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Outdoor Reset Concept
The Temperature controller can change the System Set Point based on outdoor temperature (Outdoor
Reset). The temperature controller 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 and/or sequencing of the cascade.
The Temperature controller can also control the System Circulating Pump with an adjustable Outdoor
Cut-off. When the outdoor temperature is above the Outdoor Cut-off, the pump is turned off and no
heating water is circulated through the system. When the outdoor temperature drops below the Outdoor
Cut-off, 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 the
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 heat can be input 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. With the VERSA, both ends of the slope are adjustable.
It is factory set at 21°C water temperature (Boil START), at 21°C outdoor air (OUT START), and 82°C
water temperature (Boil DESIGN) at -12°C outdoor air (OUT DESIGN).
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 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 VERSA control Reset Ratio allows for full customization 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 user can fine tune these adjustments based on the specific building need.
Reset Ratio
The control uses the four following settings to determine the reset ratio:
Water Heater Start (Boil START)
The Boil START temperature is the theoretical water heater supply water temperature that the heating
system requires when the outdoor air temperature equals the OUT START temperature setting. The
Boil START is typically set to the desired building temperature.
Outdoor Start (OUT START)
The OUT START temperature is the outdoor air temperature at which the control provides the Boil
START water temperature to the system. The OUT START is typically set to the desired building
temperature.
Outdoor Design (OUT DESIGN)
The OUT DESIGN is the outdoor air temperature that is the typical coldest annual temperature where
the building is located. This temperature is used when completing heat loss calculations for the building.
