NEGATIVE AIR – MAKEUP AIR
NEGATIVE AIR PRESSURE
When more air is found to be leaving a home than what is coming in, a partial vacuum effect occurs. We
call this effect, Negative Air Pressure. In winter, heated indoor air rises up through the home and es-
capes from upper level leaks such as windows and poor insulation etc. At the same time, we force air
out of the house with exhausting appliances such as bath fans, clothes dryers, kitchen exhaust fans, and
furnaces. All of these appliances are contributing factors to a negative air pressure problem, which is
basically a lack of “make-up air”.
Sometimes negative air pressure can lead to annoying situations such as a fireplace or stove that leaks
smoke or will not draw air at all. At other times a more serious and dangerous situation can occur, such
as an air flow reversal that draws carbon monoxide from a furnace chimney into the house. Negative air
pressure also contributes to house condensation, mould and mildew and a general unhealthy environ-
ment for the house occupants.
Most heated homes will leak some air due to pressure differences and the natural “stack effect” of the
house. Stack Effect means your entire house works like a chimney, an example being heated air rising
up the stairways. If you crack open a window, you will usually feel air coming into the house rather than
blowing out. Without planned ventilation, this negative air pressure will draw air in at uncontrolled points.
When you control ventilation,
YOU
determine when and where air enters your home, getting the maxi-
mum benefit from the least amount of air.
One example of addressing this problem would be the use of an “Air Supply Ventilator” or similar unit.
There are many manufacturers of this type of product.
If you encounter a negative air pressure problem, WinterGreen recommends you contact a local HVAC
specialist to determine the best solution to your needs.
Supplying Make-up Air
Furnaces, fireplaces, clothes dryers, exhaust fans and most other air moving appliances all draw air
from the particular room they are located in. The WinterGreen Furnace also draws air and should also
be taken into consideration when determining if there is enough fresh air to satisfy the needs of all your
appliances. An insufficient supply of fresh air will create a negative pressure in the room and potentially
starve the furnace of the necessary combustion air it requires to function properly. The following proce-
dure will help to determine the air requirements for the room that your WinterGreen Furnace is to be
located in.
Determining the Volume of Fresh Air:
1) Determine the volume of space (cu. ft.) in the room. Include all adjacent rooms in the calculation that
are not closed off by doors.
Volume (cu. Ft.) = length (ft.) x Width (ft.) x Height (ft.)
2) Determine the air input requirements of all the appliances located in the calculated room space.
(See: Manufacturer’s Plate on each appliance)
Take the sum of all the appliances and round it off
to the nearest 1000. This sum will become “BTUs per hour”.
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