When a hot water system heats up 4-7% of the volume is expected to be released via the T&P valve. If it doesnt the you get a bloody big explosion.
http://www.safteng.net/Accident%20Photos/Hot%20Water%20heater%20explosion.htm
DISCLAIMER
All advice is given with-out seeing the job, and hence all advice MUST be taken as advice with limited knowledge on the exact situation. NO responsibility can or will be taken. And yes, I am a licensed Plumber and Drainer with my own business in Brisbane Australia.
