Dear JohnTalking generally,
"Normal" PI control integrates the (proportional) error over given time, for example if the error was looking for 10% valve opening then with a 1 minute integral time the valve opening will increase by another 10% in 1 minute (if the error remains constant) eventually and would be 100% open after say 9 minutes, normally the error is reducing as the valve opening increases so the integral add on progressively reduces until there is no error and the valve will stop opening, eventually the measured value may be > the SP value (integral overshoot) so the integral effect starts reducing the valve opening until eventually the SP value is achieved.
EH must clearly interrupt this integral effect when the boiler is not (EH) switched on otherwise all the HR92s would be 100% open irrespective of their room temperatures after a relatively short period and certainly after a overnight boiler shutdown.
If, as you think, the EH does not increase/decrease the valve opening with integral effect until there is a change in temperature after a certain time then that is not true PI control so maybe this is why its called timed PI control and doesn't increase/decrease the valve opening until each burst of run time unless the temperature has actually changed.
Or maybe the EH integral time is just the on run time/cycle.
Still a bit confusing to me IF it doesn't increase the valve opening to 100% say overnight whatever way I look at it.
The HR92 units only talk to the Evohome ~every 4 minutes (so best case at 1 min on, 9 off is two updates per cycle).
Even after many of the 10 minute cycles the Evohome does not appear to open the valves further (I do not hear them change) to allow more heat in but I suspect give many many more cycles it would (certainly when in proportional working you can hear them being closed as it gets nearer the set temperature).
When just above set point the demand is zero so, as long as nothing else demanding it, the boiler remains off. It appears that when drops a bit it will cycle the boiler to try and "top up" the heat a bit (why its default is 1 minute on time) but in my case the slow boiler start up negates that, along with the fact that the Evohome does not open the HR92 far enough (yet)
I suspect that it will alter its operation as it learns (or I hope so) and will settle down but unfortunately to stop it cycling every 10 minutes for hours on end I have had to set my min on time to 4 minutes (so 6 minutes off). On another forum a user had to set his Worcester boiler to 5 minutes min on time for his to work.
Regards
Colin