1. Sludge / Scale in System
Heating and mixing the system contents - water, paste and flux residue and the combination of different metals - creates a chemical reaction that produces scale, black sludge and hydrogen gas. Eventually this can cause cold spots in a radiator and it can cause restrictions in pipework, controls and components. Sludge can make diverter valves seize or move slowly, restrict plate and main heat exchangers, will lead to poor system performance and could eventually block the system stopping it working altogether.

2. Air in Systems
Air locks or systems that are not full with water can cause circulation faults. The air can get trapped in high parts of the system stopping the flow of the water. Fitting air separators can help prevent this on open systems. Auto and manual air vents are also fitted on high parts of systems where air may get trapped to allow it to be released.

3. Pump
A slow, worn or under performing pump can cause circulation problems. This can also be caused by sludge or the general condition and age of the pump. Limpet can be used to test the operation of the pump. The pump also needs to be on the correct speed setting for the system requirements.

Prevention is Better than Cure.
Sludge and scale can greatly decrease the efficiency of a boiler and system. Replacing components may improve the system temporarily but won't eliminate the problem. A system filter, powerflush and / or pipework alterations are a more permanant solution. Measures to prevent sludge are system design and pipework layout, the correct cleansing of newly installed systems and adding inhibitor to treat the water.

Trapped air stops the water flow