Wet Patches on interior wall

[Chimp]

I have a problem with damp patches appearing on a wall in an unused bedroom of my house. I first assumed it must be something to do with my roof but after having the pointing re-done and some loose tiles replaced, the problem is exactly the same. I've checked and I've now realised that it happens when my central heating has been on for a couple of hours. However, a thorough check in the loft has revealed no issues with either of my water tanks (which are quite a distance from where the problem is occuring).

There's a problem with the radiator in that particular room and it's giving off very little heat (not sure if this is relevant).

I then thought the problem must be condensation and down to lack of ventilation and there is a vent in the room (on another wall) which I have now opened but the problem continues.

I had my house rendered about 3 years ago and I'm wondering if it's anything to do with that? However, it looks faultless outide (no visible cracks).

The water patch in not immediately noticable so it's hard to say how long it's been there but there is no sign of damp or discoloration - when it dries, it looks perfect again.

Any advice would be much appreciated.

[welsh brickie]

It could be just a cold spot in the room,if its damp after heavy rain, it may well be a wall tie thats covered in mortar and damp is bridging the cavity.
As for the radiator sounds like it need bleeding.The top of the radiator should feel cooler than the bottom,With a radiator key open the valve at the top of the rad and open slowly untill the air dissapates and water appears,then tighten it.Use a rag to prevent spillage.

[Chimp]

The wet patches appear on a daily basis, they are nothing to do with rain. They happen in the evening when the heating as been on for a while but it could also be connected to us using the shower (although it's across the landing) or cooking (the kitchen in underneath). I have a feeling it is due to condensation of some kind but no idea how to deal with it. Won't it cause damp if I just ignore it? Perhaps it will help if I fix my radiator?

[bwhomeresponse]

use the bedroom more or put the heating on more often. it is not unusual to have unexplained damp patches in a room. if it is not ringing wet try stain block and paint over.