Hot water Cylinders
If you have a hot water overflow this is usually caused by one of two things. There may be a pressure issue which will mean your valves may need replacing or servicing.
The second issue may be thermal expansion which is caused by the element heating up and the volume of the water inside the cylinder expanding causing it to vent onto the roof or out of the wall depending on which system you have.
If there is also steam then that can be a sign that the thermostat is not working correctly and needs attention.
To test thermal expansion or pressure issues turn the power on your hot water cylinder off for 10 minutes and see if the venting stops.
If the venting stops then what you have seen is perfectly normal. If not then you need the services of a plumber.
When you call the plumber please check the label on the side of your cylinder so that you can tell them if it is a mains pressure or low pressure cylinder.