e-r-a-n:
yes but i dont know for sure its 2.5KW, its an assumption
i will just measure both i & v with multimeter
but i dont want to wait too long,
can i buy a 25Amps relay and get it working even if the boiler uses lower current?
Yes you can, in fact a conservative engineer always 'de-rates' the specification he uses when obtaining relays and such as you would normally never use a 10 amp contact rated relay on a 10 amp load, but rather use a 20 or 30 amp contact rating for longer lifetime.
Lefty