Why are combi gas rates co2 and o2 levels important

[Subby]

I am not 100% on why the kW rating on a gas rate is important on a combi, and what it means if the kW is too low or high, and weather I should record it in gross or net.
Also I am not 100% on why my analyser readings for CO2 and O2 are important on maximum and minimum rate. I have spent a lot of extra time (1hr+) on my boiler services getting the levels correct when I have found them to be out and often times it is difficult get the O2 level low enough and the CO2 level high enough without sending the CO over 200ppm. Also is there a method of adjusting the gas valve that's not so to e consuming? I usually do maximum first, then minimum (which often then changes my Maximum readings) so back and forth I go using bath fulls of the customers hit water!
Thanks if anybody can help clear up these questions.

 

[Subby]

Also does anybody know where I can read up about this subject?
Thanks in advance

 

[Last Plumber]

Gas rate tells you whether the Appliance is using the correct amount of Gas.
The CO2 and O2 are not normally a major problem in my experience. There are very few I make any major alterations to.
Is your FGA calibrated?
Are you using the correct setting screw on the Gas valve?

 

[Subby]

Gas rate tells you whether the Appliance is using the correct amount of Gas.
The CO2 and O2 are not normally a major problem in my experience. There are very few I make any major alterations to.
Is your FGA calibrated?
Are you using the correct setting screw on the Gas valve?

Yes my analyser is calibrated have sent it off twice this year .
If the gas rate at the meter is incorrect how would we change it?
If the gas rate is too low / high but the CO and ratio are fine , why would this be a problem?
Sorry if it seems like I am asking basic stuff but I like to know exactly what I am doing and why I am doing it .

 

[Chalked]

Gas rate pretty much trumps anything on a boiler. It's the manufacturers design to use a predetermined amount of gas at a given pressure. This, if burning correctly, will produce The correct of heat to water that it was designed for. After that correctly settting up co2 and co will then be burning cleanly and efficiently.

 

[firemant]

OP: When did you qualify?

 

[Matthew91]

The gas rate on the boiler is to confirm how much gas the appliance is using at its maximum rate. Should always be done in test mode or on go water if test mode isn't available. If the gas rates too high it's being overgassed, if too low it's being undergassed but remember there's a tolerance of +5% and -10% which is deemed ok by manufacturers unless stated otherwise.
If the gas rates out depending on the appliance type and gas valve you can adjust this so it's burning the correct amount of gas, but again depending on the appliance & gas valve type this can effect your analyser readings.
If it does you can look at adjusting the readings for that too via the gas valve.

By adjusting the valve it can effect anything, however if you've got the gas rate correct, and then you've got the co2 reading correct, if the CO reading (PPM) is off the scale or the ratio is outwith the advised figures, you're then looking at a strip service as the gas valve is set correctly but it is burning dirty.


02 is normally measured on the air side of the flue to ensure it's providing clean enough air for combustion... sure it should be about 20.9%

 

[finchy01]

When I did my rebeqel the tutor questioned Why we do gas rates?!?! He said it's done when the boilers installed. No need to do it again. I always do them. Especially as sticking your analyser in takes all of about 2 minutes! Makes the customer feel like they are getting a bit more value for money.

 

[Subby]

The gas rate on the boiler is to confirm how much gas the appliance is using at its maximum rate. Should always be done in test mode or on go water if test mode isn't available. If the gas rates too high it's being overgassed, if too low it's being undergassed but remember there's a tolerance of +5% and -10% which is deemed ok by manufacturers unless stated otherwise.
If the gas rates out depending on the appliance type and gas valve you can adjust this so it's burning the correct amount of gas, but again depending on the appliance & gas valve type this can effect your analyser readings.
If it does you can look at adjusting the readings for that too via the gas valve.

By adjusting the valve it can effect anything, however if you've got the gas rate correct, and then you've got the co2 reading correct, if the CO reading (PPM) is off the scale or the ratio is outwith the advised figures, you're then looking at a strip service as the gas valve is set correctly but it is burning dirty.


02 is normally measured on the air side of the flue to ensure it's providing clean enough air for combustion... sure it should be about 20.9%

Thanks for the in depth reply.
Some boilers MI s ask for an o2 % on exhaust gasses.
I realise o2 on intake should be 20.8 or 20.9 otherwise there is a problem with the flue.
The particular boilers I was having problems with was remeha avanta combis, 2 of them I had serviced I was having g trouble getting the o2 low enough without sending the CO off the scale. I now know on these particular boilers a service kit is needed (£100) to do a full strip down.

 

[Subby]

When I did my rebeqel the tutor questioned Why we do gas rates?!?! He said it's done when the boilers installed. No need to do it again. I always do them. Especially as sticking your analyser in takes all of about 2 minutes! Makes the customer feel like they are getting a bit more value for money.

You should always use the analyser on a service? And you don't need to use analyser to do gas rate?
The service sheet asks for a ratio and a gas rate, these should be done as a minimum, and if you are only doing these, you can't really call it a service, just a safety check

 

[cr0ft]

By monitoring and setting O2 and CO2 in the combustion gases you are actually monitoring how efficiently the boiler is burning. For the most efficient combustion you want a small amount of excess O2 in the combustion gas. Too much excess air reduces efficiency as does not enough. There is always a relationship for any given boiler between O2 and CO2 in the combustion gas hence why most manufacturers ask you to ensure CO2/O2 is at or below a certain level during servicing. Hope this makes sense.

 

[RoryD]

Sometimes Engineers think it defeatist to talk to Manufacturers Technical helpdesks. They know their boilers intimately (in most cases). The object is to get things right and beef up your knowledge. Call helpdesks as often as you like and you'll learn much.

 

[jtsplumbing]

Sometimes Engineers think it defeatist to talk to Manufacturers Technical helpdesks. They know their boilers intimately (in most cases). The object is to get things right and beef up your knowledge. Call helpdesks as often as you like and you'll learn much.

Totally agree, quick call and they will talk you through what is acceptable on their boiler, and how to adjust it if needed, Blue tooth connection in earpiece is very handy when calling, leaves your hands free to do adjustments + customer cant here what is going on Never be to proud to call them we don't know everything !!

 

[Pssst]

These factors are important because we want the appliance to operate at as close as possible to its optimal design conditions. The other thing of course is that modern HE boilers have forced draught burners which are designed to run more closely to stoichiometric combustion conditions and therefore be more efficient,unlike traditional natural draught burners which entrained more air for combustion than is required and are much less tightly controlled.

 

[GH77]

Avanta's on NG have a target reading of 8.8% CO2 high and low, the throttle is much less sensitive than the offset, but you need to run the boiler on max rate for at least 5 minutes and get the flue temp up above 55 deg C before making adjustments, if you go too early you'll end up chasing your tail and if it's a small system it will make temp and go into H8, you can run the DHW, but unless you have a sufficient flow rate it won't be the equivalent of running in high fire... I tend to Gas Rate every appliance every time I test one, I also always do combustion and inlet working Gas Pressure, these three test confirm each other, if the regulator fails then Inlet working shows this, gas rating can pick up a faulty Gas Valve, combustion failed flue seals etc, if you're getting poor combustion i'd be doing a flue integrity test, i have found that the convolute tube from heat exchanger to condensate trap gets blocked up with debris, as such I remove the burner every year on these, hoover, then soft brush, then hoover heat exchanger, then pour water through heat exchanger to confirm condensate route is clear, full service including all checks, 1.5 hours.