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Maybe I am stupid or forgot something over the last 50 years.

If you set the boiler at 190, 200, 220 whatever does that mean the boiler will maintain that temperature.?

No, the thermostat tells the boiler to run when the space temp is low. The burner fires. If there is enough radiation (more than or = to the boiler output) most of the time the call for heat will be satisfied before the limit temp is reached.... correct? In that condition the limit control will seldom ever operate...correct.

This condition is ideal. If it occurs in a building, we need not discuss this further.

However, I have never seen it in 35 years in residential OIL. The boiler is a minimum of 85K input and the radiation PER ZONE is typically a maximum of 30K. One typically does not get the benefit of multiple zones calling unless the building is close to design.

What typically occurs is the boiler coming up to HL and cycling on the differential. If you set a 200 HL, you are correct in that the boiler will have less cycling due to the zone reaching the 'stat setting faster. If you have a proper purge so the boiler can return back to 130F, this setup MAY be reasonably efficient. I have never experimented with it.

With most oil equipment WITHOUT purge, it is grossly inefficient to run the boiler up to 200F and then have the 'stat shut it down on limit. Just set the boiler at 160F and it might cycle a few more times before the 'stat satisfies.

Where this approach fails miserably is when setback is utilized. If the HL is set lower, the boiler will definitely cycle for a much longer period (possibly several hours) due to the inability of the emitters to deliver anything close to the boiler output.

But that is not always the case. If the home is under radiated or the boiler is oversized in comparison to each other the boiler output in btus and the radiation output in btus will try and find a balance with each other. This could happen at 160, 170, 190 or any other temp in an extreme cases.

The boiler is typically oversized with a fixed output. The radiation has a variable output depending on SWT but it is rarely equal or greater than the size of the boiler (certain CI systems excepted).

There is no balance that can be achieved in this case. Whatever SWT is chosen, the boiler must cycle to maintain it, unless, as you noted, the 'stat satisfies during the climb of the boiler to the HL setting.

So if the boiler is oversized in comparison to the radiation output what is the harm in running the boiler to 190 and then purging the heat upstairs?

Generally, the higher the temperature of the boiler, the greater the overall system efficiency losses. However, you make an interesting point. IF you can go to 190F and satisfy the 'stat quickly and return the boiler to 130F in a relatively short period of time, the efficiency losses due to the 190F setting may be largely offset by the short time the boiler remains there.

I do believe that even with this approach, you can gain efficiency when the heatloss is less than half of design by reducing the HL. Why use 190F in April when you can get, effectively, the same result if you use 160F? One additional issue is comfort...........the overshoot on the radiation (especially CI) will be substantial in April if you use the 190F approach.

There are a lot of variables in the decision to reduce HL and these variables change with the season and the choice of setback. You most certainly can cause a loss of efficiency if the HL is set to 175F with a 20F differential, at a 20F overnight ambient, AND the tenant utilizes an aggressive setback because nobody occupies the building at night! The boiler will cycle for six hours in the morning before the 'stat will satisfy!! The boiler is losing energy constantly to the basement for six hours and has 48 stops and starts during that period!

This is one topic where YMMV! No specific approach will ever work for every building.