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 here ) 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


Moving to MAX!

When I moved house I started to look at expanding the heating control system, especially given that the new house was much colder than the old one. I started by looking at the FS20 kit I already had, but it was looking really expensive being around £70 per room. On top of that, I was unfortunate in that every radiator in the new house had Vaillant TRVs which are a non-standard fitting and so I’d need an adaptor for each as well.

I was pleased to stumble upon Andy Carter’s blog in which he has gone on a very similar journey, and this introduced me to the MAX! range of products (German language link but but Chrome translates it pretty well). These are produced by a German company called eQ-3 who make a whole range of home automation products. There seem to be two ranges – MAX! and HomeMatic. MAX appears to be a more budget model and is restricted to heating control. HomeMatic is a bit smarter and has a much wider range of modules for all sorts of things including relays, sensors etc but it also has the equivalent of the heating control as well.

The basic system consists of a replacement TRV head and an optional wall controller which directly associated with it. There are now three different types of these although the differences between them are pretty minor.

The big plus for me was that (unless you wanted the wall control) these were a single unit rather than two, and such were substantially cheaper (£25 all in). This made it much more viable to buy in a significant number of them, and in a number of ways they were better than the FS20 units. I liked the large backlit displays and the ease of controlling them, and it was clear that they were also well supported in software. Best of all, the CUL dongle can be used to speak the MAX protocol and so I didn’t need to buy the MAX cube controller.

As I started to put them I learned a few things:

There are various different types of TRV fitting depending on manufacturer. The MAX heads come with various adaptors for the most common ones, and will screw directly on to others. I have ended up standardising on Danfoss valves in the house, for no really good reason except that they are good quality and look nice. The supplied adaptor does work although it a bit of a pig to fit. You get this set of adaptors each time:

The Danfoss one is the far right. You have to remove the bolt, and snap the collar around the valve mechanism. Then you tighten the bolt and screw the valve itself on. Once this is done it seems to work well and is a tight fit. However, I’ve decided that in future I’ll buy valve bodies that don’t need an adaptor at all as this is much easier.

The second thing is that in the UK normal TRVs seem to generally fitted vertically. This does appear to be a matter of convention, although I have read that it makes more sense for them to be mounted horizontally instead because it moves the body of the valve away from the radiator itself and means that the airflow is more representative of the temperature in the room. Also, the MAX valve are obviously designed to be mounted the same way, as if you do mount them vertically the display is upside down. So on all my new radiators I’ve asked the plumber to fit the valves horizontally and they look much better this way. It is easier to read the display and to operate the valves. However I do have some on old valves which are upside down for the time being. It doesn’t seem to affect the way they work, fortunately.

The third thing I’ve found is that installing these valves on their own doesn’t really get you very far! I’ve had quite a few of them in the house, but I don’t think you get very much benefit until you find a way to interlock what the valves are doing with the boiler. This unfortunately is easier said than done… but I have found a way to be described in the next post.