Hypothetical: Turn up the thermostat, boiler turns on, and then immediately turn it back down?

Electricity would pass thru the thermostat and continue to the system and operate any burner that is not interrupted by some other safety device like a LWCO or other limit. After the burner starts to operate, if you turn the thermostat lower so there is no call for heat, then the open electrical circuit caused by the new temperature setting would no longer be completed and therefore the burner control would stop operating due to the lack of electric power.Do you have such a oil heated boiler? Could you do a scientific test and record the results and report back? We would all be interested in your findings. That way it won't be Hypothetical anymore.

And since you have mentioned in previous posts that you have experience as a mechanic of sorts. I would ask you a similar HYPOTHETICAL question about turning on something and then turning it off right away. For example, if you were to set the head light switch on an automobile to on, then soon after that turn the switch off, what would happen to the head light beam. Would it stop traveling at the speed of light thru the atmosphere and out into outer space? And if that switch were on a Semi Truck and you did the same thing, would there be any difference?

Was this answer detailed enough? …or was @ethicalpaul's answer sufficient?

It sounds to me like the OP is wondering if there's any kind of minimum on time with the boiler.

That's a real thing, but I'm not aware of a residential boiler that has that function.

A better car analogy might be starting the engine and putting it into drive, then putting it back into park without taking your foot off the brake, and shutting the engine back off. Not going to hurt if it happens to occur, but not something that you want to do all the time.

Swing or in control engineering, dead band. If the controlled device is on/off, you have to have it.

The whole question of control design and theory is an absolutely fascinating one — to those with a certain turn of mind! There are inevitable tradeoffs in any control scheme between accuracy in maintaining the target parameters, reliability, and cost (and complexity), and balancing those in a way acceptable to the user is… difficult.

The heating industry on the residential/small business level has been curiously slow about implementing more "advanced" controls; largely, I think (opinions may differ) because the users don't see, in general, any real need for them and they object to the cost and complexity — and regulators or other authority types have not seen, until recently, any need to force them to comply.

It is perhaps more instructive to look at the automobile industry, as there one can easily see these tradeoffs. Or perhaps it is easier for me, since I've always loved working on machines and cars. But I do have a usable timeline. Consider these machines, all sitting in the yard as I type. First, a 1951 Ferguson tractor. Second, a 1980 Chevy C10 truck. Third, a 1994 Chevy K2500. Fouth, a 2015 Chevy Cruze diesel car. The first has a manual transmission and a manual choke. There are a whopping total of 4 adjustments needed to get it running perfectly (which it still does). The second is more advanced: there is an automatic choke and a fancy solenoid which regulates idle speed. Still, that only adds one adjustment. The third has a computer which controls fuel delivery and transmission shifting. Better economy and, when everything is working right, overall easier management. If something isn't working right — you don't get out the screwdriver, you get out your wallet while you wait for the tow truck. The fourth is a joy to drive when it's working properly, which is most of the time. There are, at last count, 15 different computers in it and 3 separate serial bus networks. I've not counted how many sensors and actuators there are. Probably a couple of hundred. Nothing is controlled mechanically (normally — there is a fall back mechanical control for the brakes and the steering, but you'd better be strong). When something goes wrong, it's not repairable. Just replace it. Get out your bank account — youe wallet won't do.

The tradeoff? Number one is not all that easy to drive and control. Number four simple —hop in and go. Number one is simple, reliable, and repairable. Number four isn't.

I could say the same about airplanes — a Piper Cherokee is all mechanical. You'd better know how to fly. A Boeing has some computer controls (which give problems when they aren't working right) but still has enough mechanical backup to be flyable — if you know how to fly. An Airbus has no mechanical backup. It's easy to fly, and will keep you out of trouble if you goof up — so long as the magic works. When it stops, which it does once in a while, that's it. You are now a passenger on the way to the crash. Take your pick… tradeoffs.

End of ramble.

Many thermostats have a swing value in the range of 1.0 to 2.0 degrees F. So the system won't cycle on at 67.9 and off at 68.1.

An example would be the system turns on at 67.0 and off at 69.0 when set thermostat is set to 68. I believe this would be an example swing value of 2 degrees.

Some thermostats allow for changing the swing value (no smart thermostats I know of).

No, he's saying you (and the thermostat) want to prevent the system from doing that.

The thermostat manufacturers often hide the actual swing value from us for some reason, but the important thing is that "forced hot air" is typically the smallest, and "steam" if it has it is the largest.

Probably all of us here know that running engines for short periods is bad…I think you posted that from some kind of AI answer. I'm not sure why!

Depends on the thermostat, but it shouldn't be that tight, the equipment can't respond that fast. 1-4° would be more like it, but some (notably Honeywell) use a "cycles per hour" instead of a deadband.

Slightly off topic but does anybody else here dislike the algorithm in the current Residio (Honeywell branded) thermostats that claim to have no deadband? Cycles-per-hour is the only way to control them and I dislike them with high-mass radiators. I love the mounting/wiring base though. I've had to upgrade some for a customer and it was a snap.

At some risk of sounding like a stuck record (what's a record, Grandpa?) — neither cycles per hour nor a fixed deadband/swing is ideal for close temperature control. Neither truly (or at all) takes into account the inherent lags in both bringing the heat on or in turning it off, never mind subtleties such as actual heat loss rate at the moment (which affects potential overshoot and undershoot) and a few other variables. Not, as I've said in another thread, that a digital thermostat can't be built and programmed to do that satisfactorily, provided it has some adjustments available, but that it simply isn't worth it in a plug and play world.

The Honeywell 9000 series has PID control…

NB: I am not a Honeywell shill. I just use the 8000 series a lot, and I dislike the T series very much.