Evohome continually bringing boiler on for one minute then off for 10 minutes

[colin_]

Talking 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.

Dear John

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

 

[John.g]

If it doesn't open the valve more unless the temperature changes then it is only acting in proportional mode, it should, IMO, open the valve even if only very vey slightly more after each cycle as long as there is a "error", room temperature control is a very slow loop so the 4 minute scan time wouldn't bother me too much.
Or else it should increase the run time/cycle even if its set to a minimum of 1 minute.
Maybe this EH control is actually only TP and not TPI.
You mentioned that it was taking hours on on/off cycling to get the temperature up, one would surely expect the valve opening to be very substantial after this period, its as if the EH is assuming that the boiler temperature is at its setpoint for the duration of the on time, the nearest that this is achievable is with a oil fired boiler which fires at 100% all the time or if the water is being supplied from a stored supply like a buffer/store.

 

[colin_]

If it doesn't open the valve more unless the temperature changes then it is only acting in proportional mode, it should, IMO, open the valve even if only very vey slightly more after each cycle as long as there is a "error", room temperature control is a very slow loop so the 4 minute scan time wouldn't bother me too much.
Or else it should increase the run time/cycle even if its set to a minimum of 1 minute.
Maybe this EH control is actually only TP and not TPI.
You mentioned that it was taking hours on on/off cycling to get the temperature up, one would surely expect the valve opening to be very substantial after this period, its as if the EH is assuming that the boiler temperature is at its setpoint for the duration of the on time, the nearest that this is achievable is with a oil fired boiler which fires at 100% all the time or if the water is being supplied from a stored supply like a buffer/store.

Dear John

When in proportional mode it is cooler than about 1.5 degrees under set point and it opens the valves fully.

In TPI mode it is trying to boost a little quite often I think.

I will see if I can hear the valves moving tonight when it cools

Reagrds
Colin

 

[John.g]

So, in the morning, after a 8 hour shutdown and if one assumes a system with 3 HR92s with differential temps of 0.5C, 0.75C and 1.0C, will the respective valve openings be 33%, 50% & 67% or when the system was shut down with all 3 valves say around 22%/25% (at SP) open, do they start at these settings in the morning.

 

[colin_]

So, in the morning, after a 8 hour shutdown and if one assumes a system with 3 HR92s with differential temps of 0.5C, 0.75C and 1.0C, will the respective valve openings be 33%, 50% & 67% or when the system was shut down with all 3 valves say around 22%/25% (at SP) open, do they start at these settings in the morning.

Dear John

I will need to take a look in the morning but the % reported on Evohome is not the same as what the HR92 report on themselves for how open they are, so Evohome masks and refines the figures somewhat.

Regards
Colin

 

[Jez JF Cunningham]

It is a Worcester Greenstar 8000 Life condensing boiler (GR83000iW 30S).

The old zone valves have all been removed and it is using the Evohome wireless relay box.

I have emailed "[email protected]" asking if it is normal operation and I am awaiting a reply.

I have the multiple HR92s set for single room. There seems to be confusion on multi room operation as the Honewyell website says you can but then says it needs to be set to heating type "Zone valve" to do so (rather than "radiator valve") but when I queried Resideo they said set it to "radiator valve" and it would be ok to set to mutli room but an installer has spoken directly with Honewell and they have been told that I cannot set my configuration to multiroom ....

Thanks
Colin

I have the same boiler as a combi for hot water (always on) and heating, managed by the w-b smart thermostat.
During the summer, while heat was off, the boiler would cycle itself for a minute (showing about 55 on the temp display) and then sit quietly for an hour or so.
I called technical help and they said “it was normal- it’s avoiding water getting stale “
Still seems wrong to be doing it so often.

 

[Deleted member 123817]

Are you sure it wasn't DHW Preheat which keeps the water hot so that it is hot faster at the shower/hot tap than it would with no preheat. You should be able to turn this function off if not required.