Boiler -why does it fire up so long after demand?

olbas_oil

My system is Drayton Wiser trv's and hub, and a conventional Greenstar 8000 (s-Plan) separate zones for Heating and hot water).
My issue is that when I call for heat (and the heat element on the boiler starts flashing) there is a variable delay before the flame kicks in. Sometimes this can be as much as 4 minutes. The quickest has been about 20 seconds.

I think it is true to say that if I press on the Heating or Hot Water it is a direct call to the boiler, so I can ignore thermostat settings. (before I was using the app to create demand, that introduced more complexity, and the delays seemed longer). I think it is also true that the boiler itself doesn't distinguish between Heating/Hot Water; it's just that different zone valves operate

I think the length of the delay is fairly consistent between Heating/Hot water. When I was using the app the hot water seemed to come quicker/be more reliable, so I concluded that there might be a sticking valve. I've just replaced the head, and I'm being more patient, so I now think it does fire-up but with a long variable delay.

What happens in the boiler between a call for heat and the ignition, that could account for this?

grumbler

Because 'Preheat mode' is ON?

ThisIsWeird

grumbler said:

Because 'Preheat mode' is ON?

It isn't a combi.

O'Oil, I'm going to guess - but I don't know - that there's nothing actually amiss, but the Wiser and/or the boiler is going through a process of determining the demand, and responding in the most efficient manner. The boiler itself has some autonomy too, and will do things that it isn't being told to by the external controls. Eg, if it is told to turn off, it might choose to delay this as it needs to still shift some heated water inside it - it's called 'overrun'. Or, if told to come back on to top up the CH, it might delay firing up the burner as it knows it already has some hot water sitting inside it, and it wants to shift that out first.

It isn't always the case that if you 'tell' the CH to come on, it just 'does so'.

Wiser has 'Opentherm' capability, so if it's been wired correctly to your boiler, you can assume a lot more is going on in the calculation stakes too.

My much simpler Hive will still not always respond directly to a change in demand - it won't allow immediate on/off requests, for example, as it knows it ain't 'good' for the boiler. So, once it has fired up, it won't turn off again for a minute or so, even if I shout at it. So I'm guessing that the Wiser is doing even more than this.

You can do some checks, and it might indicate where the 'issue' (or non-issue) is.

For example, CH and DHW off. One person observing the motorised valves, and another at the boiler. Valve person uses App to turn on, say, the CH. The App says 'ON'; what does the valve do? Does it 'whirrr' right away? If so, that suggests the command from the Wiser to the valve is pretty instant.

Ok, does the valve whirrrr smoothly for 2-3 seconds, and end with am audible 'click'? If so, that suggest the valve is working just fine.

When the valve goes 'click', does the boiler immediately come to life? What does the boiler do - how does it respond?

That should show you where any 'delay' takes place - the App, the Wiser, the Valve, the Boiler.

Offski, and see how long the boiler takes to respond. Again, the Wiser should tell the valve to go off, and it should whirrrr equally smoothly back to rest, starting with that audible 'click' as the valve turns off the signal to the boiler. When the valve clicks-and-whirrrs off, what does the boiler do? I suspect it may carry on for a bit using its own built-0in control to provide a bit of overrun, until it has displaced all the heated water inside it.

Bear in mind that these tests aren't 'real world', as the boiler won't have had a chance to heat up fully, but hopefully you get the idea of the stages involved.

You can repeat this for DHW.

grumbler

olbas_oil said:

My issue is that when I call for heat (and the heat element on the boiler starts flashing) there is a variable delay before the flame kicks in.

I think, it's the pump that kicks in first. Then, when the pumped water temperature drops below the one set by the boiler thermostat, the flame kicks in.

Not sure about CH, but I remember that when my  HW cylinder thermostat was accidentally set to about the same temperature as the boiler and water in the cylinder was hot, the pump kept pumping without the flame. I increased the boiler temperature and this solved the problem.

tacpot12

The boiler will have an anti-cycling feature such that it might not fire for upto five minutes even if there is a demand for heat (this delay is configurable on modern boilers). 

FreeBear

ThisIsWeird said: Wiser has 'Opentherm' capability, so if it's been wired correctly to your boiler, you can assume a lot more is going on in the calculation stakes too.

Worcester Bosch Greenstar uses a proprietary EMS bus/protocol, not OpenTherm. Virtually the same functionality, but that still wouldn't account for any delay. Might pay to have a chat with Worcester Bosch technical support, but they may not talk to you unless you're a registered Gas Safe engineer.

Lorian

Does the radiator symbol in the top left on the 8000 start flashing within seconds of you generating the demand?

If it does the delay is in the boiler

If it doesn't the delay is in the controller/valves.