d.2 is the minimum time that the boiler will wait after cycling off before cycling back on again. There is also a required minimum temperature drop. Once both the d.2 time and the temperature drop have been reached then the burners come back on if still required.
From observations and what's written in the manual, it's the MAX time (not minimum!) that the boiler will wait before cycling... There does appear to be some relation to return temp and/or requested flow temp (from the VRT392) but I've not quite worked out what yet.
Your numbers are way huge.
Yep, d.2 is currently set to 60
(whilst I observe & monitor). - d.1 (pump overrun) is now set at 15mins.
Note however that the pump, by default, will continue to circulate the entire time that the thermostat is calling for heat even when the burners are cycled off. Double check that factoid with your fancy new controller but I suspect it still applies.
Correct. I've noticed that anytime the CH is scheduled ON, the pump is running continuously, e.g. if the CH is scheduled to start at 07:00 and end at 19:00 the pump will be running for 12hrs solid. Whilst in 'analog' (modulating) mode, the VRT392 appears to work by ALWAYS requesting heat but at various flow temperatures, so even when the room stat = the actual room temp, strangely there is still a 'demand' for heat at the boiler.
e.g. With the actual room temp at 19.5C and the VRT392 set temp at 19C, it is still requesting 'heat' but with a flow temp of only 21C. If the actual flow temp is >21C (e.g. most of the time) the burner doesn't need to fire in this situation.
It seems to me that you may be running into the inevitable consequence of gadgets that tone down the flow temperature to try and glide smoothly up to the thermostat set temperature. To maintain a low flow temperature requires a very low burner output and most boilers simply can't do it.
Whilst I was watching yesterday, the sequence of 'short cycling' went something like this:
1) Actual room temp @ 19C, VRT392 set temp @ 19C
2) Flow temperature demand was 38C
3) ecoTEC plus 418 fires for approx 5 seconds, then stops as the flow temperature reaches 42C.
4) There is then a delay of approx 10-20 seconds before the burner refires for another 5 seconds, stopping when the flow temp reaches 42C
5) Boiler then goes into pump overrun for d.1 time limit (15mins)
6) Boiler then changes to anti-cycle mode (d.2) for varying lengths of time
edit: I'm guessing at 3,4, it was looking at the return temp!? It changes so quick I couldn't move to d.41 (return temp) in time to check
It would appear that the VRT392 trys to keep the same room temperature by varing the flow temp requested. This works well when there is a big gap between actual room temp and desired temp or when the room temp drops quickly e.g. it's very cold outside (all = higher flow temp demand). The short cycling appears to happen when there is a very low flow temp demand. The boiler fires, very quickly gets to set temp, then stops.
That to me says the VRT392 is requesting too small a heat demand, below the minimum the boiler can deliver i.e. It can't modulate down to a low enough temp (Note: The 418 can modulate down to 5kW, do (m)any other boilers go lower?)
So if you want to play that game you'll have to accept that the boiler is going to cycle and potentially at quite a short interval.
I was expecting to find some kind of setting to limit this behaviour in some way, perhaps having a minimum target temp limit? or something else.... Looking through the VRT430 manual, that does have a minimum target temp setting but it's not present on the VRT392
Given the above information, does anyone have any suggestions on how to cut down / eliminate the short cycling?
