Wiser Cycling

[Joseba4242]

I recently installed Drayton Wiser Kit 2. It's set up in traditional, non-OpenTerm mode with DHW and Heating for an S-Plan in the UK. It's set to normal model (no eco, no comfort).

It is heavily cycling despite the room temperature being consistently below target temperature. The attached picture shows it.

What causes this cycling? What can I do about it?

[David_SE]

Hi @Joseba4242 ,

Thanks for your post.

If you wish your boiler to cycle less often, you can configure it as an oil boiler and not a gas boiler in the Hub's settings.

Best regards

[tbug]

The Wiser system operates on a 10-minute cycle, calling for heat for a variable number of minutes at the start of each 10-minute cycle, depending on how much its algorithm thinks it needs to reach or maintain target temperatures.  

The problem I’ve found with this design is that it doesn’t take into account any delay periods imposed by the boiler.  For example, most boilers have an anti-cycling delay which prevents the boiler from firing up again within a defined number of minutes since it last fired.  The default for my boiler (a Vaillant ecotec plus) is 20 minutes.  Which would mean that no heat would be generated until the Wiser’s 3rd 10-minute cycle.  I’ve now reduced this anti-cycling delay to 2 minutes (the minimum allowed).

It would be worth seeing what your boiler is actually doing when Wiser calls for heat.  Is it in this anti-cycling mode?  My boiler also has an additional mysterious (and fixed) 5-minute delay whenever there is a call for heat, which also prevents Wiser from getting the heat it is expecting.  And I imagine there are other delay periods built into other boiler designs.

The pattern in your picture looks very much like what I would expect in such circumstances.  The room temperature is only slightly below target, so Wiser calls for just a few minutes of heat at each 10-minute interval.  If delays at the boiler prevent heat from being generated in those few minutes, the room temperature doesn’t rise, so Wiser gradually increases the length of time it calls for heat in subsequent cycles (as you can see in the picture) until it starts to have some effect.  

It would be interesting to know if this is the cause of your issue.  I’ve raised this issue (which seems like a fundamental design problem) with Schneider, but it’s all gone quiet since it was escalated to their advance team.

[David_SE · Accepted solution]

Indeed some boiler manufacturers have their own optimization features which sometimes prevent the boiler to fire immediately when there is a call for heat or doesn't stop the boiler immediately when the call for heat stops. 

Most of the time, the rational behind these features is to prevent keeping hot water into the boiler's circuit or to minimize the circulating pump start and stops.

[tbug]

Given that is the case, the question is why on earth would the Wiser hub be designed the way it is.  The concept of calling for heat for a certain number of minutes every 10 minutes makes no sense if the boiler isn’t going to act on that call due to its own internal delays.

My Vaillant ecotec boiler has a default anti-cycling delay of 20 minutes, which I’ve reduced to the minimum allowed (2 minutes). Like all Vaillant boilers (so they inform me), it also has a fixed, non-adjustable, 5 minute delay whenever there is a call for heat.  So the boiler is rarely delivering the heat that the Wiser hub thinks is necessary in any given 10 minute period.

The hub’s software really needs to be modified to be temperature-driven, not time-driven. As a quick temporary fix, there should at least be the option in the app settings to specify the boiler anti-cycling and other delays, and for the software to take these into account when determining how many minutes to call for heat - though this would still result in more deviation in room temperature compared to a temperature-driven model.