So I have this circuit and I'm trying to heat up my heating device through my Relay that is activated through my ESP8266. Is this circuit safe?

 

The safety of the circuit is entirely dependent on the safety (isolation, etc.) of the power supply doing the 220 to 5V conversion, and incidentally the safeguards in the heating device to prevent overheating by misoperation.

 

If the Control Relay is of the standard mechanical, i.e. not SSR etc.

 

f the Control Relay is of the standard mechanical, i.e. not SSR

Why do you exclude SSR's?
They are well isolated, control input to AC output.

 

If the AC-DC adapter is placed Between the Control Relay and the Heating device, it will not work.

 

Define safe?

What happens when the heater is stuck on?

 

Will it work ?
basically , no reason why not, provided the micro is not switched off by the relay.

Is it safe ?
May be, That is voltage and current that can kill you ,
so it depends on how you build it and specify the high voltage parts .

 

Why do you exclude SSR's?

Temporary mental glitch

 

Define safe?

What happens when the heater is stuck on?

When the heater is stuck on nothing will happens, it has some proper mechanisms. When the heater is turned off the microconcroller is supposed to always be turned on