Why would a gas boiler make soot?

[Nick]

Boiler is Potterton 200K BTU. About 15 years old. Conventional fully pumped vented system. Installed and serviced annually by the same engineer.

Through the winter it had a problem. Heat output considerably down, making soot and noxious fumes. Boiler is located in a well vented external boiler room.

Engineer came out and diagnosed the gas control valve as being worn out. Next visit he serviced the boiler, de-coked the heat exchanger, dismantled and cleaned the flue, fitted and adjusted the new gas valve. Left in working order.

The boiler is certainly running much better. Good blue flame and somewhat quieter. Water output temp has risen gradually and continues to rise.

Problem is that it is still making soot in appreciable quantities. Flakes up to about 10mm across. Have spoken to the engineer again and he is a bit stumped.

My only guesses are (1) the boiler is now working more efficiently and is burning off residue from the H/E or (2) that the burner unit is worn, burning improperly, and creating soot.

If (1) I hope the problem will go away, but it has now been 3 months.

Any ideas please?

Nick.

[Roger Mills]

Soot usually comes from a yellow flame when there's not enough air to burn the gas fully - but you say the flame is blue?

[The Natural Philosopher]

I know nothing about gas boilers, but a lot about fires,,and you are right,,soot is bad combustion and the soot particles glow yellow hot and that's what makes the flames yellow.

So it sounds as it the other theory - that its simply burning old shit out - is the right one. You should see what comes out of a chimney if you set one on fire. LOTS of soot!

The last guy to service my (oil) boiler had some sort of gas analyser..presumably that's true of yer gas man, so if the combustion is within spec, that's all you need to worry aboput..ultimately the soot will burn away I guess.

[geoff]

Aah - the gas / fuel mixture

No chance of any carbon monoxide there

Dennis morphs to drivel, Harry morphs to dennis

[Gib Bogle]

Just from first principles (no specific knowledge):

soot implies incomplete combustion

incomplete combustion can give CO

e.g. 4CH4 + 6O2 -> C + 2CO + CO2 + 8H2O

[geoff]

It would if it was a gas / air mixture ...

Which is why they use a CO/CO2 meter, a Telegan for example

not really come across an oxygen meter in general use for measuring combustion efficiency

[Gib Bogle]

Missed that.

[geoff]

No - co / co2 meters - measuring the ratio

Would you like to expand on that ?

Do you really understand what you're talking about?

Not amongst your average jobbin' fitter they aren't

[YAPH]

Soot is a warning sign of incomplete combustion and your warning sign that the boiler is probably producing carbon monoxide in lethal quantities. If you had a large dog foaming at the mouth would you leave it and hope the problem would go away? Get it sorted. Now. Call 0800 111 999 and tell them about it.

[GB]

Natural gas is mostly methane, ie CH4. My recollection is that the older town gas used to include some longer chain hydrocarbons (which have a lower carbon/hydrogen ratio than methane) as well as a fair amount of CO, which of course has no hydrogen at all.

[geoff]

I think we all know that

I was trying to get at what harry thought was the relevance when it came to using a co / co2 meter

[GB]

Now I'm totally deflated.

[cynic]

The heat exchanger is difficult to clean thoroughly and it is quite possible that there is still trapped carbon but after 3 months you have more than a residual problem. Is the burner completely clean and properly jointed so that the flame bed is entirely satisfactory without any patches of yellow sooty flame across it? Does the burner gas pressure match with the data plate information? has the heat exchanger been THOROUGHLY cleaned so all the fins snd surfaces are clear of residual soot? Is the collector hood and flue clear of soot? I have seen a particularly badly sooted heat exchanger cleaned with a pressure washer but as you can imagine that made a serious mess!

[Steve Walker]

I don't know about this particular boiler, but I've often wondered whether there is any great risk to poor combustion with most boilers these days, as they are generally room sealed and under negative pressure due to the exhaust fan, therefore they should be unable to leak CO into the room.

