On Sunday, 7 February 2021 at 10:30:27 UTC, fred wrote:
Our oil burner was recently converted from diesel to kerosene. Due to a faulty level gauge the tank ran dry. By mistake I threw a couple of gallons of red diesel into the tank. I can get some kerosene from a local filling station. How much kerosene might I need to dilute the red diesel?
What damage might burning red diesel do to a kerosene system. ( I wasn't here when the burner was converted but I imagined it only involved changing he injector/nozzle.

Sods law. These problems only occur at the weekend when service isn't available never mind a refill of the tank

Depending on the model of burner you have the effects could range from sooting up the burner and photocell hourly to barely a noticeable effect. Diesel doesn't ignite as easily as kerosene so expect any conversion to have required change of nozzle to one with a different spray pattern and spray angle together with adjustment of oil pump pressure.
Running the tank dry usually results in dragging the crud in the bottom of the tank through into the fuel system. If you have a decent outlet filter at the tank you may get away with replacing the filter element if it's a paper cartridge or cleaning a nylon/metal gauze type. However there's a risk of particulate matter passing on further to the boiler. You may have a secondary filter at the boiler which could need cleaning plus most oil pumps have a strainer in the pump plus the nozzle will have a sintered strainer inlet and a very tiny hole the fuel is pumped through to create the atomised spray for the ignition arc to light. All these could become blocked
I'd suggest at least 5 parts kerosene to 1 part red diesel will help to avoid problems with ignition but low temperatures exacerbate the potential for trouble.
You should clean out the combustion chamber and flue plates when things are back on an even keel