Combi boiler serviced today with two faults identified:

1. Failed/faulty expansion vessel
2. High analyser reading

The former I understand and know my repair options. The later I don’t fully understand; I gather from some googling it’s a high CO content in the exhaust gas which I assume implies inefficient combustion. Is this a big problem?

Any help much appreciated

Depends on the boiler and the reading… Anymore info?

Yes need to know what that reading is, the ratio reading and the boiler before you can decide on action. But if reading is outside of limits then your boiler should have been either condemned or suitable remedial action carried out to enable safe operation of the appliance.

The engineer has stated a reading (both ‘initial combustion and final combustion) of 0.0091; its a Vokera Compact 35HE.

The suggested remedy for both issues was take boiler off wall and replace the expansion vessel and replace the flue. Total cost £440 (parts & labour) but no guarantee of success.

I fear it may be new boiler time…

How old is the boiler? That would influence me on whether I would spend out on the appliance. Without looking but flue kits are usually around the £75 with expansion vessels similar.
If expansion vessel is in the back of the boiler then it will need to come off the wall, making it easier to fit an external vessel in the system somewhere.
Flue would be able to be fitted with the boiler on the wall I would imagine.
Although changing the flue may not cure the combustion readings, I assume he is thinking that the inner flue is leaking into the outer flue, starving the boiler of oxygen. It would be easy to prove this with an analyser therefore proving the integrity of the flue.I assume burner pressures have been checked?

Thanks Bear,

He estimated the boiler is 9years old. Burner pressures are OK. There’s no room to fit an external vessel so yes it would have to come off the wall.

I will call him today to ask about the logic of changing the flume- since pumping the expansion vessel its not dropping to 0bar but is increasing from 1 to 2bar when running.

I don’t want to pour money at fixing it but also am weary of replacing the complete unit for no reason

Well..He has diagnosed the expansion vessel bit correctly at least by the sound of the symptoms.
Did he try to re-pressurise the vessel with air?
If it needs changing there is always a place available somewhere for a remote vessel, all you need is a 9 ltr vessel located somewhere near a heating pipe. Temp fix, Isolate a rad, let the water out and then re-open valves (dont vent it). Air in rad will act as a temporary air cushion. High CO.. by the way the reading you have is a ratio of CO/Co2. Did he give you the actual CO reading? It will be in PPM?
Could be flue, did he do all other checks i.e Case seal, Burner pressure, gas rate, check heat exchanger, fan and so-on? All part of a boiler service. Dont be too keen to throw your money at a new boiler just yet. Get clarification on what has been done. If you cant get assurances get a second opinion.

Also Vok Compact has a flue elbow on top of the boiler so no need to remove boiler to fit a new flue.
Get a second opinion, I dont like the sound of his diagnosis.
Could he be looking for work? Ask him when he can ”fit you-in” and you will have the answer.