Boiler Control using MAX hardware

So now I’ve got a number of MAX radiator valves around the house which are linked up to FHEM which is providing some monitoring and control. I’ll go into some more detail on the software side in another post. However, I realised that the amount of value that you get out of this system is pretty limited unless you can find a way of controlling the boiler.

Most of the other systems like this which I have seen do have some form of relay or control (sometimes called a ‘boiler interlock’) so that the individual zones can ‘call for hear’ and similarly the boiler shuts off automatically when heat isn’t required. Without this all you have is some fancy valves but the boiler control is still only by a basic thermostat or timer.

Unfortunately, there is nothing available in the MAX range which does this. This seems to me a major omission as all the others do (FHT, HomeMatic, evoHome etc). I had a bit of a problem, as I needed to be able to address the relay via FHEM on the computer, but the CUL device can only be used in one mode at a time. So if I wanted to use a relay from a different system (eg HomeMatic) I would need to have a second device to do so which will obviously cost a lot more.

I thought about this a lot because I really didn’t want to spend a lot of money on this and I didn’t want to overcomplicate matters. After a bit of digging around I found this:


This is one of the MAX products (details hereĀ ) although it is a little obscure. To be honest I’m struggling to work out when exactly anyone would use. It is basically a switched mains plug just like many which you find in home automation systems. However it is designed to work as part of a MAX system. So far as I can make out from the documentation it is supposed to be used with electric heaters or pumps that plug in to the wall. I’ve never seen anything quite like that… but maybe they have them in Germany.

The good news is that because it is a MAX device I can address it from FHEM without using any additional devices. However it wasn’t immediately obvious how I would use it to switch my boiler directly, which used a 240V switched circuit which normally goes to the thermostat. I had a few ideas, including stripping the whole unit down and somehow wiring the relay in directly and whilst I did get the board out to have a look there was no obvious way of doing this. The fact that it uses a European plug (aka Schuko) is another obstacle.

So I considered a few options, including buying a Schuko socket to plug it into. However as above I was trying to keep the costs down and keep it simple. My boiler already has main supply to the existing thermostat receiver so it’s fairly easy to wire things in. So what I have ended up with is starting with a UK socket, then an adaptor, then the switch unit, and then an older shaver plug with a flying lead.

The other end of the lead goes to a huge and rather OTT mains relay which I got for a few pounds from ebay:


Luckily it fits quite nicely into a blanked off single switch box, with the wires poked in through a hole in the bottom and fixed to the screw terminals on the base. The output from the flying lead on the switch goes to energise the coil, and the output contacts are wired to the thermostat input. So I’m using a mains relay to switch a relay to switch the boiler on! It’s a bit convoluted… but it works!

The end result in the boiler cupboard looks like this:


So a little inelegant perhaps… but I’m happy with the result.

In FHEM it can be configured as a MAX thermostat (so it appears exactly the same as one of the radiators) but it can be switched on and off by sending an ‘on’ or ‘off’ command to it.

I’ll cover the FHEM configuration in a bit more detail in the next post