You are OLLIE20 A POORLY TRAINED AND MISGUIDED. Just in from doing a few landlord checks this morning and was at 2 year old Baxi Duotec 33he, thought I'd have a little play to confirm what I have been saying all along is correct. Set boiler to max rate in commssioning mode as per manufacturers instructions, the results follow: Inlet pressures to boiler operation Not restricted 19.5mBar Retricted via inlet valve 11mBar O2% 5.9 9.3 CO2% 8.5 7.1 CO ppm 45 60 Flue temp 43 39 efficiency% 97 94.2 Ex Air% 39 64.8 CO/CO2 Ratio 0.0005 0.0008 Manufacturer states Co2% should be 8.7% +- 0.2 max rate and 8.4 +- 0.2 min rate. Figures were also referenced against Kane international CO/CO2 ratio matrix for determining O2, CO2 and CO ppm levels and collarate exactly. Well with restricted inlet working pressure as expected : O2 and excess air increased. CO2, flue temp and efficiency decreased. CO ppm increased negligeably by 15 parts per million which is not even dicernable in the whole scale things. I have seen it not move and go down in the past. Therefore, all in all NO change to worry about. CO/CO2 Ratio as statement above, no discernable movement. Still WELL WELL BELOW the trigger value of 0.008 What does this tell us girls then, it tells us that to much air on an air gas ratio valved appliance has NO dangerous effects. This is true in my experience for OIL and LPG appliances as well with numerous other manufacturers to boot. Gatting back to the original OP, I stand by what I said in that the guy was a fool to cap off your appliance and should not have done so. Did he carry out a flue gas analysis by the way? and if so as long as it was below 0.008 it then would be NCS not AR and definitely not ID. As for the run of 40-50 meters for a gas pipe in a 1st floor flat Mmmm NEVER seen that EVER. Somethings not fittting here, I reckon you just pulled a cowboy firm. I would like to see the certification if you could post it up? Good luck but I think you'll end up getting skinned, there are a few bad timeserved apprentice trained/experienced professionals about, however this is far exceded by short course trained, franchise or BG trained inexperienced fools who don't know one end of a boiler to the other. You just need to read what APHC, SNIPEF and CIPHE to name but a few are saying about these training courses and the bandits they produce. Whats the saying again 'A LITTLE KNOWLEDGE IS A DANGEROUS THING'