5 of 16
In order to properly control a hot water heating system, the amount of heat
supplied to the building must equal the amount of heat lost by the building.
The amount of heat delivered into a building depends on the temperature of
the water in the heating unit and the surface area of the heating unit. Heat-
ing units with a small surface area, such as baseboard radiators, require a
higher water temperature than heating units with a larger surface area such
as radiant floors.
The amount of heat lost from a building depends on the outdoor tempera-
ture. As the outdoor temperature becomes colder, the amount of heat a
building loses increases.
The operation of a hot water heating system can generally be improved by
adjusting the supply water temperature to the system as the outdoor tem-
perature changes. Using this approach, the heat input to the building can be
matched to the heat lost from the building. This method of controlling the
supply water temperature to a heating system greatly improves the comfort
of the system and is known as Outdoor Reset.
When a Taco Outdoor Sensor is connected to the Variable Speed Mixing Control (VSMC), the VSMC provides outdoor reset. When
operating in the outdoor reset mode of operation, the installer must set the Outdoor Design Temperature and the Design Supply Tem-
perature in order to establish the relationship between the outdoor temperature and the supply water temperature. This is known as
setting the Heating Curve.
SEQUENCE OF OPERATION
When the VSMC receives a Demand and it is not in warm weather shut down (WWSD), the VSMC turns on the system pump and
calculates a Mixing Target temperature. The variable speed heat source side pump is then operated to maintain the Mixing Target
temperature at the mixing supply sensor.
The heat source contact operates as described in the Heat Source Operation section. The VSMC also provides heat source protection
as described in the Heat Source Operation section.
DEMAND
The VSMC requires a demand signal before it will begin operation. The VSMC can use
either a powered or an unpowered demand signal. Once a demand signal is received, the
VSMC displays the demand icon in the display and operates as described above.
Powered Demand
The VSMC recognizes a Powered Demand Signal when 24 V (ac) is applied across the
Com and Heat Dem terminals.
Unpowered Demand
The VSMC recognizes an Unpowered Demand signal when a switch is closed between
the Com and Heat Dem terminals (relay type thermostat or end switch on zone control).
SYSTEM PUMP OPERATION
The VSMC has an internal system pump contact. This contact turns on when the VSMC
has a mixing demand and is not in a WWSD. The integrated system pump as well as an
external heat source pump may be controlled by this relay. By providing proper flow in the
heat source loop, the heat source temperature can be accurately controlled based on the
mixing load.
Outdoor Reset Mode of Operation
Outdoor Reset Settings
HEATING CURVE SETTINGS
In order to establish the heating curve, the VSMC must be given two points to work with. The first point is the Outdoor Reset Starting
Point and the second point is the Design Condition.
Outdoor Reset Starting Point
The Outdoor Reset Starting Point for the VSMC is fixed at 72
°
F. This means that when the outdoor temperature is 72
°
F, the VSMC
calculates a required supply water temperature (Mix Target) of 72
°
F.
Design Conditions
The design conditions represent the supply water temperature required to satisfy the heating system on the typical coldest day of
the year. These are the conditions that are used when calculating the size of the heating equipment needed to heat the building.
The Design Conditions are made up of an outdoor temperature (Outdoor Design) and a supply water temperature (Design Supply).
Com Heat
Dem
Boil Out
Powered
Demand
C
24 V (ac)
R
Switch
Com Heat
Dem
Boil Out
Unpowered
Demand
Switch