Yes, in fact my use is not for a boiler but for a 2kW fan heater (which is 8A at UK mains voltage). Theoretically a Sonoff Basic rated at 10A should handle that but I suspect it may not last long at that power, with the SSR it should be fine.
For my boiler I only switch the control signals to the boiler (the signals that are normally handled by the standard time switch that comes with the boiler). These are not high current and I drive them from a relay board attached to a Pi. I have normally closed relay contacts in series with the room thermostats so to use the Pi to control it I turn the room tstats up and the pi controls it. If I have a problem with the pi I can switch it off, the relay contacts all close, and the system operates as it used to do before I fitted the Pi. Additionally, with the room stats just a bit higher than they would normally go, even if the Pi goes off the rails the worst that can happen is that it can heat up a bit higher than normal, as the room stats will kick in and prevent overheating.
What you said about the Pi-relay in-series with the Room Stat is exactly what I was thinking of doing.
It avoids messing around with the controls that are already mounted or recessed on the walls.
Then, assuming that you have a temperature sensor in the room, you can use <blatant plug>node-red-contrib-pid</blatant plug> to use a PID loop to control the temperature and get decent accuracy of control.
Ha, ha - love it.
Well, thanks everyone for this, it has taught me some more stuff about mains switching that I hadn't really thought about and given me some ideas on low-cost solid-state relays - the previous one's I'd looked at were really expensive so these finds are very interesting.
There are a few loads in the house that I'm reluctant to use on a SONOFF and would prefer solid-state relays anyway. So I may get one or two of these to have a play with. Still need to add an ESP8266 for the control of course and then work out some kind of housing and think about heatsinks so not the simplest of projects - but a step closer anyway.
The spec suggests 1.6V drop, so for my 4 A load that suggests a bit less than 7 Watts. So it will get quite warm without a heatsink. However the max operating temperature is 80% so if you can put your hand on it then it is ok. I am hoping that at 4A I won't need a heatsink when operating in a domestic environment.
Julian,
The beauty of the High Voltage AC switching units is that you can use a normal Sonoff with Tasmota etc to control it, no need to mess around with the ESP8266/Wemos/NodeMCU and then a power supply, housing etc etc
Craig
Thanks Craig, I do get that and it is an interesting approach. I'm not very confident with electronics so it tends to take me a long time to get my head around things.
What would be great would be to see a blog-post or instructable with some details and a diagram so that it was all clear - he says hopefully 
OK let me see what i can do on that front
Craig
My two cents worth, from some experience.
Following on from dcjay, BE CAREFULL.
In Australia, at least, it is illegal to do any work above ELV, 120vdc/50vac unless licenced. For good reasons, electrocution springs to mind but the most common cause of death from faulty wiring is.....smoke inhalation, yes FIRE.
So, as well as your family being dead or injured, your insurance may be void due to illegal wiring.
Find a friendly electrician, please.
Hi.
Do you have experiences with Sonoff Basic2 ( FW 2.6.0)?
Who is that question addressed to and is it to do with the subject of the thread, which is about managing a boiler?
If you are talking about the Tasmota s/w then there is a good support list at https://groups.google.com/forum/#!forum/sonoffusers
It was addressed to PKGeorgiev. I am talking about original FW of Sonoff Basic with node-red-contrib-sonoff-server.
Hi. I just wanted to say great project. I have also installed Sonoff R2 with my domestic hot water tank. It has 300l and I configured it with a simple start and stop-node controlled by time (or manually) in Node-red. The sonoff turns on during the night where the energy prices is at lowest, and turns off in the morning. The internal thermostat control the temperature. I have this set to 70, and during a day it stays at a aceptable temperature.
t
I use node-red to send the values to influx, and Grafana to visualise the data, runtime, kwh used per day, week/month.
