Lets have a look at the possible causes of Aga range cookers losing heat or ‘sooting up'; it's a common problem!
By far the most common is 'overfueling'. Most of the time excessive soot becomes an issue after a service. Oil cookers use the wicks only to get going. After this the heat from the shells vaporises the oil as it arrives in the burner base. This vapour burns cleanly and relatively efficiently. Sometimes though after a service, when everything is newly cleaned, too much oil can arrive at the burner base even though the flow rate settings on the valve have not been changed. Of course if the valve has failed the same issue can present itself. When this happens the oil cannot vaporise fast enough and it begins to burn directly. This, as you can imagine, is a sooty and messy business! To solve this issue you'll need to get your engineer to adjust the flow rates on your control valve downwards. Original valves are by BM - these have now been superseded by a valve made by Toby. They are a like for like swap.
A similar issue is if the burner base has been re-installed lower after a service. The increased depth of the oil will again cause the oil to burn directly. You'll get a yellow flame - not a lovely efficient blue one! The burner base needs to be recommissioned and leveled again basically to solve this!
After over fueling the next most common cause of a sooty and low temperature Aga range cooker is a change to the flue. Insufficient draw means the carbon that forms as an inevitable byproduct of combustion does not form in the 'right' place. Your flue draw can change for many reasons even if you don’t change the length of it. Maybe a bird knocked a cowl off, maybe that hedge you removed exposed the flue to higher winds or maybe your neighbours conservatory has left it becalmed - all of these can ruin the careful balance in your range cooker and make your flue draw too high or too low. Too high and your range cooker will run cool and struggle to reach temperature, too low and it’ll ‘soot’ up quickly - bummer!
Often engineers will blame ‘low sulphur’ fuel (damn the EU for trying to save us from air pollution!) and it’s true that this can cause problems but most range cookers run perfectly well most of the time on the exact same fuel so before you try expensive fuel additives and the like have a think about what, if anything, could have changed your flue conditions or caused overfueling!