“Tales from the Sierra”
Outdoor Reset
by Alan Willi
One of our current catch phrases going
around is “outdoor reset”. Like all things
we don't understand, it sounds a little
intimidating until we get familiar with the
concept.
Outdoor reset adjusts the supply
water temperature (or the supply air
temperature) in reverse proportion to the
outdoor temperature as the outdoor
temperature falls. As the supply
temperature rises, a typical radiant reset
curve (that's the proportion) is 0.6. That
means that for every degree the outdoor
temperature falls below a set starting
point, usually 70° , the supply water
temperature increases 0.6 degrees. At 0°
outside the supply water temperature
(SWT) would be 112° . That is 70° minus
0° = 70; 70 times 0.6° = 42° ; 70° plus
42° = 112° .
Previously I’ve written about the 3/1
rule: for every three degrees the boiler
water temperature is lowered we save 1%
of fuel. A conventional boiler requires
160° supply water temperature to
maintain 140° return water temperature
(RWT) to prevent flue gas condensation.
If we take the 160° supply water
temperature for the conventional boiler
and subtract the 112° supply water
temperature for the Vitodens boiler that is
a 48° difference. 48 ¸ 3 = 16% savings
with a boiler that would handle 112° SWT,
92° RWT.
Now the sales pitch, I visited a radiant
job in a brand new home (Beaver Mt. Log
Home) heated with HeatLink radiant and
a Viessmann Vitodens boiler at 32°
outdoor temperature. The supply water
temperature was 86° . That is a reset
curve of 0.42° instead of the typical 0.6° .
At zero degrees outside temperature, the

supply water temperature would be
99.4º. At minus 7° the actual supply
water temperature was 104° . Applying the
same arithmetic 160° -140° =54°
difference. 54 ¸ 3 = 18% actual savings
for water temperature. The Vitodens
(WB2-11-44 LP) has an AFUE of 94%. A
conventional boiler has an efficiency of
87%. Take the 7% AFUE difference and
add the 18% supply water temperature
difference together and get a 25%
savings on your fuel bill.
This is why we approach our design
from a systems approach where we can
utilize our products to their ultimate. That
is the way the Vitodens hi-efficiency
condensing boiler and comfort of HeatLink
radiant systems are designed to operate
incredibly well together. Call your Yaun
Company sales representative so we can
help you design a similar system for your
next job.