Error F.12

Safety first: before doing anything switch the boiler to standby or off and isolate electrical power at the external switch if you are going to touch plumbing or the filling loop. Do not open sealed covers, do not attempt to work on gas components, and do not try to remove or modify internal parts unless you are a Gas Safe engineer. If you smell gas, leave the house immediately and call the gas emergency number.

Initial checks homeowners can do:

1) Record the error code and any display messages, then try a reset using the boiler reset button or the procedure in the boiler handbook. Note whether the fault returns immediately or after a short run.

2) Check the boiler pressure gauge. For most combi/system boilers the normal cold pressure is about 1.0–1.5 bar. If pressure is significantly below this (or at zero), that can trigger pressure/flow related block errors.

3) If pressure is low, top up the system using the filling loop following the manufacturer’s instructions until the gauge reads about 1.0–1.5 bar, then recheck the fault. Only use the filling loop provided with the boiler and follow the manual; if you do not have the manual or are unsure, stop and call an engineer.

4) Bleed a few radiators to remove any trapped air, then recheck and, if needed, re-pressurise the system. Airlocks can cause apparent block/pressure faults.

5) Inspect visible pipework, valves and the condensate/termination pipe for obvious blockages, kinks or external obstructions (including frozen condensate in cold weather). Clear any external obstructions safely.

If the fault persists after the above:

1) Do not repeatedly reset and run the boiler if the F.12 returns — this risks damage. Switch the boiler off and arrange a service visit.

2) Persistent F.12 often means an internal blockage (sludge, magnetite) in heat exchanger or pump, a failing or seized pump, a blocked filter/strainer, a stuck pressure relief/expansion issue, or a faulty pressure sensor. These require diagnostic tools and disassembly that should only be done by a Gas Safe registered heating engineer.

3) The engineer will usually: check flow/return temperatures and NTC sensors, test pressure sensor operation, inspect and, if necessary, remove and clean/replace the pump and any internal filters or strainers, check for sludge and possibly recommend a system powerflush if debris is present, and inspect the expansion vessel and pressure relief components.

When to call a professional:

If resetting and simple pressure/air bleeding does not clear the F.12, if the pressure repeatedly drops, if you hear unusual pump noise, or if you are not confident performing the simple checks, contact a Gas Safe engineer. Any work on gas or internal boiler components must be carried out by a qualified engineer. Regular servicing can prevent sludge and pump failures and reduce recurrence of block-related faults.