Anti-cycling mode and the d.2 parameter

Sometimes the boiler enters an anti-cycling mode, which is intended to prevent it from trying to re-start inappropriately quickly. Here I gratefully reproduce UpgradeME's explanation of the d.2 boiler code, which determines how long it waits before re-starting:

"This may also be of some use, which explains how the 'anti-cycle' operates. 

Vaillant d.2 code - Example of operation  

Let's assume d.2 is set at 60, the modulating controller is currently requesting a flow temperature of 40C and the burner has already fired. The boiler upon entering anti-cycle mode starts a timer, counting up from zero, with an initial 'goal' of waiting 38.5 minutes, from the following table:

[Caution - MikeH has recorded in the comments below that for his 415, installed in about 2008, the values he gets are as in the table, except in the row above the one indicated. In other words, for a target flow temperature of 70C, his boiler is using the values that the table gives in the 65C row. A 418 boiler fitted in about late 2010 uses the figures given by table without adjustment.]

If after waiting 20 mins, the room temperature drops, the controller then changes its flow temperature request from 40C to say 60C. The anti-cycle time for 60C is 17 mins, but as the timer has already reached 20 mins, there is no anti-cycle time remaining (it's actually now -3mins) so the boiler then fires the burner again.

[anti-cycle time from table] - [timer] = [time remaining before boiler is allowed to re-start]

Initially that would have been be 38.5-0 = 38.5 min then changed to 17-20=-3mins 

The above example is assuming a modulating controller set in 'analog' mode where the flow temperature demand is dynamic. If set to 'two-point' mode (or using a.n.other room stat), the flow temperature is static and set via the dial on the front of the boiler. As an example for a static flow temperature demand, if the boiler was set at 65C, after it enters anti-cycle mode the boiler will wait 11.5 minutes before firing again with d.2 set at 60, or 4.5mins with d.2 set at its default of 20. Pump is off and zone valve(s) are closed during anti-cycling period."

I've edited this description slightly from UpgradeME's text to make it clearer (and in particular have altered the equation, which seemed to have the two parameters the wrong way round). Let me know if I've got the wrong end of the stick here.

I suspect that the way the timer is described as working (counting up) is not strictly accurate, since parameter d.67 is "remaining burner anti cycling time" (i.e. a count down). I had already noticed what appeared to be a countdown as one of the vrDialog-recorded bits of data on my system, but had not worked out exactly where it came from or what it was doing. I shall post some graphs of how d.67 looks when I have time.

The way the ecoTEC manual describes this process is:

"The burner is electronically locked for a specific time after each time it is switched off ("re-start interlock") to avoid frequent switching on and off of the burner (energy losses).
The burner anti-cycle time is only activated for the heating operation. Hot water operation during a burner anti-cycle time does not affect the timer.
The individual anti-cycle time can be matched to the hydraulic and thermal properties of the heating installation. In the factory the burner anti-cycle time is set to a value of 20 minutes. It can be varied under diagnosis point "d.02" within the range 2 minutes to 60 minutes. The individual effective anti-cycle time is calculated from the momentary target feed temperature and the set maximum burner anti-cycle time.
The timer can be reset or cancelled by actuating the appliance main switch. The remaining burner anti-cycle time left after switching off by the controller in heating operation can be called up under diagnosis point "d.67". The individually effective burner anti-cycle times with respect to the feed temperature and the maximum set burner anti-cycle time can be taken from Table 7.2"

It does sound from this as though d.2 could be relevant to micro firing, since "avoid[ing] frequent switching on and off of the burner" is exactly what we aim to do. However, I cannot presently see that the d.2 setting can be the (sole) cause of micro firing in my system, for instance. I have not changed it from the factory setting of 20 min. I routinely see micro firing at a target flow temperature of 40C, which the table indicates as corresponding to an anti cycling time of 13 min. But the boiler often tries to re-start immediately (within seconds), rather than waiting anything like 13 min. The sorts of timescales indicated by the table look more like what is controlling the interval between periods of short firing.

Can anyone explain to me when the boiler does enter anti cycling mode? That is status code S.8. Sometimes when the boiler switches off, it goes to S.7 (pump over run, which presumably means that it keeps pumping for a period determined by parameter d.1)  but not to S.8. But sometimes it goes to S.8. For instance, see the status code graph during a few minutes of micro firing that is on the page called "The problem" - there it never enters S.8, so can d.2 be relevant at all during this period? Perhaps it ought to be going to S.8, since it would then wait a respectable length of time before re-firing instead of doing so straight away.