Hi - almost certainly the problem here lies with your fuel. Im assuming you have an oil fired Aga. About 3 years ago the EU changed the standard for domestic heating oil (kerosene) and dramatically removed the amount of sulphur present in the fuel. This may be fine in a injected boiler system however in a 70 year old design wick based Aga, Rayburn, Esse or other range cooker it is a big problem. What happens is that with the removal of the sulphur, it is more difficult to burn the fuel from an atomising wick the net result of this is unburnt carbon (hence the yellow flame) this unburnt fuel gradually builds up around the oil inlet pipe gradually reducing the flow of oil to the burner. Symptoms are a yellow flame, cooling Aga, spitting, spluttering and popping sounds and eventually the Aga goes out.
There is however an easy solution to this problem, my company supplies an oil additive which tackles and fixes this problem see http://www.ecofuelplus.co.uk/index.php/applications/aga-rayburn by adding a product called Aderco 5000 to your fuel, the effects of lost sulphur are minimised - your flame will return to blue, the heat will improve and the system become clean again. There are other benefits for your tank and system too - please read about these on our website above. I would also reccommend reading the experiences of Allerton villages oil buying syndicate and their experiences with Aderco http://www.allertonvillages.co.uk/Newsflash/aga-heat-problems-additive-trial.html
Eco Fuel Plus, January 2012
