Thermistor Sensors

The pump voltage is only about 70V, not enough to run the pump when the hot water tap is open.

I unplugged the sensor connector to the control board and measured the following.

DHW and PHE sensors.
Between L and N both infinite resistance.
Between L and E 5kohm
Between N and E 4kohm

By switching the pump on and off manually I can alter the temperature at the sensors but the resistance doesn't change hardly at all between hot and cold.

I notice the resistance is dependent on polarity so I assume these are semi-conductor thermistors.

Does anyone know what would affect the pump supply voltage other than the sensors?

Update: Well, after running the pump directly on 240V mains for a while I reconnected it to the control board and now it will start and run with only 70V. In fact I see now that once the pump has started and hot water is flowing, the control board varies the pump voltage according to flow, the voltage starts around 70V and goes up to about 180V with one hot water tap turned on.

Conclusions: 1) I think the problem with the pump not starting with 70V is that the system had been off for a few months and I guess the pump motor was a bit sticky. After running it at full 240V for a while it was unstuck and now the system is working fine.

2) The above values for sensor resistance would seem to be correct. The resistance change is quite small but enough to work the system.

3) Follow the advice in the manual and leave the system powered up even if the hot water is turned off, this cycles the pump every now and again and stops it sticking at start up voltages of only 70V.

Nick.

Hi just a bit of information. On this system the temperature sensors are not thermistors they are active devices with chips in, this is why they have three wires which are Gnd +5V and signal. Similar ones are used on other Gledhill units such as the Boilermate 2000 etc.. The hot water pump is also speed controlled and I believe like the Boilermate on startup the pump will run at low speed to preheat the heat exchanger, the system uses the pump speed to regulate the water temperature in normal operation. The speed control pulses the motor so voltage readings can be a bit strange. Hope this helps. There is a fault finding section in the manual, you have probably seen it, that shows how to interpret the flashes of the LED for fault finding purposes. Regards Murray

Thanks Murray, sensor info confirms my own thoughts.

Concerning the motor control, thanks for the explanation. It makes sense that the motor would be pulsed with the full 230V mains voltage, the motor is not designed to run on lower voltages. Of course my meter only shows an average voltage depending on the duty cycle. It is interesting that despite the full 230V being applied, the motor would not start after being off for several months, something Gledhill must be aware off because they recommend leaving the system power on all the time even when no hot water is required.

I did find the LED indicator description in the manual, thanks.

Overall my understanding of how it works is improving, thanks once again for the input.

Nick.