They are not. The forum has reported a number of relay failures under varying voltage conditions.
Go to an electronics distributor like Digikey or Mouser etc. Purchase a name brand industrial relay.
These devices are famous for starting fires. They are so much cheaper than properly designed thermostats that they have almost universally replaced the proper ones. You are doing well to replace them.
Assuming you are using the thermostat to control some type of electrical heater, the best design will be to put the proper relay inside the electrical heating device.
Is there room for the relay?
Why not use an HVAC defined purpose contactor? Drive the coil with 24VAC and buy the one that suits. Single pole, single pole+1, double pole, triple pole, lots of options. Fairly inexpensive, too. Probably want to ensure you use a run capacitor too, for reasons (large initial inrush of current)
What about a triac?
Now fixed sorry.
I'm using those blue hobby relays in a few installations around the house.
I have a bathroom controller system, which uses the relays to control 3 240v lights, and a 240v extractor fan. I have had no problems in over 3 years.
I also use them in a control system in the workshop. The relays are switching various lights and light load modules.
One relay gives me trouble from time to time. It sticks in the closed position. I have to lightly tap it with a screw driver, and it will work okay for a few months before another tap is required. The workshop system has been in for over 5 years.
I have had a few of those relays fail, but that has been right out of the box. Those that have worked straight away seem to be very reliable (except the sticky one).
The problem with these relays is not so much the functionality, but the safety ( see #19 ). If you're lucky, you won't even notice, if you're unlucky, you'll only notice when it's too late. With 230V, however, 'luck' is a bad strategy.
Here is a list of industrial quality relays Any one of them might be in a typical commercial thermostat.
I usually sort on the quantity in stock and then choose from there, thinking the relay with the most sales is likely a good choice.
The list has both 5Vdc coils and 12 VDC coils. Some may be driven directly through the processor and some you might want to use a simple transistor interface (basically a transistor with one resistor....simple).
I've been thinking in all you said, and I decided to better use a safety relay put in-between the electrovalve and the arduino. Not only for all the safety reasons all you said, but also because that voltage comes from the building heater circuit, not part of my home, so I don't have direct access to the general switch for it; I need to talk to our administrator to cut it whenever I want to work with those cable pairs, so putting in the middle an industrial safety relay, and sending only 24 volts to my system, is both safer and convenient.
Thanks everybody for your comments.
Be sure to tell us if that happens!
If you want simple reliable and certified (at least in Europe) setup, you could use Shelly smart relay. It has esp32 mcu, programmable with Arduino ide.
Oh!!! That looks really great!!!
They are great. Depending what you want to accomplish, even with stock firmware they often "make the job".