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Noted this morning that CH is doing the same thing...although not as much. Burner clicked on - ran for 10minutes (all rads got hot) - clicked off and cycled down as boiler stat satisfied (flashed green for the alloted one min) - clicked back on due to external call for heat (from room stat) -...

Cheers guys. Just been chatting to heating bloke who still thinks it's a zone valve issue. Valve not fully opening causing restricted flow on the circuit, which in turn causes the boiler to "surge". Not convinced at the moment, but I'm not a professional.

Think you nailed it dilalio Very recently serviced boiler with a few new parts = significantly improved efficiency. CH not coming on at moment thanks to warmer weather = only cylinder now transferring that heat. Old cylinder not transferring heat as efficiently as boiler heats it = boiler...

Boiler - Potterton Suprima 50L, with upgraded PCB (recently serviced) Cylinder - Santon Premier Unvented - S config (x2 2 port zone valves)

Thanks guys..... Not sure of the exact temps as they are both mechanical knobs with no temp markings (helpful), but if the manuals are to be believed: Cylinder = circa 60C Boiler = circa 75C I have heating engineer coming on Thursday who's convinced its the zone valve/microswitch, but that...

Can failing thermostats work intermittently? I'm working on the assumption that thermostats are binary - they either work or they don't. However an intermittent fault on the CYL Stat would explain what I'm experiencing. I really don't thinks its: 1) Faulty boiler stat as that would impact CH...

Thought we had it - alas not. HW has been switched on at the programmer all evening with the higher temp set on boiler stat and no short cycling. Great!! Boiler just fired up after about 3+ hours of silence and unfortunately it's doing the same as before! Burner came on for 4-5 mins...

Thanks Harry. I've started to think the same thing. Not ever touched the CYL Stat (until today) since we moved into the house several years ago, however the boiler was serviced shorty before I noticed this issue. Looking at the boiler temp control its definitely been lowered (was set nearly...

Thanks. I don't actually use the timer for HW - at the programmer it's on 24/7 (or at least it was until this issue started - now I'm manually flicking it on and off as and when hot water is required) and its left to run from the CYL Stat. Will try the stat test though - next time I switch...

And that was my initial thought too, but if it was a sticking microswitch (as you say, fed by a permanent live feed), the switch would remain stuck, even after I cut the power to the stat. So, programmer would show as HW off, but the stuck switch would mean the end circuit is still closed and...

And that's my understand too. The programmer sends live to CYL Stat. If the stat is satisfied, nothing should happen - no heat required. When stat registers need for heat, stat completes circuit to energise HW zone valve - valve opens (for water to flow) and micro switch closes end circuit to...

Hello New to the forum so please be gentle I noticed a few days ago that our gas boiler was short cycling. Burner comes on for 1-2 mins, cycles down and shuts off.... Just for it to immediately get another call for heat and turn back on again. .. It does this over and over and never turns off...