[Greenbuilding] One more question on AC

Alan Abrams alan at abramsdesignbuild.com
Tue Jul 9 15:31:46 MDT 2013


on my current passive house project (pre-certified in May; permit issued
today!) I opted for separate distribution systems for ventilation and for
heating and ac.  there were too many unique conditions throughout the house
(different floor levels; different solar exposures and shading conditions;
different functions, such as cooking, bathing, laundering, etc) to rely on
a gently puff of ventilation air to heat, cool, and dehumidify--while still
exhausting stale air and bringing in fresh air in appropriate volumes
throughout a complex floor plan.  Conversely, if the ducts are sized for
max loads, then it would be optimistic to try to push properly metered
ventilation air through that system while the heat pump was not running.
 Thus dual ducting.

the heating and air will be an inline Fujitsu, about 18KBtu capacity.  It's
oversized for the calculated loads, but only to push enough volume to
handle dehumidification and prevent hot spots.  One concern is overheating
in shoulder months, when the leaf cover is gone and the sun is lower than
the overhangs--which are tuned for summer shading and max winter
gain--IOW's we anticipate days where some cooling may be required in the
afternoon, and heating is required later at night.

at a ton and a half, it raises the question of why not just go with a 2 ton
Carrier Infinity, with almost twice the fan capacity--but still able to
wind down to maybe 3KBtu.  But we decided to do a fine tuned duct system,
reducing the trunk at each tap, with sweeping takeoffs and boots, to reduce
friction where-ever possible.

this all is counter to the passive house legend--that when you hit some
sweet spot of insulation, you only need a small scale ventilation air
distribution system to distribute heat and cooling--thus justifying the
premium for the 9 inches of insulation under the slab--but I think that
dealing with our latent loads dispels that myth.

-AA


On Tue, Jul 9, 2013 at 4:55 PM, Gennaro Brooks-Church - Eco Brooklyn <
info at ecobrooklyn.com> wrote:

> Combing ERV/HRV and cooling is a huge question. Does anyone have
> experience with a new system where you can plan for the ductwork? In that
> can could you install large ducts and run the ERV and some sort of cooler
> in the same ducts? In that case I think it may be better to find some
> inline cooling system instead of jerryrigging a minisplit to work inline?
>
> Gennaro Brooks-Church
> Director, Eco Brooklyn Inc.
> Cell: 1 347 244 3016 USA
> www.EcoBrooklyn.com
> 22 2nd St; Brooklyn, NY 11231
>
>
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