Flue CO2 question?

[Harry Bloomfield]

Hi,

I'm not personally involved with this problem so supplying more information will be difficult...

A permanently installed flue gas instrument (LED display) measuring CO2 levels indicates 20% with no combustion gases passing up the flue. With the burner system in operation this level declines to an indicated 9%.

My immediate response is that the meter is obviously working the wrong way, if CO2 levels go down with combustion. The instrument is suggested to be clearly marked that it reads 'CO2'.

What is the normal level of C02 in the atmosphere as a percentage and will the instrument be designed to detect the additional amount of CO2 (above the atmospheric normal), or to measure the overall level?

[Ian Stirling]

If it's oxygen, that might be broadly plausible. Oxygen levels are 20% in normal atmosphere, and 9% would not be utterly unreasonable (though maybe a bit high) for exhaust.

For CO2, well, there is a fault. IIRC normal range is 10-20ppm.

[Mike Harrison]

Sounds like a dodgy meter -

Co2 is normally 0.036% of atmosphere.

[Morten]

Are you sure it's not measuring O2 ie. Oxygen, that would explain the figures and they sort of make sense. 20% is normal levels of Oxygen and 9 sound about right when the boiler is running, depending on how the boiler has been setup.

If' that is the case then the meter needs calibration, or is connected to a high flue that stratisfies because CO2 is heavier than the rest of the air.

Does it decrease below 9% when the boiler runs the fan before ignition?

2-3% if I remember correctly...

/Morten

[Roger]

The message from Harry Bloomfield contains these words:

I am not sure of the exact amount but it sure as hell isn't 20%.

.03% CO2.

However Oxygen is at 20.95% so is the meter measuring oxygen as it makes sense for that to be considerably lower in combustion gases.

[Andrew Gabriel]

As others have said, this sounds like the oxygen reading. Combustion analysers often don't measure CO2 directly because it's hard to do continuously in that type of unit. Instead they measure O2 and C0 levels, and then calculate the CO2 from that based on the O2 input assumed to be 21% and type of fuel in use.

[Harry Bloomfield]

Thanks all...

As several have suggested, I think it must be an oxygen measuring instrument rather than CO2. It was just a description via the phone and the op insisted it was clearly marked as measuring CO2. Clearly he was mistaken.