A.O. Smith VF VB 1000 Service Handbook page 67

BOILER CONTROLS (CONT)
EXTERNAL CONTROLS (CONT)
Types of External Controls
An External Control can be a Primary System Control or a Supervisory Control or both. The
enable/disable circuit described on page 65 would be field wired to the external control's dry
contacts. Field control wiring must be installed in a dedicated conduit.
When an external control is used as a Primary System Control the Operating Set Point on
the boiler (pages 8 and 63) must be set at least 20°F higher than the external control's
temperature setting. This prevents the boiler's Operating Set Point from cycling the boiler off
before the Primary System Control's Set Point has been reached (short-cycling the boiler).
Aquastat is one form of an external Primary System Control. A simple mechanical
thermostat installed in a storage tank (domestic water) or in a T fitting on a heating loop
(heating system).
EMS (energy management system) is another type of external Primary System Control.
EMS controls can also provide supervisory enable/disable functionality. System temperature
can be controlled by an EMS control equipped with it's own Tank /Loop temperature sensor.
Supervisory functionality such as occupied/unoccupied and/or freeze protection can be
programmed in EMS systems to enable or disable boiler operation based on temperature,
time, and event programing.
Outdoor Air Reset Control is another functionality EMS controllers often provide. The EMS
system will have an outdoor temperature sensor or external load sensor in addition to a
system temperature sensor. Typical Hydronic Reset Control operation; as the outdoor
temperature drops, the system set point is automatically raised to provide more heating
capacity as the load increases. As the outdoor temperature rises, the system set point is
automatically lowered to provide energy savings during low load conditions.
Boiler Sequencing Controllers are another type of external control that can sequence
multiple boilers on and off as the system load fluctuates using their own Tank or Loop
temperature sensing device. These types of controls offer lead/lag operation of multiple
boilers to provide equal run time rotation (also available in many EMS controllers) which
evens the wear on all boilers over time. Many boiler sequence controllers are also available
with Outdoor Air Reset Control functionality described above.
Multiple Boiler Control
EMS or Boiler Sequencing Controllers are strongly recommend in multiple boiler
applications.
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VF BOILER SERVICE MANUAL
Servicing should only be performed by a Qualified Service Agent
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