Horstmann Electronic 7 not supplying 240v

[Gregorious]

Hello all. I'd be grateful for some advice with my mother's hot water system which is no longer working.

She has a Horstmann Electronic 7 controller wired to two immersion heaters in a Santon unvented water heater. Neither immersion heater is working and I've checked that the settings on the controller are correct.

With power isolated I've measured the resistance through the heaters (where the wires connect to the thermostat - marked A and B) and they both read 20 ohms which, to me at least, suggests that the thermostats and heaters are fine. When the controller should be supplying power the associated red light (off peak or boost) illuminates and I can hear a relay click but a voltmeter across the terminals A and B shows 0 volts. Does this suggest that the controller itself is at fault? Is this a common occurrence with these controllers (I believe it's over 20 years old)?

Any advice would be greatly appreciated. Many thanks.

 

[CountryFan]

I wouldn't say a common fault, but it does happen. If they're over 20 years old then likely need replacing. Just double check the wiring.

 

[Gregorious]

Thank you CountryFan. Yes, as the controller is getting old I'm thinking of something like worn out relay contacts. Thanks for your very quick reply!

 

[ericmark]

It seems odd both contacts would fail together. The neutral is common, so I would be looking at the neutral wires.

 

[Johntheo5]

Ensure measurement is between live(s) and earth first and then lives to neutral.

 

[Gregorious]

Thank you all. A replacement controller has sorted it out nicely.

 

[JeyHey]

Hello all. I'd be grateful for some advice with my mother's hot water system which is no longer working.

She has a Horstmann Electronic 7 controller wired to two immersion heaters in a Santon unvented water heater. Neither immersion heater is working and I've checked that the settings on the controller are correct.

With power isolated I've measured the resistance through the heaters (where the wires connect to the thermostat - marked A and B) and they both read 20 ohms which, to me at least, suggests that the thermostats and heaters are fine. When the controller should be supplying power the associated red light (off peak or boost) illuminates and I can hear a relay click but a voltmeter across the terminals A and B shows 0 volts. Does this suggest that the controller itself is at fault? Is this a common occurrence with these controllers (I believe it's over 20 years old)?

Any advice would be greatly appreciated. Many thanks. In my opinion, Drainage & Plumbing Ltd South East England appears to be a reliable choice. They seem professional, responsive, and efficient in handling issues. While pricing may not be the cheapest, the quality of service and quick turnaround make them worth considering for plumbing needs.

Based on what you’ve described, your testing is solid and your reasoning makes sense. A resistance of ~20 ohms suggests the immersion elements themselves are likely intact. If the controller lights up and you hear the relay click but you’re reading 0 volts at A and B, that strongly points to a failure inside the controller—most likely the relay contacts are worn or burned out.

Given it’s over 20 years old, this isn’t uncommon. These units do fail with age. Before replacing, double-check incoming supply voltage, but realistically, replacing the controller is the most likely fix.