Potterton Suprima no hot water unless central heating on too

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I have a Potterton Suprima 60L boiler. When the HW (only) comes on, the gas ignites. After about a minute, the red LED comes on with green LED flashing (to indicate there is no longer a demand for heat) and gas is extinguished. Shortly after that the red LED goes out with green still flashing (so demand for heat). After about 3-4 mins the boiler ignites and repeats the process.

Needless to say it doesn't manage to generate much hot water in the process. The thermostat is set to about half-way (although I've tried it in min/max combination also). I'm not convinced it is necessarily the PCB, because I'm not getting the red-flashing lock-out that seems to be common with these boilers.

If HW is on together with CH, then everything seems to work much better (stayed on for approx 15 mins after 25 mins of just HW cycling) and I get hot water/radiators (so I don't think it is a problem with the hot water tank). I had a heating engineer come to do a service and he reckons that everything's set-up OK (he was the one who put me on to the PCB problems).

Any ideas? Is it worth replacing the thermistor? Surely the motorised valve defaults to HW, so it can't be that? Any other diagnostics I can do?
 
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I took the PCB out and had a look at it. Couldn't spot any cracked solder joints (but didn't have a magnifying glass to hand...).
 
turn your boiler stat to max then check to see if the stat on the side of your hot water cylinder is set to 60 and see what happens
 
Thanks for the suggestion, but unfortunately it didn't help (except that maybe the boiler stays on for a minute or so longer before claiming there is no longer a demand for heat).
 
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then I would check the cylinder stat , it could have gone out of calibration
 
OK, how do I go about doing that?

Could it be that far out of whack? The cylinder is barely getting warm.
 
Could probably borrow a multimeter from work. Am I looking at an impedance measurement?
 
nope your looking to see when the stat is satisfied, ie if the cylinder is hot enough when the stat changes over from calling to satisfied.
 
Ok, so what am I expecting? Nothing seems to change.

The cable from the stat has three wires (L/N/E colouring).
* Measuring L wrt N reads 0V, this doesn't change from boiler ignited to temp satisfied.
* Measuring L wrt E reads 240V, again doesn't change.

From what I can gather, the Neutral connects to the boiler and actuator (MZV?), Live connects to the control panel and Earth to common ground.
 
Try linking the wires on the cylinder stat Common and terminal 1 (connects back to MV orange)

See if the boiler stays on longer
Just watch fingers
 
Try linking the wires on the cylinder stat Common and terminal 1 (connects back to MV orange)

If I understand correctly, these are linked. The neutral from cylinder stat (if that's what you mean by common), is already linked to boiler and MV orange.
 
the colours dont relate to live/neutral/earth ,one of the cables is common from the programmer, one is connected to the grey (hot water off) and the other is connected to the orange (boiler on) you need to connect com to 1 and see if the boiler stays on longer, this is bypassing the stat and is only to prove the stat is faulty, it cannot be left like this.
 
Thanks, will take a closer look at the wiring and try shorting common and terminal 1 (probably tomorrow).

Will update with progress - no doubt as a pretext for more help... :)
 
Hi, thanks for tips. Decided it'd be best if I had a read about the wiring (surprised I never thought to do that before...!).

So, I guess what you're suggesting is to link orange to programmer HW ON output. Thus forcing orange (what the system thinks is coming from cyl stat ON) always high if programmer wants HW.

Does that mean I have to disconnect grey from the OFF output of the cyl stat (but leave it connected to programmer HW OFF output)? Otherwise, surely I'm shorting orange and grey through the cyl stat (which doesn't seem good).

EDIT: Now that I have a vague idea of what I'm measuring, I measured again. Grey never went high when the boiler switched off (and orange was still 240V).

mid-position-zone-valve-external-wiring.png

Thanks to gasheating.co.uk
 

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