An Aga puzzle.

[The Natural Philosopher]

Oil agas are simple brutes. Fill them up with oil, decoke once a year,
and they JustWork™.

Except mine has taken to popping its fire valve.
Let me describe, to those who dont know how one works, the basic operation.
Kerosene is gravity fed into a carburettor via two fire valves. One is
external to the property and its sensor is in the house structure. It is
designed to shut off oil when the house catches fie. It is not the issue
in this case.

The other fire valve is just upstream of the carburretor and its sensor
is located just outside the burner door but inside the Aga. This is the
problem child.

What the carburettor does is fill a float chamber with a perfectly
ordinary float valve until the fuel level is high enough to start
trickling down a needle valve assembly to the burner. The needle valve
has two positions that correspond to high and low oil flow rate - ISTR
its 8ml/minute and 4ml/minute - that can be selected in the original
design by moving a lever from low to high. (This lever is latching in
the way a lever on a motorised valve is latching). During installation
the burner height, the carburettor height and an adjustment screw are
set to get the flow rates of the two positions correct.

On top of this carburettor as an add on extra post IIRC the mid 1980s,
which has never actually been integrated into the design, is a expanding
wax capsule heated by a resistive element controlled by an on off
thermostat in the main oven, that expands and pushes a lever down to go
from high oil to low oil flow.

So, in use the Aga toggles between high and low flow to maintain an
approximately even temperature.

The burner itself comprises a large cast iron base assembly with a
central sump that fills up with oil and that then percolates into an
arrangement of concentric rings - like ditches - and some those in
addition to oil carry an assembly of concentric perforated metal
cylinders. There are also round wicks, at least one - I forget - whose
only use is to light the thing from cold. Once lit the perforated rings
get dull red hot, the oil vaporises and the vapour burns ,more or less
with a clear blue flame and very little soot.

The hot gasses then do the rounds of the Aga internally and exit via a
ventilated flue pipe at the top.

Twice the internal fire valve has tripped, meaning the burner got too
hot. Or flames got outside it.

Last night it happened again. It was windy.

Inside the burner was absolutely caked in soot. I have never seen it
like that before.

I had recently - before all this happened - serviced and decoked the
burner and filled it up with as much kerosene as the tank would take,
kero was cheap...

There has been a slight irregularity in the sound of the burner
recently., As if it were 'flickering'.

But the burner is burning a lovely blue flame, showing plenty of oxygen.
The cylinders are red hot as they should be.

I did remove the flue vent cover and there was a lot of soot in the
flue. I have arranged to get it swept, but it seems clear thanks to Mr
Henry, for now, at least in the base area .

Although the boiler has been serviced the Aga seemed to be working
thereafter correctly and although I cleaned the carburettor thing
yesterday that was after the problem not before. No settings have been
changed.

Online wisdom all says the same things - too much oil , too little air
or crap low sulphur fuel *all indicated by a yellow flame.* I don't have
that at the moment after relighting.

So I am it must be said a bit stumped.

The only thing I can think of is that at some point soot has built up in
the flue - I last had it swept about 3 years ago - after over a decade
of never being swept *at all* - possible as a result if the inferiors
fuel now being sold - and that it has become dislodged and blocked the
flue and the wind has dislodged a bit more, strangle the burner,
carboned it up and tripped the valve.

I did notice a reluctance to get hot quickly over the last couple of years.

Of course one leaps to conclusion, You serviced it. You fucked it. Tank
full of BAD oil caused it. The Russian did it. Aliens from Space are
camping in the chimney. etc.

Oh the chimney has a conical cowl over the top of it and no bird has
ever fallen down it.

My best guess to date is that somewhere in the internal flue system soot
has built up and is/has been blocking the passage of gases. And some of
it fell down and blocked it even more causing the burner to go all
smokey and trip the fire valve. But why now? After twenty years...?


--
"Strange as it seems, no amount of learning can cure stupidity, and
higher education positively fortifies it."

- Stephen Vizinczey

[Michael Chare]

Are you able to sweep the flue? I bought a kit from Wicks to do this.
It is also suitable for cleaning drains with different attachments on
the end.


--
Michael Chare

[Fredxx]

On 28/09/2023 13:53, The Natural Philosopher wrote:

<snip>

> But the burner is burning a lovely blue flame, showing plenty of oxygen.
> The cylinders are red hot as they should be.

Do you have visibility of this through a sight glass? Or does this check
affect air or exhaust flow?

Perhaps a recorded camera to see any variation in colour?

[Tim+]

The Natural Philosopher <t...@invalid.invalid> wrote:
>
>
> My best guess to date is that somewhere in the internal flue system soot
> has built up and is/has been blocking the passage of gases. And some of
> it fell down and blocked it even more causing the burner to go all
> smokey and trip the fire valve. But why now? After twenty years...?
>
>

Have you changed the air-tightness of your house or kitchen recently?

Tim

--
Please don't feed the trolls

[The Natural Philosopher]

Well Michael, my favourite Sweep is coming, just not yet - I have two
more chimblys that need his attention. but yesterday I did take out the
decorative part of the lower flue and the baffle plate and vacuumed as
much soot from there as I could, relit it and it seems to be holding...

I think there is a flue restriction somewhere. That is now *less* than
it was.

If the chimney sweep doesn't fix it, may need to take the plates out of
the top and clear any soot from in there. its a mystery why after years
and years this has happened unless the low sulphur fuel is simply more sooty

--
Of what good are dead warriors? … Warriors are those who desire battle
more than peace. Those who seek battle despite peace. Those who thump
their spears on the ground and talk of honor. Those who leap high the
battle dance and dream of glory … The good of dead warriors, Mother, is
that they are dead.
Sheri S Tepper: The Awakeners.

[Brian Gaff]

Its the same with anything. Nobody has tested every conceivable issue for
long enough to be sure its not a long term issue.
You get much the same in electronics. You have had a unit running like a
well oiled machine for many years, then it starts doing odd things, No one
voltage looks wrong , but after a lot of probing around you find that
almosest every electrolytic capacitor is either leaky or under value. A
complete change of these normal fixes it, but the labour involved costs
nearly as much as a new unit, which of course is now no longer made and the
newer model is more flimsy, has a self destructing remote control and goes
wrong when the warranty has expired.
See it all the time with TVs, and the like.
In your case it should in theory be purely a mechanical problem as you have
identified, but modern electronics seem to be built with surface mount
components never designed to be replaced.
Bah Humbug.
Brian

--

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>
> Oil agas are simple brutes. Fill them up with oil, decoke once a year, and

> they JustWorkT.

[John J]

I had an oil aga on "my patch" that suffered the same fate. After extensive cleaning and sweeping the soot formation was traced to a weeping joint in the under burner oil pipe where a parasitic flame formed. Remaking the problem joint stopped the weep and the problem went away.