Trying to reduce my oil usage

It may seem obvious but it is still incorrect. In a sense you have touched on the answer in relation to the flue.

Short cycling directly increases the boiler temperature (in that the heat is not been dispersed from the boiler quick enough), thus increasing the flame chamber temperature thus increasing the flue temperature. Flue temperature is directly proportional to efficiency, which is turn is directly proportional to fuel consumption. The higher the flue temperature the lower the efficiency of a boiler and thus more fuel will be wasted to the atmosphere and not into your home where it should be going. For example a standard efficiency boiler will have on average 200C flue temperatures. A high efficiency condensing boiler will have a flue temperature of approx 50C. The extra heat is being transferred into the heating system and thus being more efficient.

To check this have the boiler running with all radiators open and check the flue gas temperature with a FGA and then close down most of the radiators. An distinct increase in FG temperature will occur.

As I previously stated, another directly proportional side effect of consistent high boiler temperature is component failure. Metal expands as we know when heated. It generally reverts back to it designed state when cooled. Heat excessively and it does not, leading to undesired performances and eventual failure. This brings unnecessary cost to the homeowner.

Shane007 said:

It may seem obvious but it is still incorrect. In a sense you have touched on the answer in relation to the flue.

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No Shane007 what I advocate is correct, it may not be efficient or the best way to run a heat source but nevertheless is correct.

As stated, assuming all uncalled for energy is not lost/dissipated through the flue, or indeed through other losses, then that energy will be sent to existing loads, and not withstanding boiler short cycling it still remains that if I reduce the load put on an energy source by 33% then if above is accepted it follows that input is also reduced.

As I already said input may or may not be reduced accordingly but it will be reduced.

It would need to be a very inefficient system if the load is reduced by the tune of 33% and no corresponding reduction in input is measured.

So at what point does your theory not work? Should I fit a 100kW boiler to heat one radiator. Would this use less oil? So if you are correct and a 100kW boiler uses more oil to heat one rad then at what point does the balance tip?

I think you are getting too hung up on the demand rather than the whole picture. It is more to do with how a domestic oil boiler behaves and how it is designed for the job in hand. Over-sizing is an incorrect practice. It will heat the system quicker than a smaller sized boiler but it will use more once heated.