Worcester Bosch [EF 349] fault codewhat it means and how to fix it

EF 349 appears in the blocking tables of the Compact family as "central heating, boil detect — large delta T despite minimum burner — the boiler is operating at minimum burner load and there is a greater than 18 °C flow and return temperature difference". Delta T is the gap between the temperature of water leaving the boiler and the temperature coming back. A well-circulating system running flat out might show a gap of 20 °C; the same system on the burner's minimum setting should show a small one, because there is very little heat going in.

A large gap at minimum burner means the little heat being added is going into a very small amount of water, which is another way of saying almost nothing is flowing. The word Worcester choose, boil detect, is the clue to why they watch for it: water that is barely moving through a heat exchanger will boil locally even at low burner output, and boiling is what produces the kettling noise and, eventually, exchanger damage.

Because it is a blocking code, the boiler pauses and tries again rather than locking out, and you may only see 349 in the info menu or hear its effects as a boiler that runs briefly, stops, and runs again. It sits with D4 271 and the letterless 212 and 213 as the Compact range's family of "the water is not moving" reports, and the causes are the same ones: air, pressure, pump, and a circuit that has been closed down.

1. 1

   Open a few radiator valves fully, including one in the room where the thermostat is, so the boiler has a circuit to push water round.

2. 2

   Check the pressure gauge and top up to about 1.5 bar cold if it is low. Bleed any radiators that are cold at the top.

3. 3

   Let the boiler run and listen. If it kettles — a rumbling or hissing from the case — or keeps stopping and starting, switch it off and book an engineer. If you have a magnetic filter, ask them to clean it at the same visit.