Potterton 100 not turning off

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I've just replaced the wax capsule head as it was leaking. Now the hot water works fine but the central heating wont turn off. Put it on constant and it just fires up for 20+minutes. Any idea what could be going on here?
 
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Why do think it should have shut off..if the heating output has been range rated (matched to the building heat loss) it could take a while to come up to temperature.
When you first turn on the heating the boiler should fire up for a minute or so...then the burner should shut down for a short time and then fire up again.
 
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The boiler fires up at a minimum gas and then shoots to full gas for that long (20mins+). It was short cycling before (possibly related to the wax capsule head). The CH HW outlet and CH return are very hot after 20mins and the pressure goes up to around 2.5 bar. I'm concerned something may be sticking.

I was functioning OK before except a dripping wax capsule head and slightly short cycling.
 
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The frost thermostat is usually open circuit, if it gets gold, it makes a circuit and brings the boiler on until it gets hot enough, and then goes open circuit again, and the boiler will shut off. I have had them go faulty and go close circuit so the boiler stays on. Removing a wire will ensure it is open circuit, if it still stays on, then it is not the problem.
 
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Ah ok. Makes sense. Just tried it out and don't think that's it. It was hot from before, ran for 10 minutes and didnt cut out. Return pipe scorching hot.
 

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Ah ok. Makes sense. Just tried it out and don't think that's it. It was hot from before, ran for 10 minutes and didnt cut out. Return pipe scorching hot.

initial ignition and burner heats the primary water that circulates within the boiler. Pump will be running and flame size will adjust or go out to suit. The wax capsule you replaced, it’s spindle is ejected which causes the diverter valve to open to the heating circuit. Flame will adjust to suit. Check the pump is running. Return should be cold and flow red hot
 
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The solder joints on the pcb are the worst in the industry....every connector pin, relay pin, high watt resistor joint needs reflowing...then see if you've got a problem.
 
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Yes I've heard that, was one of the first things I tried to sort out the short cycling. Replaced the PCB and replaced the pump as there was poor flow in the far rads.
 
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The expansion vessel needs recharging...pressure too high when rads hot.
The heat is going into the rads..it can't disappear. Give it time and the boiler should modulate down or shutdown on overheat.
It's always a good idea to set the output to match the heat load...set the spindle on the pcb to an appropriate setting...minimum for flats and 1/2 way for larger homes...unlikely to ever need it at max given it's a combi.
You're better off junking it. It was a disaster the day it was launched around 1996 and most were ripped out within 5 years....soo many design faults.
 
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The expansion vessel needs recharging...pressure too high when rads hot.
The heat is going into the rads..it can't disappear. Give it time and the boiler should modulate down or shutdown on overheat.
It's always a good idea to set the output to match the heat load...set the spindle on the pcb to an appropriate setting...minimum for flats and 1/2 way for larger homes...unlikely to ever need it at max given it's a combi.
You're better off junking it. It was a disaster the day it was launched around 1996 and most were ripped out within 5 years....soo many design faults.

Ah ok, what's the thinking behind that? And you mean the expansion vessel on the hot water system?
 
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The expansion vessel behind the boiler.
The boilers energy is going into the radiators...the burner will only modulate down once the target temperature set on the front panel is approached.
I'm not convinced yet you have a problem.
You do have a room thermostat in an area with a radiator with fixed valves ie. you don't adjust them.
 
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