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Xs air at 500

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richardd

I carried out a check on a flue today.all other measurements were fine ratio o.oo25. The xs air seemed very high thou 500 as I have stated . The boiler was a Worcester si model 10year old and the flue run vertically for 2.5meters until it exited the property on a pitched roof.
Could someone explain xs air in more detail and this reading. I know high reading can effect fuel consumption. But is there any safety issues here thanks . I am in my first year on the register.
 
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Hey always learning ... I now have a handle on excess air .. :) Amazing what technolgy does! So what is a "normal" reading on the Xs scale ??? :)
 
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My usual reading is around 30 on a horizontal flue straight out the wall. I got 480 on this flue I assumed it was because it was vertical and had a run of 2.5 meters.

I have done a bit of research myself it seems it is the amount of extra air present to assist combustion. So therebis no safety issue I presume but would like to hear further opinions.
 
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Okay done a bit more reading on the excess air to get my own understanding on it. It's a measurement of "air" present in the exhaust flue. The higher the percentage reading the greater the dilution of flue gasses and is calculated by measuring the presence of oxygen in the flue. the less oxygen present the lower the excess air percentage reading.

A reading of 500% tells you there was a lot of air present... Possibility for this could be that the reading was taken on low fire with a fan flued boiler?

How does that sound? It makes sense for me anyhoo's!!! :)
 
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Ok. My understanding of excess air is that it's the measurement off too much air over stychiometric combustion. So the average is about 30

Too much air will severely affect the efficiency of the appliance

To me. In this situation I would say that you have a undergassed appliance of you have a fault in your flue

You will want to get that ex air right down to have good efficiency. Check you on high fire
Gas rate
You may have by now found your fault
 
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SimonJohns said:
In this situation I would say that you have a undergassed appliance of you have a fault in your flue

You will want to get that ex air right down to have good efficiency. Check you on high fire
Gas rate
You may have by now found your fault
Click to expand...

Hi SimonJohn ... The boiler in question is fan flued not pre-mix which may explain the Xs air readings. I don't think it neccesseraly means there's a fault :) These fans aren't variable speed, they're either fixed or 2 speed whereas the boilers are modulating so as the burner pressure drops you're gonna get an increase in Xs air if the fan is constant yeh?
 
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But 500% excess air is far too much

Even on low fire of a non premix that is crazy. I would be looking at 125% ish for low and 20-30 on high. Doesnt seen right to me
 
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SimonJohns said:
But 500% excess air is far too much

Even on low fire of a non premix that is crazy. I would be looking at 125% ish for low and 20-30 on high. Doesnt seen right to me
Click to expand...

Just reading back through and you say you have a co2 of 16%. That's very high. But won't explain the excess air. If anything you should have very little excess air. What was your O2?
 
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SimonJohns said:
But 500% excess air is far too much

Even on low fire of a non premix that is crazy. I would be looking at 125% ish for low and 20-30 on high. Doesnt seen right to me
Click to expand...

My FGA is a dinosaur mate and doesn't do this Xs air thing! I'm learning something new here to be honest as it's not a term I've come across in this sheltered self employed life I lead :) Is it possible for a poorly positioned analyser probe to give these sorts of readings? I'll be paying more attention in future to the O2 readings I'm taking though as i now have the calculation you do to determine Xs :) Always learning eh :)
 
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What analyser do you have?

The analyser should be positioned where manufacturer says

On certain Hamworthy boilers. If the FGA prove is not in the right area of the combustion chamber it will give incorrect readings

On a OF boiler. If you sample after draught divertor then you will only ever achieve readings of co2 at about 3.5% and you will have a stupid excess air

But on domestic all you should be worrying about is what the mi's say.

500% could even mean that his O2 cell is broken.

Most of the time you would be looking 9-11% co2
4-7%o2
25 -120 excess air. Boiler dependant
0-200 ppm
Ft non cond 120 cond 90 (30c above flow temp)


Obviously these are average figures. Always refer to mi!!
 
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Got a TESTO 325 bought 2nd hand 3 years ago!

Thanks for the info ... I'd guessed that you'd have stupid numbers with open flue in secondary flueway :)
 
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diamondgas said:
Got a TESTO 325 bought 2nd hand 3 years ago!

Thanks for the info ... I'd guessed that you'd have stupid numbers with open flue in secondary flueway :)
Click to expand...

The 325 can do excess air. It's the funny looking y shape. I'll double check thou. My apprentice has one
 
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Just checked and it does do excess air. It's the EA option

If your not sure how to set it up I'll send you the link to the manual
 
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Please SimonJohn ... I've got the thing in front of me and it doesn't seem to have anything regards xs... it's the testo 325-M ... whether the 'M' makes any diff I don't know :)
 
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Hey, Richard. I know you said this was an older Si, but do you have a GC number for it? You've got me all kinds of curious about 14% CO2 per MIs. Natural gas has a CO2 max of 11.9% so I'm dead curious how the manus expect you to get 14%!
 
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CMairiD said:
Hey, Richard. I know you said this was an older Si, but do you have a GC number for it? You've got me all kinds of curious about 14% CO2 per MIs. Natural gas has a CO2 max of 11.9% so I'm dead curious how the manus expect you to get 14%!
Click to expand...

Good shout CMariD ... I have the mi's for GCno 47-311-65 and it states 5.3 - 6.2 CO2 ---- Someone's FGA needs calibrating maybe :)
 
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Diamond, I have 47-311-49 & 51 for the 25/28si (not Greenstar!) and they're asking for 5.5%-8.5% CO2.
 
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