Hi

I have a Vaillant combi about 25 years old. The pressure had dropped and the boiler stopped working so I topped the pressure up via the filling loop. After topping up, the hot water circuit works - although the heat of the water isn't as hot as it had been. When the heating comes on the boiler fires and the rads get luke warm. Then the boiler cuts out. The heating "cycles" with the burner igniting then shutting off after one minute 2 more times. Then an orange light comes on at the front of the boiler next to a "flame" image on the cover of the lower part of the boiler and a sprung button pops out.

I am going to get the boiler seen by hopefully a competent engineer. But forewarned is forearmed. Is this problem probably (possibly) a pump failure on the CH circuit? if yes, how much should I expect to pay (London prices, guv) for a replacement pump?

I accept at some point the boiler will become uneconomic to repair but I am not prepared to junk it if I can get it fixed for a couple of hundred.

Separately, how can a combi lose pressure if there is no leak on the pipes to the radiators nor any water discharged out of the "pressure outlet pipe" (or whatever it is called) out of the back of the boiler?

Thanks