Heating and hot water troubleshooting

This type of system has a small black tank and is often found in the loft or at the top of a cupboard.

If the ball valve float in the black tank becomes stuck and the system's water drops, water cannot circulate. 

If you can safely and easily enter your loft, check if the ball valve is stuck. You will see no or low water in the cistern if this is the case.

Gently move the ball float valve arm to release it (don't force it). This may solve the problem.