Boiler cycling query

[Johnytuono]

Ok I have a WB 30 Si compact
The minimum modulation is just shy of 7kw. My rads are about 9.5 kw in total.
I can adjust anti cycle time or temp hysteresis via the menu.
I know that unless the outside temps are 0 and below the boiler can't modulate low enough to keep running.
What I'm unsure about, as I've read about both arguments (r.e. long or shorter cycles) is what would be optimal.
Am I better off having a longer 'off' time between firing so the flow temp eventually goes right down enabling another long burn (if room temps is obviously not satisfied) or am I better having an 'off' time that enables the boiler to burn for a few minutes (say 7 minutes(ish).
I'm running 45c and what I'm trying to achieve is a steady temp for most of the day rather than hitting room stat and then the room cooling down to the point when you actually 'feel the pinch). I can see the logic for a long off time if I want to satisfy the roomstat in the next 'hit'. Bare in mind I can control the anti cycle time or temp hysteresis to achieve this.
All rads are balanced and show about an 8c difference (at the rads) at this flow temp but no matter what I do, when 45c flow is running (and modulating) the return at the boiler only shows about a 4 to 5 c difference eventually, obviously it is more difference when coming up to the flow temp.
Thoughts please as I am a little confused.

 

[Johntheo5]

Boiler efficiency vs comfort?

Operational Time
per Cycle Base Efficiency Gross Loss
Secs % %
3600 85.9 0
180 84.4 1.5
120 83.6 2.3
60 81.8 4.1
30 79.1 6.8
10 74.1 11.8

 

[Johnytuono]

Boiler efficiency vs comfort?

Operational Time
per Cycle Base Efficiency Gross Loss
Secs % %
3600 85.9 0
180 84.4 1.5
120 83.6 2.3
60 81.8 4.1
30 79.1 6.8
10 74.1 11.8

So which would be an 'average' run time John. If I used the 3600 secs (60 mins) then surely the rads would be too cool and then wouldn't the boiler be firing at a high level again (as it does first thing in the morning, the first burn is very long) and therefore using a high volume of gas. I really want to try trickling along as we definitely feel the difference with a much higher flow temp (i.e hot, overshoot, then feeling 'cold')
My rads are definitely oversized btw (especially where the room stat is, the bedroom ones are smaller but we don't like it too hot anyway in there).
Are you basically saying anything over 3 mins run time is fine?

 

[Johntheo5]

A run time of 3600 decs is bordering on constant running, gas fired boilers have a very small HEX volume of ~ 2L or so, so will refire in a few minutes at most, not much effect on room temperature/comfort, if the burner off time (assuming roomstat not satisfied) is say even 3 minutes, 180 secs then the cycle time is 3600+180, 3780 secs, and the % burner run time is, 3600/3780, 95.24%. 180 seconds on might result in a cycle time of 50%, one reason for loss of efficiency with gas boilers is possibly because I think the burner does a post purge every time in stops firing, your boiler has a minimum output of 7.0kW so a 50% burner on time might reflect a heat demand of ~ 3.5kW and so on, I would think most gas boilers even ones with a fixed anti cycle time will have a minimum anticycle time of 3 minutes or so.

 

[Johnytuono]

So, with the 'off' times, basically it needs to be long enough so the radiators don't cool down (too much) and this will give a longer burn time until the room stat is reached (eventually).
Isn't this a kind of way of range rating ? (as I can't do that with mine)

 

[Johntheo5]

Not quite range rating as this only affects the maximum boiler output but will result in a longer run time, also the minimum Off time has to be sufficient to ensure that the boiler flow temperature doesn't exceed the target temperature by more than a degree or two as the burner will cut out normally at target temp+5C, most gas boilers fire up at ~ 60% of their max output (whether range rated or not) and then modulate down, some more rapidly than others which is good.

 

[Johnytuono]

Very informative, thank you John.
It's really trying to find the happy medium.
When I was trying the flow at 55c (which still should have been fairly efficient), the gas use didn't reflect that, I realised the temp hysteresis was set at 6 C so when the boiler hit set point (+4 on mine) it was only off for less than 2 mins (obviously) and then going through the cycling process again and again too quickly (it needed to be at least 9C at this flow temp for a longer off time and hence a longer next burn).

