10.14 Heater Circuit
The water temperature in the pressure tank is controlled by a thermistor probe. The
thermistor has a variable resistance; when cold it has a high resistance and when hot
it has a low resistance.
1. The thermistor probe sits directly in the water and continuously senses the water
temperature. The resistance of the thermistor is interpreted by the controller
as a temperature reading.
Note: The resistance of the thermistor when at ambient (room) temperature
should measure about 3000 ohms, when hot (205°F/96°C) it should measure
approximately 220 ohms.
2. If the water needs to be heated, a signal from the controller is sent down the
Comms. link to the I/O board (heater on signal). The I/O board then switches a
24 volt negative output to the solid state relay. The solid state relay then switches
a 115V supply to the heater element. The element then starts to heat up the
water.
Note: Heating element fitted in the pressure tank is rated at 1250W.
82
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