Caution
The boiler is dead or running without producing anything: the whole system has stopped, not one radiator.
You top the gauge up to 1.5 bar and days or hours later it has dropped again.
Low risk
The boiler cut out overnight in a hard frost and is gurgling or showing a fault code: usually a frozen condensate pipe.
The bottom of the radiator is hot and the top half stays cold: air is trapped in the panel.
The top of the radiator is hot but the bottom stays cold: sludge has settled inside the panel.
The boiler rumbles, whistles or bangs like a kettle coming to the boil when the heating fires.
Taps run hot as normal but the radiators stay stone cold: the heating side alone has failed.
The programmer, timer or thermostat is ignoring you: heat arrives when you did not ask for it, or never.
High risk
Water is running or dripping from the tundish pipe outside: an unvented hot water cylinder is releasing pressure.
The heat pump is running but the house never gets to temperature, or it struggles as soon as it turns cold.
Describe the job and get a fixed price from a local heating engineer.
