What to check if a boiler is working but the water temperature is low?

If an oil boiler is working but the water temperature is low, a heating engineer should check the following. Please note, it is important to always isolate the electricity supply to the boiler before working on the boiler.

• Check if the nozzle is undersized nozzle and/or low fuel pressure.
• Check the condition of boiler's heat exchanger and clean if necessary.
• Check the boiler thermostat.
• Check the combustion settings.
• Check the condition of the fuel filter.