Doyle most band a boilers have adjustment on the gas valve, you put the analyser in the exhaust sensing hole and adjust at low output to whatever the MI's dictate. Can be different for each model some manufacturers have same figures for all models. Most manufacturers have different figures to each other. Then you check the high output seting which in many cases is not adjustable. If after adjusting low seting the high is outside tolerance you need to check you haven't mullered the flue seal or something. If it's still out of tolerance there is nothing you can do, call the manufacturer and put the ball in his court. Some gas valves allow for max rate adjustment, which makes life a lot easier.
Alternative to gas valve adjustment of co2 some manufacturers facilitate the adjustment by varying fan speed using a pot on the pcb.
It may even be possible to juggle the two.
Buderus don't expect any adjustment, and they don't expect you to measure co2 with a fga, but they do provide a means by which it can be checked using a sensetive differential manometer.
Worcester (the last time I commissioned one [always check for updates in MI's they change frequently even though the boiler appears to be the same]) don't expect you to even check it, specifically stating in the MIs that it is set up in the factory and they don't want you messing with it. I have seen it being done in the factory by far better equipment you or I have, every single boiler is set up at a final workstation, from memory there were about 4 of them being tested and set up correctly at a time.
Your band B boilers just need the min max and in some cases ignition gas pressures checking. And of course it's a good idea to gas rate all new installs, though if you have checked burner pressure you don't strictly have to gas rate.
You need to shop for the kit, there was a Kane 400 standing at £50 on ebay go check on it. In your case you could buy the kit like that , get used to it, but not use it seriously, then have it callibrated before going live with it. I don't advise you in your position to get a new one.
I picked up a BW Gasprobe on eBay fully callibrated with a new CO sensor just fitted and the old one to prove it for £270 while my Kane was being repaired [AGAIN](it's been nowt but trouble to be honest). The BW gasprobe gives same figures as Kane but not so sensetive as a differential pressure guage, though adequate for most tasks.
I am justified in having two as I have to use it on a daily basis and just couldn't do my job without it. Two is a luxury you don't need.
In all cases you must read the MI's to check for changes and also check data badges, some band b boilers' burner pressures are specific to that particular boiler if so it is stated on the data badge, allways look on it (if you can find it) first for such data.
Of course if the gas assessment of competense system wasn't a 100 years out of date you would have passed competence in this.
