heat-timer Mini-MOD-CNC Installation And Operation Manual page 9

The Mini-MOD-CNC has multiple operating modes that satisfy most hydronic systems.
It changes the System Set Point based on outdoor temperature (Outdoor Reset). The
Mini-MOD-CNC varies the temperature of the circulating heating water in response
to changes in the outdoor temperature. The heating water temperature is controlled
through the modulation or sequencing of the stages.
The Mini-MOD-CNC also controls the system circulating pump with an adjustable
Outdoor Cutoff. When the outdoor temperature is above the Outdoor Cutoff, the
pump is off and no heating water is circulated through the system. When the outdoor
temperature drops below the Outdoor Cutoff, the system pump relay is activated and
the heating water circulates through the system. The temperature of the heating water
is controlled by the Reset Ratio, Water Offset, and changes with Outdoor temperature.
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 temperatures 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 temperature has fallen 20° from
the starting point of 70°F. Therefore, the heating 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 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 dropped the water
temperature will increase 2 degrees. The Mini-MOD-CNC 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 the 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
Radiators (Steel & Cast Iron)
Baseboard (Finned copper tube& Cast Iron)
Radiant (High Mass/Concrete)
Radiant (Low Mass/Joists)
Fan Coils & Air Handlers
059102-00 REV. C
05
OUTDOOR RESET CONCEPT
1:4
130
120
110
100
70 60
Outdoor Temperature
RESET RATIO OFFSET
1.00 (OD) : 1.00 (SYS)
1.00 (OD) : 1.00 (SYS)
4.00 (OD) : 1.00 (SYS)
2.00 (OD) : 1.00 (SYS)
1.00 (OD) : 1.00 (SYS)
220
210
200
190
180
170
160
150
140
130
120
110
100
70
Outdoor Temperature (in °F)
Outdoor Temp. : Water Temp. Ratio
Reset Ratio Curves
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.
1:1
Warmer
Colder
4:1
50 40
0˚F
0˚F
-10˚F
-10˚F
20˚F
1:4 1:3 1:2 1:1.5
1:1.25
1:1
1.25:1
1.5:1
2:1
3:1
4:1
50 30 0 -10
60 40 20 10 -20
Reset Ratio is Presented as
1:4 1:1
130
120
110
4:1
100
70 60 50 40
Outdoor Temperature
1:4 1:1
110
100
90
-20 Offset
4:1
80
70 60 50 40
Outdoor Temperature
1:4 1:1
150
140
130
4:1
120
90
+20 Offset
100
70 60 50 40
Outdoor Temperature
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HEAT-TIMER CORP. 9