High Limit Trips - looking for ideas please

[Nick C]

Hi there, I'm looking to book an engineer for a general service and check of flue gases etc but would like if I can NOT to have to present him with an intermittent snag namely high limit trips on a Worcester Greenstar Heatslave 25/32 oil combi boiler, if anyone is willing to share their expertise please.

The boiler has been doing this once every few weeks for a long time. It always starts as soon as the high limit is reset. I cannot categorically say whether it is tripping at the beginning middle or end of a CH or HW demand cycle as its in the garage, but I THINK it is tripping at the beginning of a HW cycle, based on the fact that whenever one of the family report no HW, the heating will have been operating normally at that point, and they'll have only tepid water from the store. The only other related symptom I can think of is that very occasionally like once every couple of months the hot water feels unusually hot (but no trip).

Unrelated (I think) things I've done just to help the joining of dots:
1. Recently we had a couple of burner lockouts (Riello RDB 2.2). It hasn't been serviced for a year so I changed the nozzle, checked electrode gaps, cleaned flame sensor, pulled the baffles and decoked. Still had a further lockout, so changed the coil and starter cap as they were easy and cheap to do. Noticed that someone when checking flue gases has lost the original plug because the brass one that's in there now is (not very well) secured with ptfe and doesn't have any locating lugs. How critical is a tight seal there as an aside whilst I'm boring you?? No further lockouts seen in the couple of weeks since.
2. I did have a weird period 3 or 4 months ago when rads where hot when HW heating, checked the diverter valve, disassembled the head, didn't find any explanation but it is possible I had put it into the A+B manual setting myself inadvertently before the snag.
3. The high limit is a popout plastic button which should protrude by 4 or 5mm but for the last couple of years doesn't not really push out at all when tripping, it pops out by about 0.5mm, but it is still doing its job and is reset easily.

Sorry this is a bit long, thanks for reading this - just trying to give everything I can. I think there are a number of things that could be causing this, but if there is a simple diagnostic step I can do like changing the stat or something that would make the engineer's life easier please educate me

 

[oilhead]

If the overheat stat is only 'popping out' by 0.5mm, I think I'd start with that. High limit trips on HW cycles are not unknown especially if short bursts are commonly used frequently and the HW stat is set high. Try reducing the DHW control stat.

 

[Nick C]

Thanks very much for the reply Oilhead, really appreciated. Just to clarify how I read your advice: I've just gone and reduced DHW rotary control from approx 3.5 out of 6 to around 2.75. Are you also suggesting replacing the overheat stat's electrical cutout as it is behaving slightly oddly by not popping out cleanly?

Update: don't worry I've ordered a replacement anyway. One question though: all the various copper leads for the various stats are a birds nest of bends and tangles which touch all sorts of hot components on the top of the boiler. I assumed because they're copper their job is to conduct heat back to the stat where some bi-metallic strip does its thing. Could the birds nest touching other hot surfaces be a contributor to this tripping symptom or is the jacket around the burn chamber the hottest point and therefore touching stuff on the top of the boiler on the way back to the overheat stat is irrelevant and I'm barking up the wrong tree?

Will replace it in a couple of days and route its wire carefully when I do. Thanks again

 

[Terrywookfit]

The capillaries should only be touching where they are actually monitoring the temp.
Check the control stats cut off temp first as the limit stat may be working as intended.

 

[Nick C]

Good point Terry and thanks ref my birds nests

 

[Nick C]

Hmmm the plot thickens. We are now definitely getting DHW that is way too hot vs what the DHW is set to. So the high limit (replaced) is doing its job. I changed both DHW and CH stats last night, on the basis that when the condition exists I could not hear the teltale click of switch operation on turning either of them but testing both on the bench they seem to be fine. I now realise that when the boiler is super hot, I won't get a click out either stat because they have long since opened the switch and are staying open no matter what I set the rotary controls to. My question is where to go next... why are the rotary controlled stats being intermittently ignored? I can stand there and control the boiler with them as normal, triggering on off according to what demand I set. Then at some point later on, it will ignore the stat completely and heat to super hot until high limit saves us.