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Introduction
I recently started with domotica. I use a Raspberry Pi with Domoticz and the RFXCOM rfxtrx433e for this to control KlikAanKlikUit switches. Works perfect.

However, i also would like to control my central heating. I have a Remeha iSense thermostat connected to my Agpo Econpact 225c heater (which talks OpenTherm, just like the iSense thermostat).
But i wasn't willing to pay the 45 euros (kit) or 84 euros (assembled gateway) for the OpenTherm gateway.

DIGI-input of Remeha iSense
I started searching for other solutions, and found out the iSense has a so called DIGI-input. This is a input that is meant to attach a switch or a PIR to (connect the two wires together to let the thermostat know something is happening).
In the iSense you can then configure how the thermostat reacts on this. The user manual provides an example of the usage of a PIR. No motion for xx minutes = turn heating low. But the nice aspect of the iSense, is that you can configure what you want to do. You can also configure that if the input is shorted for at least one minute, the heating turns to the day programme, and otherwise falls back to the night programme.

Using a relay
I thought by myself: Wow, this is a super low-tech way to connect it to my domotica system!! I can simply short the wires with a relay, which is controlled by the GPIO pins of my Raspberry Pi !

I digged through my parts stock, and found a relay that i once ordered for usage with an Arduino. A quick test with an old 5V phone charger connected to the relay, and the relay shorting the pins of the ‘DIGI-input’ of the iSense turned out that it worked great! YEAH!

Relay with optocoupler
However, the relay turned out to be pulling too much current, which the GPIO pins can’t supply. I searched around on internet, and found somewhere that if you want to drive a relay directly with the Raspberry, you need to have a relay module which has an optocoupler. The optocoupler (which only needs a very little amount of current to go into saturation) then drives the relay. The relay is connected to the 5V line of the Raspberry, which can supply more than enough power to drive the relay.

So i ordered a new relay module, with optocoupler, from eBay. Waited two weeks for it to arrive (China, items for cheap, but long shipping times). When it arrived i quickly connected it to the GPIO pins of my Pi and connected the DIGI-input of the iSense. And it works PERFECT! I can now automatically turn off the heating when we leave home, or turn it on from remote.

The relay module was less than $3 on eBay. The only 'hard' thing was to replace the wire running to my thermostat. It was a 2-wire cable, but i needed two extra wires for the 'DIGI-input'. I tied a network cable i had lying to the old thermostat cable and pulled until the network cable showed up on the other end.

Putting DS18B20 inside thermostat
The network cable has 8 wires, but i only needed 4 (2 for OpenTherm, 2 for DIGI-input). So i still had 4 wires left. When i had my thermostat dismounted from the wall i also sneaked in a DS18B20 sensor in the thermostats housing, and put it on 3 of the 4 wires i had left. So i can also read the room temperature from remote now.

Conclusion
To conclude: You don’t need an expensive OpenTherm gateway if you just want to turn on/off your heating which is controlled by an Remeha iSense thermostat.

Schematic

Nice alternative, I had an iSense once, but ditched it cause it didn't do what I wanted it to - can't recall what it was anymore...

Nice cheap way to control the isense!

If you open the relais, do the thermostat Goes back to the usual program, where it was before you close the relais?

Yep, by default it is in the 'nightmode' and when the contact is closed it goes to dayprogram. Turn off the relay = thermostat returns to nightprogram.

So this works really great, and also very cheap! Only investment was the optocoupler relayboard from eBay.