I'm adding an electric boiler 240 volt. Can I use the same breaker from the dryer? I don't have room to add another breaker on my electric panel.
If you live in the US. Then NEC Article 210.23(2) states that you cannot exceed the circuit ampere rating more than 50% of the total rating of the dryer. NEC Article 220.54 states you shall rate the dryer at 5000W (V/A) or the name plate rating of the dryer whichever is larger.
So 5000W/240V = 20.8A (more than 50% of 30A). That means NO you cannot use the circuit to power both your boiler and the dryer on the same circuit.
You might try to revisit your Panel and see if it accepts half size breakers and see if you can create some space in it, or the only other practical solution I can see, would be to install a subpanel.
Hope this helps.
The short answer is no. You will either have to replace your panel with a new larger one, install a sub panel for additional space or install twin style breakers.