Boiler Air pre heater

Love to help, but we need a little more info.

Do you have some sort of air filter system for this air before it gets pre-heated? if the answer is yes, the filters may be blocked.....therefore you would need higher RPM to give the same amount of air to be burnt!!

This is exactly why a car needs to have its air filter replaced from time to time - years ago the petrol mileage dropped as the filter got blocked, today the mixture (petrol to air ratio)is kept correct by the onboard motor management system, but as the air volume gets less and less, eventually motor maximum power/torque starts to suffer.....or a bypass is used and then motor damage occurrs from the ingested dust.

I suspect that if the filters are the source of the problem, they are not being replaced at the correct intervals, or the dust levels are simply too high...change them sooner then....