A.O. Smith BFC Cyclone 120, Installation, User And Service Manual

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5.4 Operating cycle
The temperature sensor T
1
(7) measures the water temperature at the top of the water
heater (T
top
). This temperature is sent to the controller. As soon as T
1
is lower than the
set water temperature (T
set
), the controller registers a "heat demand". The fan (18) starts
running and the gas control valve (16) is opened. The fan runs at full speed and the gas is
mixed with the air in the venturi (30). This mixture is ignited by the hot surface igniter
(20) and the water becomes heated. As soon as the water temperature approaches T
set
the fan rpm drops to the partial load speed (this is also referred to as modulating). Once
T
1
rises above T
set
, the heat demand ends and the controller stops the operating cycle.
When responding to both the start and end of the heat demand, the controller assumes a
certain margin. We refer to this margin as the hysteresis (see section 9.2.1).
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