Re: Commonhouse air ducts
From: Neena Jud (
Date: Sat, 16 Jul 2011 08:39:36 -0700 (PDT)
Sharon Villines wrote:

> We have two fresh air ducts on our HVAC system for the
> large dining room and kitchen of our commonhouse. The
> dampers were always open but we only needed that much
> fresh air when the room was fully occupied. We asked the
> HVAC engineer to wire the dampers and put a switch
> upstairs so we could open the dampers when we were
> expecting a large crowd but otherwise not be heating and
> cooling outdoor air.

>He said that was against code and instead put a CO2 sensor 
>on the vent that is supposed to open the damper when the 
>CO2 is above 1000 (we think). The other night, with no one
>in the CH and no cooking going on, it was 954.

Hans replied:
>I am not an expert but here is my experience from recent
>remodeling.  Old gas furnaces are constructed in such a way
>that they cannot function without allowing large amounts of
>energy to escape through the flue.  It seems modern furnaces
>are much better (they need PVC or stainless steel flues
>because of condensation water).  Just closing the flue is
>against code for a good reason; the combustion gases from
>Natural gas are of course much better than oil or coal but
>still have very nasty stuff in them, not the least of which
>is PM2.5 (ultra-fine particulate matter).  This is also an
>issue with gas stoves.  Install vents which do not
>recirculate but go outside and have a powerful fan.  300 CFM
>cubic feet per minute) is too little.  I found some with
>440 CFM in the store I was looking, that's what I picked.
>When I heard about the health dangers of gas stoves (which
>of course nobody wants to tell you because profits are at
>stake), I wanted to replace it with an electric induction
>stove, but didn't have the required 220 volts in my home.

>Another energy saving measure which seems obvious but has
>its drawbacks is tightening up the building, i.e., plugging
>all the little holes through which air can get through the
>walls.  If the building is too tight, then the outgassing of
>carpets and paints etc in the building will be a health
>hazard.  Solution: tighten the building and at the same time
>install a heat recovery ventilator (in arid regions as in
>SLC) or an energy recovery ventilator when moisture is an

>All this is much easier with new buildings than as
>retrofits.  If I were member of a cohousing group building
>now I would insist on following the passive house standards.
>They are stricter and more relevant than LEED.  It
>increases the original building costs by 15% or so, but I
>think this will pay off tenfold over the life of the
>building in energy savings, comfort, and resale value.
>Finding an architect who is familiar may be the biggest
>hurdle, but consciousness about this is increasing rapidly.

Sharon's issue was about the fresh air required by the building code for the
occupants of the building, regardless of which kind of stove is in the
kitchen, or the heat source for the water heater or the HVAC system. I think
the recommendation from the HVAC engineer for the carbon dioxide sensor was
appropriate - although it may not be set correctly, it may be defective, or
it may not be installed in the best place. Nonetheless, the principle is
correct - the detector will sense the higher amounts of carbon dioxide in
the air when there are more people in the space and allow for more fresh

This is not to say that Hans' comments were wrong. The HRV or ERV suggestion
is addressing the issue at the heart of this discussion - the need to temper
the outside air economically. Such a ventilator does have an up-front cost,
but there is long term payback.

Honestly, I think in the Washington D.C. area you will find architects (and
some engineers) who know about PassivHaus design as well as very tight
construction. Sometimes the bigger challenge is to find contractors who are
willing to pay close attention to the hidden details of their work to plug
the holes, and then it comes down to the level of the workers - are they
willing to pay attention.

Sorry this does not give you hard data about whether plants have been shown
to improve the air quality (which I think they have), and what are the
acceptable or optimal levels of carbon dioxide in an occupied space. Better
air quality is well known to improve worker's productivity (and is
frequently used as an argument for upgraded HVAC systems) and hospital
patient health - so of course it will improve life for all of us. I wonder
what the ambient air quality is like in the D.C. area? Fresh air brought in
is only as good as that which is outside, even if it is filtered. Could
planting trees near your fresh air intakes be appropriate?

Ahh, more questions. Thanks for reading.

Neena Jud
Harmony Architecture
Cincinnati, Ohio

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