 

[Johnytuono]

Don't forget the anti cycle time on mine includes the total time of a full cycle (on/off) so, for me, I prefer to use the hysteresis.
i.e. after the initial start up (from cold) even an anti cycle time of 45 minutes would be used up (probably).

 

[Johntheo5]

I thought the anti cycle time would be as stated below, the amount of time between stops and starts?.

The hysteresis function is a nice touch, as you can match it to the boiler flow temperature to prevent it exceeding the target temp after refiring, presume which ever one is satisfied first "wins"?

What target temp and hysteresis temp drop are you using now?.

 

[desktop987456]

Have you tried adjusting the pump speed? I have an external pump on my heat only boiler and at lower pump speed the burn time is lengthy.

 

[Johnytuono]

Have you tried adjusting the pump speed? I have an external pump on my heat only boiler and at lower pump speed the burn time is lengthy.

Yes, but the bigger rads don't get satisfied even with balancing the smaller rads.

 

[Johnytuono]

I thought the anti cycle time would be as stated below, the amount of time between stops and starts?.

The hysteresis function is a nice touch, as you can match it to the boiler flow temperature to prevent it exceeding the target temp after refiring, presume which ever one is satisfied first "wins"?

What target temp and hysteresis temp drop are you using now?.


View attachment 397631

No John, whatever is the longer 'wins'. The ACT would only start (code 202) if the ACT total time isn't satisfied by the time the hysteresis drops (i.e the ACT is then obviously longer).
Say I set an ACT time of 10 mins, if the boiler fires for say 3 mins then it has 7 minutes remaining BUT if I had the hysteresis say at a ridiculously low figure 15k then the boiler wouldn't retire until that temp drop is achieved.
Running at flow 45c now, after the initial longer morning 'burn', I'm getting off times of about 9 mins and an on time of 13 mins at a temp hysteresis of 7k (so thats about 60% boiler output) . When I had it at 6k the off time was about 7 mins and on of about 11 half minutes. It both works out about 60% boiler output, give or take. Room stat is satisfied now at this flow temp (albeit slowly), so currently it is off.

 

[Johntheo5]

Find a bit difficult to understand this, you set 2.3B to 10 seconds, the boiler may run for only 3 minutes so this leaves minutes of of the set 10 minutes remaining, but this is looking at the boiler run time?, seems strange to me, so after the hysterisis time has elapsed and the boiler refires and just say (if the hysterisis was set to achieve this), the boiler again refired for 3 minutes, what happens then?, is there now 4 minutes left of "2.3B" or what? and what happens on each succesive refiring?, (assuming roomstat not satisfied) or does it start from scratch again?

 

[Johnytuono]

It starts from scratch again.
It seems very different than say, the Vaillant anti cycle mode.

If I set 10 mines ACT and the boiler runs for longer than this (which it obviously does from the first morning start) then no ACT time is left therefore it looks at the hysteresis, which in this case is longer. Boiler then cools to say 39 C (Hysteresis 6k, Flow 45C), it then refires, if it then runs for 7 minutes, then 3 mins ACT is remaining, the Hysteresis drop is a longer time @6k drop, so the boiler stays off until that drop is reached.
I emailed WB and this is the reply:
Dear John

The anti-cycle time is to protect against multiple burner start ups and so is calculated from the time in which the burner starts. If the burner is lit for a longer duration than the anti-cycle time, then this element has been satisfied when the burner shuts down and so the central heating hysteresis based on temperature will determine when the burner re-activates.

The recommended settings would be based on system requirements which in most cases would deem that the boiler should not cycle during a demand which would extend the demand period and not satisfy requirements. The boiler should modulate to maintain the required flow temperature.

Poor circulation or too greater heat output from the appliance could cause the boiler to cycle as the temperature is not being dispersed.

Best regards,

Technical Support
(HC/SGB-I2R-TC)

 

[Johntheo5]

A little clearer now, if you set the hysteresis at something daft like 2C, will the boiler just stay OFF for the (10 minute) ACT time or what?