Hot water coming from a pipe on the roof
A hot water cylinder builds up pressure as it heats. For safety, there is a pressure-relief valve (ajax valve) near the cylinder, often under the floorboards, leading into an outlet pipe, usually feeding up to the roof. If hot water is coming out of the outlet there is too much pressure coming from your cylinder. Call a plumber. Do not leave it long, as an ajax valve can spring a leak. Losing hot water is also expensive on your power bill!
Water supply (click here).
A spurting unpredictable hot tap (click here).
What temperature should the hot water be? (click here).
Adjusting the temperature of the hot water (click here).