EHH week 08: HVAC precon

This house will be heated (and sometimes cooled) by air running through ducts out to each of the rooms and back from some of the rooms.  There are actually quite a lot of ducts in this house, and the trick is routing them through the structure so they stay hidden above the ceiling or inside the walls.  Easier said than done.

Anticipating this common design problem, VELOCIPEDE made a 3D model of all the ducts to assure there was a viable route from central mechanical room to perimeter windows.  Then once the main floor was framed, the architect, mechanical engineer, general contractor, and HVAC contractor spent a couple hours at the site spotting the route of each and every duct.

This on-site planning effort is called a “precon” for preconstruction meeting.  We considered not just the ducts, but also the plumbing pipe routes, structural obstacles, and architectural niceties like centering grilles below windows.  There were (as there always are) a few problems that we had to work out in the precon.  For example, we had to change four 8” ducts serving the great room to three 9” ducts to better fit in the available joist bays.

Then we marked the chosen routes and grille locations right on the structure for the subcontractor to follow at install.  A couple hours of planning effort should avoid problems and make things go smoothly later on.  Remarkably, duct layout is usually left to the installers to figure out the day they arrive on site with a truckload of ducts and fittings.


EHH week 36: heating system

With a superinsulated enclosure and triple glazed windows, the heating system does not need to be big.  We considered and rejected both ground coupled heat (often called “geothermal”) and radiant floor heat because both would be overkill for the minimal needs of this house.


Ecotope, the mechanical engineer, recommended an inverter driven split system heat pump, made by Mitsubishi.  For each kilowatt of electricity input to a heat pump, it will provide about 2.5 kilowatts of heat, because a heat pump moves heat instead of generates heat.


A heat pump operates on the refrigeration cycle, but instead of cooling the inside of a refrigerator, it heats the inside of a room.  Heat pumps can provide air conditioning too, in which case they act just like a refrigerator. 


There is one outdoor unit, which is the compressor, attached to the garage.  Inside we have six zones, each with its own programmable thermostat.  Three of the zones have condenser units in utility rooms and their heat is distributed by ducts.  The ducts are air sealed at every joint with a gray mastic to eliminate leaks.  Aluminum diffusers at the end of the ducts are either set flush in the wood floors or on top of the concrete floors, so they look great and familiar.


The other three zones are heated by wall mounted condensers that emit low velocity heated air.  These white units, called “cassettes,” must sit high on a wall, usually over a door.  They are not as elegant as floor diffusers, so we used them in less important rooms.