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 f***ed 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...?