tekmar 274 Boiler Control - Manual (Page 14)

© 2012 274_D - 11/12 14 of 44
Mode 3 - Setpoint in Primary/Secondary with No Priority
Whenever a Setpoint Demand is present, the primary pump
is turned on and the boilers are operated to maintain the
setpoint target.
Primary
Pump
Setpoint
Mode = 3
Mode 4 - Setpoint in Primary/Secondary with Priority
Whenever a Setpoint Demand is present, the primary
pump is turned on and the boilers are operated to maintain
the setpoint target. Space heating zones will be shut
off if the boilers are unable to maintain the boiler target
temperature.
Setpoint
Primary
Pump
OFF
Mode = 4
Setpoint Priority Override Setting in Adjust Menu
Setpoint Priority Override applies to SETPOINT MODE 2 and
MODE 4. To prevent the building from cooling off too much or
the possibility of a potential freeze up during setpoint priority,
the control limits the amount of time for setpoint priority.
When set to auto, the priority time is calculated based
on outdoor temperature. At or below the design outdoor
temperature, 15 minutes are allowed for Setpoint priority.
At or above 70°F, 2 hours are allowed for Setpoint priority.
The time allowed for Setpoint priority varies linearly
between the above two points. There is a manual setting
also available in the adjust menu.
The priority timer does not start timing until priority is
selected and both a Setpoint Demand and a Boiler Demand
exist together. Once the allowed time for priority has
elapsed, the control overrides the Setpoint priority and
resumes space heating.
Automatic Priority Override
15 mins
2 hours
Design Temperature 70°F (21°C)
Conditional DHW Priority
If the boiler supply temperature is maintained at or above
the required temperature during setpoint generation, this
indicates that the boiler has enough capacity for setpoint
and possibly heating as well. As long as the boiler target
temperature is maintained, setpoint and heating occur at
the same time.
Setpoint Post Purge
After a tN4 Setpoint Demand is removed, the control
performs a purge. The control shuts off the boilers and
continues to operate the Setpoint Pump and the primary
pump if applicable. This purges the residual heat from the
boilers into the Setpoint load. The control continues this
purge until one of the following occurs:
1. A Boiler Demand is detected
2. The boiler supply drops 20°F (11°C) below the Setpoint
target temperature
3. Two minutes elapse