Potterton Suprima (basic model):

The original Potterton Suprima is not marked "Suprima Original" or “Suprima basic”, just plain "Suprima". Understandable as at the time of launch they had no idea if there would be re-designs and new versions. As it happens it was a very successful boiler. Unusually simple but even so, a growing number of plumbers are telling their customers it's old, inefficient and needs replacing. Not the case in my personal opinion as although it might be as old as 1997, the fuel efficiency is in the order of 78% and all spare parts are freely available.

Here are the common problems, in order of likelihood:

1) Control board failure

This only applies to the early version control boards. (Not many of these left now, most have been upgraded to the new version made by Siemens.) You get a solid red light showing on the front panel and the re-set button needs to be pressed. Although the manual says this means the boiler overheated, it usually didn't and the control board is beginning to fail. The boiler will work again for a random length of time then lock out again with the same solid red light symptom, and the button will need pressing again. This was such a common problem (and Potterton denied it) that this boiler featured on BBC "Watchdog" programmes and eventually Potterton redesigned and marketed a completely different control board. Most Suprimas have now had this new control board fitted. The 'red light' problem is not so common these days and only occurs of Suprimas with the old style control board.

The new Siemens control board is also perfectly capable of failure but in different ways to above. I’ve encountered Siemens boards which behave randomly then work correctly for a while after pressing the re-set button. This correct operation of the boiler after re-setting tends to be temporary and if further re-sets are required, another new Siemens board will be needed. The second way in which the Siemens board can fail is the boiler will heat to a random and different temperature each time it lights. Users with conventional central heating systems often don’t notice this at all but when a boiler with this fault is connected to a thermal store e.g. a Gledhill Boilermate 2000, the result will present as random, intermittent and unexplained central heating failure. The cause being the store randomly being heated to the adequate temp for the thermal store control board to permit heating to come on, or not, depending on each cycle temperature reached by the Suprima. 

A third mode of failure of the Siemens board is where the ignition sequence starts and either stops before the burner ignites, or the burner shuts down shortly after ignition and before the boiler has properly heated up. Sometimes confused with thermistor failure.

2) Flue Fan Failure

As with most fanned-draught boilers, flue fan wears out and fails occasionally.  Combustion gas is blown out of the boiler with an electric fan - a safety-critical component. If the fan fails to start, the safety device which tests to see if the fan is running before allowing the boiler to light. To the user the boiler will stay silent (or possibly hum quietly) instead of bursting into life as normal when a demand for heat is turned on. The fan is still freely available as a spare part and is fairly straightforward to install. Early fan failure can be caused by the fan outlet seal becoming embrittled and falling apart. This allows hot flue gas to escape back into the boiler which degrades the motor bearings. One problem that crops up with fan replacement is there are lots of sizes of Suprima each needing a slightly different fan (11 different fans are listed actually), and the exact model number and version should be on the data label above the control knob, behind the tall vertical hinged door. When the PCB and chassis are replaced, the installer is supposed to copy this data onto to the new blank data label on the new PCB chassis from the old label but but often this is not done as in the photo here. Note the blank spaces where the model, serial number, size etc are supposed to have been written in. When we don’t have this data, identifying which fan to fit can be difficult. Sometimes there is a data label on the fan motor itself which identifies the fan. The boiler model can then be reverse-looked up from this, the manual found on line and the data plate filled in from the manual.

3) Air Pressure Switch failure

The safety device which tests the fan for correct operation is the air pressure switch. These are know to fail occasionally and the result is the fan runs, the boiler sounds for all the world as though it is about to light, but no ignition ever occurs. A new air pressure switch is usually required.

4) Solenoid gas valve failure.

The symptoms appear to be the same as air pressure switch failure above (i.e. the fan runs, the boiler seems about to light but never does). The difference is that air pressure switch has detected the fan and power is being sent to the gas control valve, but the progression to lighting the main burner never happens. The solenoid in the gas valve has usually failed and a new gas valve is needed. 

8) Thermistor failure

The thermistor is a semiconductor device that senses the temperature of the flow pipe from the heat exchanger so the control board can turn the gas flames on and off to maintain a broadly steady flow temperature. The resistance of the thermistor rises and falls with temperature and the control board reads this resistance and uses the information to work out when to turn the gas flames on and off. Its quite common for the thermistor to stop changing resistance linearly which confuses the control board, leading the the gas flames turning OFF at too low a temperature. The boiler still lights and runs briefly but turns OFF at too low a temperature. Quite perplexing to the user!

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If you'd like me to fix your Suprima, call or text me on 07866 766364.

Mike the Boilerman

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