On Thu, 9 Mar 2006 19:01:04 -0600, Mike Dobony
wrote:

You obviously never heard of Sick Building Syndrome! Indoor air is highly
polluted and needs a replenishment of fresh air. In the winter time the
house is usually shut up more than in the summer and contaminants just
build up. This is especially bad if someone has allergies to indoor
contaminants, a growing cause of asthma! Also the air exchanged with
opening a door is contributing unheated air to the house. The outdoor air
supply is being heated before entering the house.


I went through a building design construction course. Community
college level, once a week for 12 weeks if I remember.

On the furnace heating system picture it this way. Your house is an
insulated box with insulation for the outside walls only and on the
ceiling. The furnace fires up and the heated is recirculated
within this tightly insulated box to bring the air up to your set
(comfort) temperature.

Heated air will cool on its own mainly conduction loss with some loss
through leakage. The (heated) leakage air loss has to be replaced by
outside air. The best route for this outside air to enter the house
is to duct outside air into the return air duct so that it is heated
first before it joins the circulation.

It is very dangerous to over seal the house, ie make it too airtight,
and not have this outside fresh air intake. The reason is that heated
air normally rises up the chimney. But when the room air cools air
contracts and it will suck air (in the reverse direction) down the
chimney. This suction includes toxic burnt gases present in the
chimney and can be blown back into the house. You breath this stuff.
This is slow poisoning.

Another reason for the fresh air intake is that in the absence of one
the heated air inside the house will be drawn up the chimney together
with the furnace gasses. You lose expensive heated air up the chimney
every time the furnace fires up. The best solution is to have a
separate fresh air supply to the furnace burners. This way outside
air goes direct to the burners and goes up the chimney immediately
after combustion. The burners do not use up warmed room air.

Often all the contractors do is to have an open duct from the outside
to an opening near the burner assembly. Of course in winter this lets
in freezing cold air. Many homeowners plug this duct up with
fiberglass or remove the duct altogether. It will work but you will
be sending warmed room air up the chimney. I fabricated an enclosed
duct to trunk the duct air into the furnace. Cold air is drawn in
only when the furnace fires up, a per normal convection, and the cold
air never gets into the basement. I understand newer furnaces already
have this feature.

To recap there should be a fresh air duct from the outside and
connected to the furnace return air trunk. There should be a separate
fresh air duct to supply fresh air to the furnace burners.