Heat rising up into room above water tank - why?

Hi I've noticed today that there is alot of heat being generated in the box room above a water tank which is housed within the house downstairs but accessed by an outside door. It's some kind of storage system linked to the hot water system I'm told that prevents pressure drops.

At first I thought the excess heat was being generated by humidity from recent plastering but even after drying out the heat is there. This evening I noticed hear was coming from the corner of the room directly above the tank. When shining a torch on the exposed floorboards I noticed it was steam/light mist rising. Not good I thought!

Now can someone more experienced than me explain what could be causing this, what the solution is and if it is dangerous at the present time. It wasn't there when we moved in but since turning on the combi boiler in the kitchen and switching on a mystery fused spur with cabling that goes up into that room the heat has become apparent.

Errr...have you tried turning the fused spur off again to see if that stops the heat?

Water tank and a combi boiler? Maybe a back up water heater in loft powered from you spur?

Hi I'm not sure what the system is or how it all works as I'm used to having a single large combi boiler that runs the whole house. There is a boiler in the kitchen which supplies hot water and central heatinig I beieve, then this Vailiant tank in an outdoor accessed housing. I will take a photo and upload when I can because not sure what system it is to be honest.

The heat stopped this morning when spur was switched off which I noticed definatelty feeds the tank. Water is dripping from a pipe into a funnel shaped device underneath which is then evaporating and the steam rising up I guess this is the steam I saw last night. The device is overflowing too and dripping onto the floor.

Can anyone make sense of this before I post up pics?

Sounds like an unvented cylinder that is overheating, causing the water to expand and out through the pressure relief valve. It should be turned off immediately. Most likely a new thermostat is required, but you'd be best to get a plumber with a G3 ticket to look at it.

Whatever you do, do NOT try to block the dripping valve. These cylinders can go off with enough force to remove the external walls of your house if there is nowhere for the water to expand to.

Thank you guys it's now switched off pending a check from plumber once the house gets all painted up and the radiators go back on the Walls. Inwanthed the whe system to get checked before we move in so will get him to replace the thermostat if necessary.