Min Burner Re-igntion Period 30 secs v 3 minutes

[GoodDIYjob]

I've got a 10 year old combi boiler and for the last 3 years have used a Honeywell CM927 programmable thermostat, which, to be fair, does keep the room at a pretty constant temp.

I've read quite a few posts on the stat and understand a bit about the algorithm and how it fires to achieve the set point. And obviously, without weather comp, it fires either more frequently depending on how far off set point, to get the rads to dissipate just enough heat to achieve the set point.

However, what is pretty clear is the boiler obviously cycles a lot more than the case with the old conventional stat. Especially as it nears the set point. The boiler is pretty oversized being a combi, 31kwh output and rads only about 9kwh at most and the boiler only modulates to min 12.7kwh.

The defaults on the stat have been left as is, so the stat allows 6 cycles per hour (1 every 10 min) and ,min 1 min firing time.

Sometimes, when it is really cold in the morning, you can hear the boiler momentarily shut down after 10min and immediately like 1 second delay, come back on again so firing time obviously being calced at 10 min but it swithces on off and immediately back on.. But If the boiler had already been running and burner just stopped firing it will immediately provoke the boiler to refire after cycling on and off which seems a bit excessive and contrary to what the boiler would of been doing had it just been left uninterupted.

The boiler has a dip switch setting for min burner re-ignition period. It is selectable between 30 sec or 3 min via a dip switch.

So, to my queastion, Is there any value on choosing the higher value of 3 min? Would this make any difference with the stat momentary switch off and immediate switch ons?

As said, the boiler is 10 plus years, we have lived here 4.5 years. It has been fairly reliable in the scheme of things but has required a diverter valve group set, (understandable) and an ignition pcb. Wondering if the repeated firing cycles is not helping the latter?

Thanks.

 

[scoobydo123]

The two logics are clashing

 

[GoodDIYjob]

Yes, the fight for control. It is just a bit strange that initially it will run straight through leaving the boiler on, and the boiler choosing when to fire to keep the water temp within range, as obviously the room temp is a long way off the set point on the stat. That makes sense.

After about 20-25 minutes it does this stupid cycle where a few times it literally turns the boiler off and then on again the very next second. It makes more sense when eventually it turns it off for a minute or two or longer as it gets closer. But 1 second. It just confuses the boiler which even if just been firing then usually re-fires after the cycle.

Anyway, re the re-ignition dip switch - any point trying 3 minutes rather than 30 second setting?

 

[ianmcd]

no harm in trying to see if it makes any difference got a feeling it is actually a boiler fault though, which boiler do you have ?

 

[GoodDIYjob]

no harm in trying to see if it makes any difference got a feeling it is actually a boiler fault though, which boiler do you have ?

Yes, thanks I might give it a go.

I guess a fault is possible but it did not do it with the old school stat and from what I can remember it started straight away with the CM927. And the CM927 does control the room temp well, and once near set point it does eventually turn the boiler off for several minutes of the 10 min cycle. The one second on / off cycles are strange. But they seems to co-incide with a 10 min interval apart which is also why I assumed the stat. The boiler is set to always on and the stat connections [edit: relay box connections] across the thermostat connections in the boiler.

Looking at documentation, the Boiler it is closer to 13 years. Not generally well liked on these forums I know but has been pretty serviceable. It has had services since I've owned it plus the repairs I mentioned. Biasi Garda M90F 32s.

 

[GoodDIYjob]

no harm in trying to see if it makes any difference got a feeling it is actually a boiler fault though, which boiler do you have ?

Just to close this discussion out and post with some resolution.

ianmcd was correct. There was also a fault with the boiler. It became more noticeable in cold weather. The burner would light but only stay lit for 12 seconds, then as the boiler flow temps continued to decrease, it would try again after another 3 min ignition delay. Sometimes it would go through this 3 or more times, and sometimes the boiler would momentarily shut down, pump stop and restart with the burner igniting again. It would only do this after being left overnight off (standby).

After 6 visits (long story), the control pcb was replaced yesterday (it is a split board, the ignition board had already been replaced 2 yrs ago). And the problem has seemed to go away and the control for DHW is a lot more responsive it almost did nothing before.

Bizzarely, the flame out fault, would always right itself in the end and operate through the rest of the day ok. This did have the engineer a bit rattled.

I'm now wondering if lowering the boiler max output on CH (i.e. range limiting on CH) would help by running the boiler with less burner cycling, as it is way oversized for the radiators in our house which are probably 9kw at most. But I'll post another thread about that, with some info on initial boiler firing time and burner cycles that I timed this morning.

There is not much I can do about the CM927 on / off cycles approaching set point, as that is how it controls the temp and it does do it well in terms of keeping room at temp, unless I was to go to another boiler with some sort of flow temp control process, which there presently is not the funds for unfortunately.

Thanks for replies.

 

[snb]

we have this problem with oil boilers which are fixed output. changing the settings to 5min on period and 3 cycles will reduce your cycling dramatically and you won't notice any heating losses.

 

[GoodDIYjob]

we have this problem with oil boilers which are fixed output. changing the settings to 5min on period and 3 cycles will reduce your cycling dramatically and you won't notice any heating loses.

Yes, thanks. I read some old posts on here suggesting that so I have altered it to 3 cycles per hour and 4 min I think. It does seem to be better. As the boiler is old now and the 1 min cycle time seems ridiculous plus there is only about 10 sec pump over run so when it was firing for 1 min there was barely enough time for any of the heat generated to circulate.

Thanks