Selgas
Yes – If when you turn on the hot water in the shower room the fan automatically starts up it is possible the problem is caused by a shorted fan electrical circuit either at the fan motor or the controlling circuit switch.
I take it the problem happens when you turn on the Lights 1 electrical ciruit which points me in the direction of the electrical wiring to the extraction fan.
The gas water heater has no electrical connections whatsoever so it will not be anything to do with the water heater – just coincidence with a faulty electrical circuit. 
Selgas Services Ltd
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