Suprema 50 blowing mains fuse

[jungtony]

Hi, this is my first post, hope someone can help.
Arrived home yesterday to a cold house, no lights on boiler and programmer dead.
Checked mains fuse to find it had blown. fitted new but boiler dead, checked pcb fuse, this had also blown,resolder all dry joints i could see, fitted new fuse and test, blew instantly. After much messing about, if the overheat thermostat is taken out of the circuit, everything stays ok, ie, pump will run, diverter lights up and boiler fan runs, except there is no ignition and the red led is flashing. How do you test the boiler o/h stat, using meter across spade terminals it reads short circuit, is this correct. Is it best to change the pcb with the latest upgrade kit.
Any help is much appreciated

 

[Baxi-Remeha]

o/h stat is a thermistor, so works on resistance, use a multi meter to check........

 

[ChrisR]

I had precisely the same situation yesterday! The insulation to earth on the overheat stat broke down at mains voltage but measures 20 Megohms+ on a multimeter.

The stat should be 0 Ohms when cool, opening at high temp.
I wouldn't expect the boiler to start with the stat not connected at all, trace your way throught the online fault finding chart.

On my boiler the pcb had been messing about for a while, with intermittent lockouts, so I changed the pcb as well as the stat.

 

[ChrisR]

I thought the OH stat was a switch. Jason ought to know though!

 

[Baxi-Remeha]

SORRY, i eat my word's. just read what i wrote, and its a load of tosh....(toomuch red wine with me dinna)

chris' ur right. the oh stat is a switch........basicly its a make or break switch. which means it might not be your oh stat blowing the fuse. when you have disconnected the oh stat, the boiler wont go past that point of ignition, therefore it could be anything after that point.

dose the fan move at all b4 the fuse blows.....

could be the pcb (email if you need one)

there again, it could be ur stat. without the boiler in front of us, were hard pushed to find out exacly what it is......

 

[jungtony]

Thanks guy's for the replies

ChrisR - The switch is closed, even with it connected and removed from its screwed location, fuse still blows. Where is the online fault finding chart ?

jasonvaughan - Yesterday with the o/h stat disconnected, fan was running at full speed, pump running, lights on diverter were on, room stat set over 25, EP2002 ch/hw lights on, this happened as soon as the fused spur was switched on, ie, no delay of any kind.
I've ordered an upgrade kit off Ebay yesterday, i think its you, as the names the same. going to fit new o/h stat and pcb and see what happens. Get it to me ASAP, cheers jungtony

 

[ChrisR]

Try Potterton Website! Or partsarena/baxi

 

[jungtony]

All sorted, new upgraded pcb arrived today in the post, fitted tonight along with new o/h stat, bingo, no blown mains fuse and all heat is restored

 

[ollski]

Just to say another thing to look for on suprimas is that a batch of flow switches were faulty about two years ago. They used to intermittently short out and blow fuses or trip RCD's.