Installation / Operation Instruction Manual

Page 71
Low demand: The rst boiler in sequence ignites and
gradually increases ring rate to satify the heat demand.
Increased demand: Once the rst boiler reaches the Base
Load Value (50%) ring rate, the second boiler ignites. After
ignition, both units modulate to half of the cascade ring rate,
then gradually increase the ring rate together, up to the
Base Load Value.
Increased demand: Once the rst two boilers reach the
Base Load Value (50%) ring rate, the third boiler ignites.
After ignition, the three units modulate at 1/3 of the cascade
ring rate, then gradually increase ring rate together, up to
the Base Load Value.
Approaching max demand: Once all three boilers reach
the Base Load Value (50%), all units are allowed to increase
ring rate (same at all boilers) up to maximum ring rate.
Decreasing demand: As the demand decreases, once the
requested ring rate reaches the Drop Load Value, the third
boiler turns o󰀨.
Continued Decreasing demand: As the demand
decreases, once the requested ring rate reaches the Drop
Load Value, the second boiler turns o󰀨.
DemandSatised: When the heat demand is satised or
the temperature reaches set point + o󰀨 hysteresis, the nal
boiler will turn o󰀨.
As the load increases:
• Until all units are ring, no unit is requested to exceed
the base load value.
• Additional units are added once the Base Load Value
has been reached and the Min On Time timer has expired.
• As long as all boilers/heaters are ring, the base load
value can be exceeded, as long as all units maintain the
same ring rate.
Figure 38. Lead / Lag, 3 Boilers
As the load decreases:
• As long as all units are ring – the base load value can
be exceeded, as long as all units maintain the same ring
rate.
• As the ring rate decreases below the Drop Load
Value, the last unit to ignite turns o󰀨 rst (last on/rst o󰀨),
following this pattern until the heat demand is satised and
all units are o󰀨.
Units in Lead / Lag mode maintain local boiler/heater
limiting features (ring rate limiting based on outlet or ue
temperature) when in Lead / Lag mode operations.
1 2 3
30%
1 2 3 1 2 3
50%
25% 25%
1 2 3 1 2 3
50% 50%
33% 33% 33%
1 2 3 1 2 3
100% 100% 100%
50% 50% 50%
1 2 3 1 2 3
20% 20% 20% 20% 20%
1 2 3 1 2 3
20% 20% 20%
1 2 3 1 2 3
20%
Low demand: The first boiler in sequence ignites and
gradually increases firing rate to satisfy the heat demand.
Continued decreasing demand: As the demand decreases,
once the requested firing rate reaches the Drop Load Value
, the
second boiler turns off.
Lead/Lag - 3 Boilers
Increased demand: Once the first boiler reaches the Base
Load Value ( 50%) firing rate, the second boiler ignites. After
ignition, both units modulate to half of the cascade firing rate,
then gradually increase the firing rate together, up to the Base
Load Value .
Approaching max demand: Once all three boilers reach the
Base Load Value (50%), all units are allowed to increase firing
rate (same at all boilers) up to maximum firing rate.
Demand Satisfied: When the heat demand is satisfied or the
temperature reaches set point + off hysteresis, the final boiler
will turn off.
Decreasing demand: As the demand decreases, once the
requested firing rate reaches the Drop Load Value , the third
boiler turns off.
Increased demand: Once the first two boilers reach the Base
Load Value (50%) firing rate, the third boiler ignites. After
ignition, the three units modulate at 1/3 of the cascade firing
rate, then gradually increase firing rate together, up to the Base
Load Value.