Rank: Forum user 
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This question was raised today during a training session. Have any colleagues considered this problem? When gas boiler service engineers are carrying out gas analysing tests on flues using their equipment they may find evidence that combustion gases are spilling back into the fresh air feed to the boiler. In some circumstances this can cause spillage into the room in which the boiler is sited. This means that inadvertently the engineer is entering a harmful environment without warning. There appears to be no standard acceptable procedure for preventing the situation, unless they have reason to be suspicious Have any of you dealt with this problem, by requiring engineers to wear a personal gas monitor?
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Rank: Super forum user
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Is this an emergency situation or part of routine boiler servicing?
Why does the gas engineer need a personal monitor when he is already has a gas analyser meter/analyser?
For a routine combustion test/check after servicing a boiler I don't think what you are suggesting is credible. As part of the boiler service the boiler engineer should check the air supply (if necessary any ducting) is fit for purpose and not obstructed so that the boiler can obtain the air it needs for complete combustion.
If after this the combustion still isn't correct after ignition, he will be using the gas analyser to check the combustion gas CO and Co2 levels to confirm either way. If the CO levels are 'high' then the build up of CO does take some time, so is not likely to be immeadiatley life threatening. The engineer can adjust the air/fuel ratio so that correct combustion takes place, which he will re-check on his gas analyser.
Even if the CO levels are high, the boiler gas outlet should still vent to the outside as the engineer should have checked the flue is clear of obstructions and not leaking back into the room.
In an emergency situation, if high CO levels are suspected then room ventilation will be required before entry.
Remember before working on a gas boiler the engineer must be trained and qualified to work on gas equipment, so should have better safety awareness of CO levels than the average person.
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Rank: Forum user
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Thank you Ian. The scenario envisaged is during routine inspection, where the engineer enters a room, such as a plant room, as normal, without switching on his analyser, before entry - because he has no reason to be suspicious. The analyser can be used if he is suspicious, and I believe the Vipa guidance refers to this situation. My query centres around the engineer entering a space which has an excess of 100 ppm of CO - a gas which as we know is invisible and offers no smell without his analyser on, as is normal practice
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Rank: Super forum user
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100ppm is not likely to result in ill health/toxicity in the short term
https://en.m.wikipedia.org/wiki/Carbon_monoxide_poisoning
Looking at the table, probably in excess of 1000ppm would be required for an immediate short term effect.
If you think the room is so confined to present this risk, then treat as a confined space - with full training/PtW etc. In which case a boiler would not be installed in a confined space. Again remember a boiler must be installed following gas safety legislation, as well as manufacturers recommendations etc.
Boilers require an excess of air for combustion. A properly installed boiler where the ventilation has been assessed and sized correctly for the heat required and hence air/ventilation required - for a concious & competent engineer to be overcome by CO is pretty unlikely.
I often get involved with assessing ventilation as I undertake alot of DSEAR risk asessments from the hazardous area /explosive atmosphere point of view. This often includes gas boilers. So while I'm not calculating CO levels, similar principles apply. Such calculations usually lead to gas concentrations taking hours to buildup to dangerous levels.
If sufficient ventilation is available to remove the risk of a flammable atmosphere, then combustion gases leaking into a space will also be likely to be removed as well in the general mix of gases.
Clearly CO poisoing can occur if a boiler is poorly serviced, flues blocked, persons sleeping etc etc - but in the situation envisaged - a routine check, trained engineer, gas monitor used - then only a very remote possibility.
Your example of a plant room - often have air bricks etc to allow natural ventilation.
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Rank: Forum user
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Ian, very helpful. Thank you.
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