I would have thought that the main worry would be increased fuel bills and gradually reducing heat output.

SteveW

[tony sayer]

Bit worrying that, never seen soot from our boiler and If I did I'd soon shut the thing down..

Mind you it can do that sometimes 'tho is been well behaved since it had a new board from Geoff no surprise there needing one after all it's a Suprima;!...

[Chris Hogg]

I've no specific expertise on domestic gas appliances, but I do operate a gas-fired pottery kiln where firing with insufficient air for complete gas combustion is commonplace above kiln temperatures of say 850C*. Soot production can be excessive if you get the conditions wrong.

Think 'Bunsen burner'. Most simple burners have air supplied in two ways, primary and secondary air. Primary air is drawn in through the bottom of the burner tube via an adjustable opening. Secondary air is what reaches the flame from all around it. The sootiness of a flame is very dependent on the amount of primary air that mixes with the gas in the burner tube just before it reaches the flame. The more primary air, the bluer and less sooty the flame (too much primary air and the flame velocity exceeds the flow rate of the gas/air mix up the burner tube and the flame strikes back down the tube and burns on the jet).

Even though you say your flame is blue, my guess is that you don't have enough primary air. As well as a generally blue flame, I would expect to see a brighter blue cone at the base of the flame, or flamelets if there are lots of them as in a gas ring, for example.

• For those of you familiar with domestic gas appliances and CO emission regulations etc. gas-fired pottery kilns are an entirely different ball-game. CO production is deliberately induced at higher temperatures to achieve particular colours in the glazes. The CO burns as it leaves the top of the kiln and meets the outside air if the kiln temperature is high enough. My kiln is in a well ventilated shed in the garden. There is a 'cooker hood' type of arrangement over the kiln, with a chimney, to collect and carry away the combustion products. In fact the general arrangement of such a kiln is very like a high-temperature gas oven, going up to 1300C, but I wouldn't have it in the house!

[YAPH]

It's not as dangerous as with a non-room-sealed type but it's still putting CO into the atmosphere outside which can - given prevailing wind conditions - get back into the house. Or a neighbours' house.

There was a case recently where a boiler had been installed with its flue close to a window, which had been screwed shut to prevent Products Of Combustion (POCs in the trade) getting into the room. Then the owner of the building had the window replaced, with one which had normal openers, and the occupant of the room died of CO poisoning. (IIRC the boiler was also badly malfunctioning - you wouldn't expect it of a normally- operating boiler.)

And, more innocuously, I got called out a few months ago, when it was freezing cold, to a house where a CO alarm in a room with a boiler had gone off. The boiler seemed fine - burning cleanly (as measured on the Flue Gas Analyser, and visual inspection - certainly no sign of sooting). It was an upstairs room. No sign of any way gases from the cooker downstairs could have got up to trigger the alarm. But the CO alarm was next to a vent to outside, and I can only think that a car had been idling outside for a while, with the engine running to warming up/keep warm while waiting for someone, and the exhaust gases drifted up in the cold still air and found there way in through the vent to the alarm.

[geoff]

The soot also progressively clogs the vanes of the heat exchanger

[Andrew Gabriel]

My flue gas analyser has an oxygen cell and a CO cell. CO2 is calculated from the oxygen content, knowing that it's measuring natural gas flue gasses (I think you can also set it for propane and butane, but I never have). Mine doesn't calculate the CO/CO2 ratio - you have to do that yourself, but the next model up does that for you, for those who don't know how to convert units and do a division;-).

I don't know what the situation is nowadays, but gas installers didn't generally have flue gas analysers a few years back. I bought mine when I installed my condensing boiler 10 years ago, as although it was supposed to come pre-adjusted, it was actually miles off the right settings, and it made a significant difference getting it setup correctly.

[Andy Champ]

Just FYI mine's under positive pressure from an inlet fan. But the combustion chamber is completely enclosed by the fresh air plenum except the exhaust - so leaks would be cold fresh air into the room, anything combusted can only go out of the flue.

Andy