Second controller in parallel with a Honeywell ST9400?

18 Jan 2016
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United Kingdom
Greetings, experts. Is it possible to bypass a Honeywell ST9400 programmer by applying 240V to the appropriate output (pin 4) without blowing it up?

This is in a rented house with TRVs but no room thermostat. I'd like to be able to control the heating remotely, preferably without disturbing the existing system too much.

The Honeywell wiring seems simple: pins 1 and 2 are not connected, pin 3 goes live for HW and pin 4 goes live for CH. So if I use a second controller to apply 240V directly to pin 4, that should turn the heating on – but could it damage the ST9400?

The second controller would be either an Internet-linked Arduino with a proper relay board, or perhaps a Nest/Hive/whatever if these can switch the appropriate voltage and current directly.

Any advice or suggestions for alternative approaches will be appreciated.
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Thank you Dan. Yes, it's a class 1 bodge. But I'm the tenant, and I simply want a more flexible way to control my heating without making any changes that would be hard to reverse, or spending money on anything I can't take with me.

I guess I could find a smart controller that would fit the existing Honeywell baseplate, but that would be a waste of money if I soon move to a house with a different kind of controller.
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Honeywell RF2 pack 3

Very tempting, since the house badly needs a room thermostat. But I'm looking forward to Internet control.

Though I could use my Arduino for that, I suppose … it's currently set up with a 433 MHz transmitter to control electric heaters in the village hall. It turns them on OK, but then fights with the existing wireless controllers, which broadcast a regular "off" signal. Relays are simpler :)

Anyway, many thanks for the feedback.

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