As I understand it, the 50% damper is a manually operated unit that
helps to control the amount of cool air coming in the house through the
4" fresh air intake of the gas furnace. It's placed near the gas
furnace for easy access. This will really only be useful in cold
climates. From May - October the air temperature outside is 'roughly'
similar to that inside the home, i.e., not significantly different for
the most part. During this time the damper is left open at 100% air
flow. From November - April the air is generally below freezing, and
usually much below that, at least in Alberta. Because cold air expands
when it heats, less is required for fresh air exchange within the
house. The 50% damper is manually engaged during November - April,
reducing the amount of cold air coming in the house, but not the total
amount of air required for air exchanges. As a bonus, apparently it
also saves 10's of dollars in heat savings every year.
Some contractors like to install a Hoyme damper for a similar purpose.
It operates as follows (from the Hoyme website):
"The COMBUSTION FRESH AIR DAMPER, Series 'HOM' is motorized and used
for the control of fresh air for combustion. It is patented and Code
Certified to be field installed and interlocked with a thermostat of a
furnace or aquastat of a boiler, using either oil, propane or natural
gas. The damper is placed at the terminating end of an insulated fresh
air inlet duct beside the furnace. When the thermostat asks for heat,
this motorized damper opens to allow fresh air for complete combustion.
When the furnace stops, the damper closes to prevent the entry of
unnecessary cold air. This results in added comfort with a definite
saving of fuel."
Installer 1 (from my first post) who wants to install the 50% damper
does not believe the Hoyme damper is useful in residential applications
(he will install them in some commercial applications). In fact, he
believes that they may be dangerous. I must admit that I don't
completely follow his logic (other websites laud the Hoyme Damper), but
he seems to know what he's talking about. Perhaps it's related to the
fact that he wants to install a two stage variable speed furnace and
recommends that the motor run constantly.
Hope that explains the 50% damper more fully. And thanks for the