Vaillant boiler, heating does not stay on, override "pops out"

Hi
I have a Vaillant combi about 25 years old. The pressure had dropped and th e 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 w ater isn't as hot as it had been. When the heating comes on the boiler fire s 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 pop s out.
I am going to get the boiler seen by hopefully a competent engineer. But fo rewarned is forearmed. Is this problem probably (possibly) a pump failure o n the CH circuit? if yes, how much should I expect to pay (London prices, g uv) 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
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I have bled all the rads and pressurised the system to about 1.5 bar.
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On 04/02/2018 18:53, snipped-for-privacy@gmail.com wrote:

Leaky joint hidden under the floor or some other obscure place.
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On Sunday, 4 February 2018 18:53:37 UTC, snipped-for-privacy@gmail.com wrote:

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 fi res and the rads get luke warm. Then the boiler cuts out. The heating "cycl es" with the burner igniting then shutting off after one minute 2 more time s. Then an orange light comes on at the front of the boiler next to a "flam e" image on the cover of the lower part of the boiler and a sprung button p ops out.

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?

am not prepared to junk it if I can get it fixed for a couple of hundred.

s to the radiators nor any water discharged out of the "pressure outlet pip e" (or whatever it is called) out of the back of the boiler?

it does sound like a pump problem, or possibly crud blocking something. Che ck the hot pipe coming out of the boiler, if it's poor flow it'll get hot w hile the rest of the system isn't.
New pumps are about £80 for fancy ones, used unintelligent ones much l ess. It may just need decrudding or unsticking: http://wiki.diyfaq.org.uk/index.php/Central_heating_pump_repair Also time to look at the filter if it has one.
NT
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snipped-for-privacy@gmail.com was thinking very hard :

You do not need to lose that much water, for the pressure to reduce below the level. Often the loss can go unnoticed, for years.
Possible sources of leaks, are radiators, radiator valves, pipes and pipe joints, the expansion vessel having failed possibly causing the expanded water to be blown out of the over pressure release valve outside the building, or in fact an internal leak in the boiler itself. Put a plastic bag over the end of the outlet pipe, to be sure that is not passing. If a water leak cannot be found with it filled with water, it can be easier to find with the system drained and pumped up with air pressure. The noise of (much more) air escaping is easier to trace.
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