Hamworthy Purewell VariHeat he Installation, Commisioning And Maintenance Instructions page 29

Similarly, if 2 boilers are on and providing less power than P
modulate further down until the divided power reaches P
8.5.2.4.3 Blocking timer
When a boiler is switched ON, a 'blocking off' timer prevents the boiler from immediately switching OFF. Within
this time, another boiler can switch ON, but a boiler can only switch OFF when the timer has expired. Similarly,
when a boiler is switched OFF, a 'blocking on' timer prevents the boiler from immediately switching ON. Again
within this time, another boiler can switch OFF, but a boiler can only switch ON when the timer has expired.
These rules are ONLY overridden when the power of the cascade becomes 0 (eg. open circuit on Remote On/
Off). In this situation all boilers will be switched OFF regardless of 'blocking timer' settings.
8.5.2.5 Control of Cascade (Primary) Pump
When a primary pump is used, it must be connected to the Master. By setting the relevant parameter, it is
possible to control the primary pump as follows:
Pump runs continuously, even with no heat demand, or
Pump Off. Normally this is used when the Primary pump is under external control, or
Pump runs according to the heat demand and switches Off after the pump overrun time has elapsed.
Frost protection of the Primary Pump is not provided by the Master.
8.5.3 Boiler Management System
This mode of operation relates to a 0~10V dc control signal from a BMS to individual boiler modules, and by
setting the relevant parameter, the level of modulation is determined by the control signal, with only the Frost
Protection function remaining active in the boiler control. Two modes of control are possible;
Temperature control
Power control
8.5.3.1 Flow Temperature Control (BMS)
The analogue input signal is translated to a setpoint for the flow temperature. Translation is done by setting the
relevant parameters according to a linear line between 0-10V and a temperature range between 0-100°C.
( Maximum flow temperature is factory set to 85°C ).
When the Remote On/Off input is open circuit, the burner stops and will start again when the Remote On/Off
circuit is closed.
8.5.3.2 Power Control (BMS)
The analogue signal is translated to a setpoint for the combustion fan speed (boiler input) by setting the relevant
parameters according to a linear line between points A & B - see Figure 8.5.3.2
When the input voltage is less than A, the burner stops. When the input voltage is higher than A + Hysteresis, the
burner starts. Should the flow temperature exceed the setpoint + it's Offset, the heat demand is 'blocked' and the
anti-cycling timer starts. The 'block' is removed as soon as the flow temperature has dropped 10°C and a start will
resume when the anti-cycling timer has expired.
HAMWORTHY HEATING LTD
- the modulation is a function of the setpoint temperature.
- the modulation is a function of the determined power requirement of
the cascade.
PUREWELL VariHeat he
(2 * 60 = 120%), the 2 boilers will stay on and
down
(or the divided power <= 100% for 1 boiler)
min
24
500001155/F