d.67 - the countdown to boiler re-start

Here's an example of what I take to be d.67, which is the countdown referred to in the last post. In my vrDialog data, I think it must be the parameter called RemainingBurnerblocktime_DK, which is in red in the graph below.

The graph shows a transition (at about 17:00) from micro firing to short firing. Note that the micro firing period does not show all the detail of what happens on a timescale of less than about a minute, so it is missing many periods of boiler status S.4 (i.e. ignited) and many of the short-lived peaks in flow temperature.

This begins to answer some of my questions from the last post. In particular, note that d.67 starts to count down when the boiler goes to state S.7, not when it goes to state S.8. However, I have noticed that it only does so when it is going to go to S.8 next (unless it decides it can re-fire first, as in the micro firing region). Somehow it must already know when it goes to S.7 that it is going to go to S.8 (which it certainly does not always do). The only example in this particular graph is at about 18:44, when it goes to S.7 but not then to S.8 - it did not start the d.67 countdown when it went to S.7, so it must have known it was going to do something different next.

As for which row of the d.2 table my boiler is using (a 438 installed in March 2011), it seems to be as shown in the table - at least when there is a requested flow temperature. See e.g. the second peak in each of the saw tooths, at about 17:30 and 18:12. The target flow temperature is 43C. If we're supposed to round down to the nearest multiple of 5, we should be using the 40C row. My d.2 is the default max of 20min, so the table gives a blocking time of 13min, which is what we see on the graph.

However, I do not understand what is happening on the first peak of each of the saw tooths (i.e. at about 17:16 and 17:58). This is at the end of a period of firing, when the requested flow temperature drops to zero (although there's no need for it to do so, which is why this is short firing rather than normal cycling). In both cases d.67 goes to 19min. This may be entirely unrelated to d.2. It was at a time when my Pump Delay Time on the VRC430f was set to the default of 15min - if had been set to OFF, I'm sure I would have seen a continuous burn here, without any short firing. But why did d.67 go to 19min? Is it related to the 15min setting of Pump Delay Time? I don't know.

What is happening in the micro firing region? The boiler very rapidly tries refiring, despite the d.67 countdown having barely begun. This is presumably because (once the tiny spurt of heat from the few seconds of firing has dissipated) the flow temperature is back at 27C, which must be low enough for the boiler to think it can have another go. But when it does, it immediately exceeds the target flow temperature and cuts off. I'm therefore still not convinced that increasing d.2 will help at all with micro firing, but I shall certainly try when the weather warms up again and micro firing reappears.

Frustratingly my vrDialog data does not appear to include the parameter that corresponds to d.41 (i.e. return temperature), which would enable me to see what the difference is between d.40 and d.41, despite the boiler being capable of displaying d.41 on its screen. The only candidate appears to be ReturnTemperature_DK.Temperature, but it always returns a reading of -1.8 (indicating it is non-functional, which is presumably what is meant by the next parameter, ReturnTemperature_DK.SensorState, returning a value of "cutoff" - contrast e.g. BMU_FlowTempOrVF_1.SensorState, which returns a value of "Ok").