Furthermore, the blockage is rarely pure sludge. It is a composite material: magnetite particles bind with limescale (calcium carbonate) in hard water areas and with flux residues left over from the original installation. When a system is repeatedly turned on and off, the sludge undergoes thermal cycling, hardening into a cement-like substance known as “copper carbonate” or simply “hard sludge.” This metamorphosis transforms a removable deposit into a near-permanent obstruction that can withstand pressures of up to 3 bar.
The most pernicious consequence is boiler short-cycling . Modern condensing boilers are equipped with overheat thermostats and flow sensors. A blocked microbore circuit reduces overall system flow rate to a trickle. The boiler heats the static water in its heat exchanger to setpoint within seconds, then shuts down to prevent boiling, only to reignite a minute later. This rapid cycling destroys the boiler’s heat exchanger and fan, wastes gas, and fails to heat the property. In extreme cases, the blockage can cause the pump to cavitate, producing a characteristic “gravelly” noise as it churns air and debris. microbore central heating blockage
The Hydraulic Heart Attack: Understanding and Resolving Blockages in Microbore Central Heating Systems Furthermore, the blockage is rarely pure sludge