Please provide a complete circuit diagram, your description is only confusing.
This. 6 times like this, and then one with 3 actuators in parallel. The actuators are little motors that run on AC and fully close and fully open the valve with very little current over about 3min. "PWM" with these things and a floor heating is on the scale of 30min periods. (One reason to PWM these instead of half-opening the valve is that it works better with ultra-efficient dynamic pumps.)
I already put on the check list that my SSRs have to be able to deal with the low current.
First thing you need to do is document that with numbers - changes in outdoor temperature, changes in room temperature, response to thermostat settings etc - over two or three weeks. Keep in mind that underfloor heating will be slow to respond and that has nothing to do with the type of thermostat.
A few of these thermostats are downright broken. They also behave differently in every room and there does not seem to be any way to calibrate them. I replaced one, but I kind of want something more intelligent, with accurate sensor readings and integration into the home automation. I absolutely know the physics of a floor heating.
I suspect it will not be as easy to get an improvement as you may think.
I would expect that it is quite easy to get the same performance as a bi-metal based bang-bang controller, seeing that is about 3 lines of code to simulate one. That would already be an improvement, since I had remote control and consistent sensors. From there, I could work my way to a PID controller with above-mentioned very slow PWM.
If the present system is old you may be able to buy off-the-shelf improved control units. Don't be fooled into thinking you can do it cheaper if you put even a minimum hourly value on your time.
The issue is that current intelligent system usually want 3 wires for mains. I am willing to put a 5V signal on a PE-colored wire, but not 230V mains. If I use PE-colored wires for GND and actually put them on earth potential, this would not cause a safety issue even IF someone would interrupt the cable somewhere in the wall and randomly decides to use it as PE.
Some models have batteries and only need two wires, but I want to avoid batteries. They are also quite crappy looking and have no smart features. I assume they switched the bi-metallic switch with a relay, a (questionable) sensor and put on buttons and an cheap LCD.
If anybody knows an of-the-shelf solution with these requirements, go ahead.
PPS. Make sure you can put the system back the way it is now if your project does not work.
That should be no issue. I am not cutting any cables.
PPPS. Check with your home and contents insurer and your landlord if DIY heating controls are acceptable. If they aren't you will have to pay a professional to install an acceptable system before you can sell the apartment.
This is my apartment. There is no landlord. Assume that insurance is fine.
