© 2013
400_D - 09/13
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Auto Differential
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Both on-off and modulating boilers are operated with
a differential. In some cases, a modulating boiler must
be operated with a differential while operating at low
fire because this indicates the load is smaller than
the minimum modulation of the boiler. For modulating
boilers, the differential no longer applies once operating
the boiler above low fire.
The differential operates by closing the boiler contact
when the boiler supply water temperature is 1/2 of the
differential below the boiler target temperature. As the
supply temperature reaches 1/2 of the differential above
the boiler target temperature, the boiler is shut off.
+1/2 Differential
- 1/2 Differential
Boiler Target
Boiler On
Boiler Off
In order to decrease temperature swings and increase boiler
efficiency, the Auto Differential feature automatically adjusts
the operating differential of the boiler based on the heating
load. As the load increases, the differential will decrease
to minimize temperature swings. As the load decreases,
the differential will increase to prevent short cycling. This
can significantly improve operating efficiency and prevent
equipment failures through excessive cycling.
Off
Differential
Time
Heating Load
On
Pump Operation
Section F
Boiler System Pump
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The boiler system pump operates whenever all the
following conditions are met:
A Boiler Call is present.
The thermostat calling for heat has the Heat 1 pump
setting set to On.
There is no DHW priority.
There is no Setpoint Call present.
The heating system is not in Warm Weather Shut Down.
DHW Pump
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The DHW pump operates whenever all the following
conditions are met:
A DHW Call is present.
The heating system is not in DHW priority override.
DHW post purge.
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Pump Exercising
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The control operates each pump every three days for 10
seconds to help prevent corrosion and / or precipitate build
up on the pump components that can lead to pump seizure.
The control ensures that no heat is supplied to the zones
during exercising.
