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If you have been having efficiency problems with your oil AGA Cooker, Rayburn Cooker or oil home heating boiler in last six months or so, say it's being smoking or going cold or just not going at all the problem is probably the properties of the kerosene you are using. Heating oil suppliers have been forced to reduce the polluting sulphur in domestic heating oil from 0.2% to 0.1% with effect from the 1st January 2008 to meet EU regulations. This requirement is contained within the 1999 EU Sulphur Content of Liquid Fuel Directives. In England and Wales the Directive is being implemented under The Sulphur Content of Liquid Fuels (England and Wales) Regulations 2007. See Kerosene Sulphur Change
The resulting product has a much higher ‘char value’ than before and this will cause your oil AGA cooker to coke up quicker and more often. It may have taken a while for you to notice this and you've probably been living with it blaming it on other things or you've being using the old kerosene stored in your oil your tank and have only bought and used the new stuff more recently.
Suppliers have no choice but to provide this new formula kerosene. New AGA cookers work OK with the new oil and a replacement burner is available at a cost to owners of oil fired AGA I'm not sure about Stanley cookers, Esse cookers, Redfyre cookers, Rayburn cookers, Sandyford cookers etc. However if you have an old converted solid fuel AGA cooker (not sure about Rayburn cookers) the new burner does not fit and I've heard that the new burners will cost a bomb. I'd expect these to be the ones with Dons conversion kits. I think because of these the older 'standard' AGA cookers will die out as they will be very difficult to live with.
The older AGA cookers that have been converted from solid fuel have always been a nightmare anyway regularly coking up, yellow flame, smoking, going cold when loaded etc. This new fuel with make things worse.
My advice if you have an AGA cooker that was once a solid fuel one but has now been converted and you are having serious problems with it is to fit a new AGA pressure jet burner unit from www.tradcookers.co.uk. You may have to buy a new AGA off ebay and then get the unit installed in that one.