furnace.  A MERV 13 filter is between the return duct on the right and the furnace.
One set of trunk ducts run from the furnace along the front side of the basement.  The duct on the right is the return trunk.  One of the ducts on the left supplies the first floor zone while the other serves the basement zone.  A shorter set of trunk ducts goes the opposite direction from the furnace to serve the second floor zone.
Zone dampers are installed in each supply duct where it connects to the furnace plenum.  There are four zone dampers, but two of them will be wired to open and close at the same time such that there are only three zones.
This shows a return duct and a supply duct routed through an interior wall on the first floor to the master bathroom on the second floor.  Most of the return ducts are created by closing off a stud bay.  Some articles advise against this practice, but the reasons seem most applicable to ducts located outside of conditioned space.  Return ducts formed this way must be on interior walls.  The supply ducts use oval metal duct within a stud bay.  It is better to route supply ducts through interior walls, but we were forced to use exterior walls in some locations.  Fortunately, even in those locations, the ducts are still inside of the rigid foam insulation.
This is the only usable interior wall on the second floor, so the return ducts from the master bedroom and master closet had to be placed in this wall.  The PVC pipe in the other stud bay is a radon vent from the sump pump to the roof.  We don't have any reason to suspect a radon problem, but adding a radon vent later if radon is detected would be much more expensive than now.
Finding a place for the supply duct for the master bathroom was particularly challenging.  Wall registers are preferred in rooms like bathrooms that get wet floors.  Due to the structure under this room, there was no way to route ducts from the basement into any of the walls of this room.  After much head scratching, we concluded that the tub base was a viable option.
The kitchen, at least, was straight forward.  The supply duct will be under the sink in the island.
Lots of things compete for space along the basement ceiling.  Two supply trunk ducts are on the right.  The two parallel PVC pipes are the combustion air and vent for the furnace.  The PVC pipe on the left side of the black beam carries water away from the sump pump.  The two round ducts are supply ducts for the powder room and the basement ceiling respectively.  The red and blue PEX tubing are hot and cold water lines to the powder room and the guest bathroom.  The PVC pipe running parallel to the joists is the powder room sink drain.  The black pipe is the natural gas supply to the furnace.  That small white tube is, uh, I guess I would have to trace that one.

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