On 18/12/2016 12:42, wrote:

1.414x 13A = 18.38A, well above the 13A rms current. If mains voltage
goes to the upper end of its permitted range, the current will be
higher again. So the OP needs a 20A relay at least.

Nonsense! Calculating the repetitive peak current is irrelevant. Relay
current ratings are RMS values. An immersion heater is a very benign
load - it's AC-21 service [1] with no significant inrush current. A 16
A relay of reputable manufacture should be fine, assuming any sensible
number of switching events per day/hour.

[1] https://en.wikipedia.org/wiki/Utilization_categories

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Andy