Ideal [FD] fault codewhat it means and how to fix it

FD means the appliance cannot detect water flowing through it. That is a step beyond low pressure — there may be plenty of water in the system, but it is not moving. Ideal appliances monitor this because firing a heat exchanger with static water in it is the fastest way to destroy one. The usual explanation is the pump: failed electrically, seized mechanically, or simply not receiving power. Seizure after a long idle summer is the classic version: a rotor that has stood still for months sticks on the first genuinely cold week of autumn, when the heating is asked for again.

There is a specific point worth making about FD. Because a seized pump can sometimes be freed briefly, an appliance may appear to recover for a day and then fail again. That is not the fault clearing. A pump that has seized once has told you what it intends to do, and the second failure usually comes at a less convenient moment than the first.

1. 1

   Check the pressure reads 1.0 to 1.5 bar cold. A pump cannot move water that is not there, and low pressure can produce this on its own.

2. 2

   Open every thermostatic radiator valve fully and check the service valves under the appliance are in line with the pipework.

3. 3

   Call for heat and listen at the appliance for the first thirty seconds. A working pump gives a steady low hum. Silence, or a buzz with no hum, both point at the pump.

4. 4

   Bleed the radiators from the ground floor upwards and repressurise afterwards, in case a severe airlock is the cause.

5. 5

   If FD persists, book an engineer rather than resetting repeatedly. Running the appliance without flow is what turns this into an L1 overheat and then into a heat exchanger bill.