oil boiler efficiency

[Rotunda82]

Hello

Just a query really. I work on oil boilers/Rayburns and use an analyser on a daily basis.
I worked on a relatively old Trianco Eurostar earlier which hadnt been serviced in years. Did all the usual, replaced nozzle (with a 'W' as didnt have the correct 'S'), checked oil pressure and set Co2 to 12.5% as advised by the literature. When I printed off the readouts I noticed that the efficiency was 91% ?? How can this be possible on an old, non condensing boiler ? Are these efficiency readings reliable in the slightest ? oh I had a flue temperature of around 220 degrees also so I just cant see how that would be possible ?
Analyser calibrated around a two months ago

Many thanks

 

[tamz]

Was it set for gas?

Take the efficiency readouts with a pinch of salt.
As long as you get the smoke down and the correct CO2 and flue temp it should be fine.

 

[mickyg]

its not boiler efficiency its burner efficiency. its not the same thing.

 

[tamz]

Got any snow down there yet micky?

 

[Rotunda82]

Yeah it was set for oil. Not concerned with the operation of the boiler just baffled at some of these efficiency readings Im getting and wanted to understand why ?

So it burner efficency not boiler ? Because when you set up a Grant Vortex say for example they tell you to analyse at the heat exchanger for Co2 and at the flue for temperature and efficiency so it must be a fairly accurate reading (all be it at the flue and not the standard test point)

 

[mickyg]

Got any snow down there yet micky?

a little bit. More flu than snow tbh

 

[simond]

Was it Gross or Nett?

 

[fredfredfred]

Hi, I'm new to this forum but have been reading it for a long time.

Thought I'd field out a question on observations that any commissioning people may have had. I'm a boiler engineer, as in the designer, not installer or plumber. The comments I read on here make for some very useful feedback when it comes to 'altering' designs on new oil boilers.

I'm curious to see what plumbers have seen in their quests when it comes to measuring efficiency vs CO2 readings. In an ideal world, the CO2 would be maxed out, with zero CO and zero O2, with a flue temp of ambient (outside air temp). That would give you 100% efficiency. That is what I always aim for. But in practice we say set the corrected CO2 levels to be say 12% to compensate for soot build up or a messy spray pattern. Basically running a little bit lean or rich.

Has anyone noticed how the efficiency varies on older boilers depending on what air flow you allow through the venturi... If you ignored the manufacturer's CO2 commissioning settings, what would you set the boiler CO2 reading to, and why?

cheers, fred

 

[Agile]

I dont work on oil boilers but I think you will find that most would set to the manufacturer's CO2 figure or a similar figure they have found by experience on a particular model.

Tell me, to what extent do manufacturers recommend figures more appropriate to a dirtier boiler compared with a clean new boiler?

Tony

 

[Boilerman2]

The 0% O2 that is being referred to is "Stoichometric" Combustion - in reallity this is only possible under laboritory conditions!

A boiler heat exchanger is very compact and the combustion gasses are cooled too rapidly for perfect combustion.

therefore an "excess air" allowance of around 35-40% is required which works out at 4.8- 5.5% O2

the reason you got 91% Efficiency is that your analyser is set to "Nett" this should only be uswd on Condensing appliances when Combustion testing standard efficiency boilers the analyser set up should be changed to "Gross", this would have given a figure of around 86% which is more realistic for a Eurostar Boiler!

 

[fredfredfred]

Tell me, to what extent do manufacturers recommend figures more appropriate to a dirtier boiler compared with a clean new boiler? Tony

Generally what we find is that as soot builds up, the boundary effects (motionless gasses around the edge of the 'firebox') increase, raising the gas temperatures. This in turn moves the heat profile further and further away from the burner, which is why baffle plates are so useful, as they slow down the gas velocity through the system. In an ideal world, all the fuel is burnt in the first few cms near the nozzle, and then the job of the boiler is to extract the heat from the hot gases before they leave. Generally, the longer the gas path, the more heat is extracted.

But in reality, most oil boilers don't suffer from much soot at all, so the figures don't move. BUT dust, cobwebs and fluff do reduce the air throughput, as the impellor gets clogged. This in turn causes the mixture to become rich. So most manufacturers suggest a slightly leaner setting. Soot buildup in a condensing exchanger is a big no-no.

A new, clean boiler, with no fluff or soot can be as much as 10% more efficient than an older, dirtier clogged one. This is why an annual service is recommended. All the components in a boiler are completely service free. If giant filters were fitted to the air flow and active carbon filters to the Kero, the boiler would stay in calibration for years (assuming the filters stay clear).

-fred

 

[namnori]

I'm curious to see what plumbers have seen in their quests when it comes to measuring efficiency vs CO2 readings. In an ideal world, the CO2 would be maxed out, with zero CO and zero O2,

The products of combustion for a 'stoichiometric' air-to-fuel ratio (CO2max) will produce high levels of CO , hence the reason 'excess air' is added to the combustion process , as you correctly pointed out 'too much' excess air will decrease efficiency of combustion in the quest for lower CO/CO2 percentages..............as regards 'ignoring manufactures commissioning settings' i guess we can to a certain extent (the buck stops here) , as long as the commissioning engineer is 'happy' CO2 is around 12%vol (light oil) or less along with O2 in the region of 5-7%vol.

Stoich combustion is extremely unstable/volatile & very much dependant , this is the reason excess air is introduced.

At the end of the day it's a balancing act & obviously safety is paramount in respect to combustion efficiency.