To answer the question in simple terms I would advise the following : When water is heated it expands and unless there is an expansion tank in the system the only place the water can expand freely to is the Temperature, Pressure Relief Valve (TPRV). Without this device working the water can superheat and the whole tank could become a time bomb under tremendous pressure and no body wants that huh?
To correct the problem you have there may require an inspection of the plumbing pipework to establish the best way to overcome the problem – have a chat with your local Plumber to get an idea of likely costs.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians