Preparing for some Home Automation – Heating

Today I decided to start planning for the install of some home automation kit, specifically related to heating, but while planning on that I decided to take a closer look at a problem I’ve been having for some time – the thermostat hasn’t been doing very much.

This is a standard dial thermostat – and as you would expect, the numbers on the dial are a little off (~6°C) compared to the actual temperature. This isn’t the actual issue, since once you find the comfortable point, it doesn’t really matter what the reading on the dial is.

The issue I was having was that, regardless of the thermostat position, the radiators were always on (except in the rooms with thermostatic valves) – I had managed before the summer by running with the timer on the boiler, and reducing the boiler temperature.

This has the side effect of (1) the boiler operating less efficiently, and (2) I can’t use it for heating the hot water as the temperature isn’t high enough.

After some digging, I got to this simple diagram of the basic setup:

As you can see, the thermostat doesn’t directly influence the boiler at all – instead it controls the actuator, which in turn allows hot water to flow, or not, through the radiator loop.

Taking a multimeter to the two wires in the Thermostat, I could see that it was in fact switching – 240V between the two when turned off vs 0V between the two terminals when it was on.

So with the thermostat basically functional, I then moved on to the Actuator (SZ2301) – this is a basic 2 port actuator. When there is current applied it opens, and when the current is stopped a spring pushes it back closed – generally speaking these things either work or they don’t.

But… on closer inspection, I found that someone had set the manual lever to flushing – basically forcing the valve open at all times. I moved it back to the “normal” position, and bingo – the thermostat could now control the flow of hot water through the radiators.