Carrying out repairs and then condemning a boiler

Now that it is getting cold in Yorkshire, I arranged for a heating engineer to come and service my boiler. He replaced an expansion vessel that was external to my combi boiler. I don't actually know why it needed replacing (but maybe it was leaking past the diaphragm - who knows). First of all, I don't know why an external expansion vessel (new house) had been installed in the first place OR why the engineer replaced the external one instead of getting rid of it altogether and replacing the one in the boiler itself.

After fitting the expansion vessel, he then carried out the boiler service, during which he measured carbon monoxide in the exhaust gases. He decided that the boiler would be too expensive to repair and recommended getting a new boiler installed. Now, to me, with my limited understanding, it seems a bit unfortunate that he detected the CO and wrote off the boiler AFTER he had carried out a repair (£200). The work involved in the service was obviously necessary for discovering the fault. Can someone with greater understanding please explain if it was necessary to replace the expansion tank in order to detect the CO and/or service the boiler. If so, I would be grateful if you could explain why. If this was a reasonable approach, then fair enough, but I would like to understand.

Thank you

What boiler, and how old ?
Did he do a full strip down service or just used a flue gas analysiser

The boiler must be at least 10 years old. He did use a flue gas analyser. I'm not sure how far he got with the rest of the service. He certainly wants to charge for the full service.

It could be the boiler just needs a proper service, what make and model

I'm really sorry. I'm away from home at the moment. I can let you know when I get back in a couple of days. However, it is a Glow Worm. Possibly the Energy 30C.

Combi boilers have a expansion vessel fitted inside, sometimes on a large system a additional one is fitted externally, why he replaced this you will have to ask him.
He could use a analyser before servicing to get a indication of the condition of the boiler. A strip down service and checking of the flue plus possible adjustments of gas valve would normally correct any problems.
You could always try the boiler manufacturer's for a service or a recommended engineer if not happy with the present one

Thank you danfre. (It isn't a large system, by the way - just a 3-bed semi)

nowthendiy said:

which he measured carbon monoxide in the exhaust gases.

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Yes, present in all boilers exhaust gases, measured in parts per million (ppm). Was it high?

nowthendiy said:

He decided that the boiler would be too expensive to repair and recommended getting a new boiler installed.

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In his opinion - get another

nowthendiy said:

please explain if it was necessary to replace the expansion tank in order to detect the CO and/or service the boiler.

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Can’t really say, but ime I’d say no. Expansion vessels usually require the air pumping back into them using a suitable pump.

Was the boiler left operational? If yes, why if it’s supposedly dangerous? If no, was paperwork left? Do you know the Ppm levels?

I'd be asking for a full written report on what he has done, what he found and why it needs replaced at that price.

Could be a number of reasons why the CO reading is excessively high and most should be adjustable/repairable and if it isn't then I'd want him to please explain why not.

2nd opinion would be way cheaper than a new boiler..