The question is, do you trust your own handiwork to the extent you're not worried it'll malfunction and burn your house down, or freeze your pipes during winter? Personally, I'm reluctant to have electronics I built myself connected to mains and running unsupervised for longer periods of time.
Well, some parts of it are a lot more likely to fail-deadly than others. A commercial power supply that outputs 12V or whatever your servo wants basically can't burn anything.
As for bugs in the control code freezing your pipes? Well, just give the code as much care as you give to driving and you should be fine. ;)
(The serious solution to that is to have a dead simple failsafe controller that switches between on, off and auto, where auto is controlled by your crazy inventions.)
For projects like that, you don't connect anything to the mains at all. Specifically:
- At least in US, thermostats control is low-voltage (24 V AC), and come from low-power transformer which will just stop working on overload (ask me how I know :) ), so there is no electrical fire danger
- You never plug anything into the AC outlet directly -- you always use AC/DC adapters (and not the ones bought on ebay for minimum price, too)
- To prevent freeze hazard, you also leave original thermostat hooked up, but set it to really low temperature (10C/50F).
- Overheating house is possible, and you definitely need to consider this in your software (for example, require periodic control messages to keep your boiler on). However, I do not think it is an actual fire hazard -- likely, there are hardware safeties in your heater to prevent that. Still, if you are worried about this, you can install a second thermostat in series with your relay and set it to max temperature you ever want.
That's a very interesting point, I built my own system, but I don't trust my own handiwork enough, so I went the middle route:
I got a Siemens RDF302 thermostat (which is a dumb thermostat with a Modbus RTU interface for remote control) and then I use a raspberry pi to change the setpoint temperature according to a remote signal or schedule.
The actual decision to turn on/off the boiler however, is still left to a "real" thermostat made by a professional company, and a malfunction of the automated system is likely to result in a failure to change the setpoint, not in infinite heating.
