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059705–00 REV. B
02
HYDRONIC HEATING
RESET RATIO/OUTDOOR RESET
When a building is being heated, heat escapes through the walls, doors, and windows
to the colder outside air. The colder the outside temperature, the more heat escapes.
If you can input heat into the building at the same rate that it is lost out of the building,
then the building temperature will remain constant. The Reset Ratio is an adjustment
that lets you achieve this equilibrium between heat input and heat loss.
The starting point for most systems is the 1.00 (OD) : 1.00 (SYS) (Outdoor Temperature:
Heating Water Temperature) ratio. This means that for every degree the outdoor
temperature drops, the temperature of the heating water will increase one degree.
The starting point of the curve is adjustable, but comes factory selected at 70°F
Outdoor Temp. and 100°F Water Temp. For example with a 1.00 (OD) : 1.00 (SYS) ratio,
if the outdoor temperature is 50°F, this means the outdoor temperature has fallen 20°
from the starting point of 70°F. Therefore, the system water temperature will increase
20° to 120°F.
Each building has different heat loss characteristics. A very well insulated building
will not lose much heat to the outside air, and may need a Reset Ratio of
2.00 (OD) : 1.00 (SYS) (Outdoor:Water). This means the outdoor temperature
would have to drop 2 degrees to increase the system water temperature 1 degree.
On the other hand, a poorly insulated building with insufficient radiation may need
a Reset Ratio of 1.00 (OD) : 2.00 (SYS). This means that for each degree the outdoor
temperature drops the system water temperature will increase 2 degrees.
The Genesis Heat-Timer
®
has a full range of Reset Ratios to match any buildings
heat loss characteristics.
A heating curve that relies not only on Outdoor temperature but also on type of
radiation will improve heat comfort. The following are suggested initial settings for
different types of radiation based on average building insulation and heat loss. The
contractor can fine tune these adjustments based on the specific building need.
TYPE OF RADIATION IN BUILDING
RESET RATIO
OFFSET
Radiators (Steel & Cast Iron)
1.00 (OD) : 1.00 (SYS)
0˚F
Baseboard (Finned copper tube& Cast Iron)
1.00 (OD) : 1.00 (SYS)
0˚F
Radiant (High Mass/Concrete)
4.00 (OD) : 1.00 (SYS)
–10˚F
Radiant (Low Mass/Joists)
2.00 (OD) : 1.00 (SYS)
–10˚F
Fan Coils & Air Handlers
1.00 (OD) : 1.00 (SYS)
20˚F
WARNING
When controlling a non condensing
boiler directly without the use
of a mixing valve, minimum
boiler water temperature must
be set to boiler manufacturer
specifications. In that case,
system temperature must not go
below such temperature.
Water Temperature
70
60
50
40
Outdoor Temperature
130
120
110
100
Water Temperature
70
60
50
40
Outdoor Temperature
110
100
90
80
Water Temperature
70
60
50
40
Outdoor Temperature
150
140
130
120
1:4
With a 0° Offset,
the Reset curves
begin at 100°
Water Temperature.
With a -20° Offset,
the Reset curves
begin at 80° Water
Temperature.
With a +20° Offset,
the Reset curves
begin at 120° Water
Temperature.
1:4
1:4
1:1
1:1
1:1
4:1
4:1
4:1
90
100
+20 Offset
-20 Offset
Outdoor Temperature
W
at
er
T
em
per
at
ur
e
70
40
50
60
130
120
110
100
1:4
1:1
4:1
Colder
Warmer
70 60 50 40 30 20 10
0 -10 -20
100
120
110
130
140
150
160
180
170
190
200
210
220
1:3
1:2
1:1.5
1:1.25
1:1
1.25:1
1.5:1
2:1
3:1
4:1
Water
Temperature (in °F)
Outdoor Temperature (in °F)
Reset Ratio is Presented as
Outdoor Temp. : Water Temp. Ratio
Reset Ratio Curves
1:4
