A difficult Potterton Suprima repair in south east London

One of the subjects that heating engineers can argue over til the cows come home without ever coming to a conclusion, is the use of modern, mostly digital equipment for boiler repair and maintenance. Some insist on using tools like flue gas analysers for every repair and every service whilst others only get them out when there is absolutely no other way of doing the job. Strangely enough, it doesn’t seem to matter much whether you do mostly service work in the heart of south London, or mainly repairs out in the country side, nor whether the guys are fresh out of plumbing school or nearing retirement.

As with many subjects, personally I think it depends more on the requirements of a particular job than that it is a matter of you should do this or it is always better to do that. A good example of this principle is in a Potterton Suprima boiler I repaired recently in south east London. The owner complained about irregular lock-outs that seemed to have absolutely no particular pattern. In case the term is not familiar to you, a (volatile) lock-out means that one of the boiler’s safety devices has detected a problem with a potential safety risk, and turned itself off. This can be caused by numerous different faults, from an interruption of the gas-supply, via overheating to an unsteady flame due to the burner being dirty.

The standard approach with intermittent lock-outs, is to have a quick look for anything obvious and if there isn’t something that stands out as the likely cause, you have no other option than to start at A and carry on to Z until you find the culprit. Using conventional tools, I did just that and found several things that were not quite right and corrected them. This made the boiler behave a lot better, but it would still fail if it was set to absolute maximum with a very low load. In this particular case, that was a 100,000 BTU or 30 odd kilowatt boiler running only the hot water cylinder that was nearly at the required temperature absorbing only an estimated 6 kilowatt or so. I could simply have advised to keep the boiler set at a three quarters for most of the year and only go to maximum in the midst of winter. I have worked on more Potterton Suprimas than I can remember, and I know that they should be able to run on maximum under any conditions, as long as the manufacturer’s instructions are fully met.

I had already established that all the sensors worked as required, the boiler was fully serviced and clean as whistle inside, and the input and working pressure were inside the prescribed tolerance. The odd thing was that where these two values normally move parallel, in this case one was slightly below the middle and the other slightly above. For some reason, this lead to the CO levels (Carbon Monoxide) to get near the maximum allowed when the boiler approached the very end of the temperature scale it was designed to work on. A minute adjustment of the burner-pressure lowered the Carbon Monoxide levels by about 80%, and the boiler stopped locking out.

The design of Potterton Suprimas is quite old and in those days flue gas analysers were quite rare, so the manufacturer does not give recommended guide lines for the CO and CO2 levels.


The measurements I found were inside general recommendations, so that in itself was no answer either. Only the fact that I knew from experience that the boiler normally can work without locking out at maximum, AND I would expect a lower CO level on this model, AND that conventional tests had shown the working conditions were met, led me to the solution. Most of my lobs are straight forward, but sometimes repairing boilers is really more art than science.

Potterton Suprima boiler repair in Ashburton CR0.

The Potterton Suprima is another excellent example of boilers that are far too often written off as beyond repair. They can have their problems, and sometimes it can be a bit of a challenge to find where the trouble began.

This is one of those boilers where one part that goes wrong can take out another. When the heating engineer working on it doesn’t know the trick, he will replace the broken part when he finds it. But because it is not the only problem, the boiler still doesn’t work. The combination of seeing his investment in parts go up in smoke, combined with the treat of defeat can now easily lead to the verdict: “Beyond (economic) repair”. The report to you will be that unfortunately it is time for a new boiler.


The Potterton Suprima is usually set up as a so called open-vent boiler and has external controls. In at least nine out of ten cases this is either a y-plan or s-plan. Both versions have, or at least should have, a timer/programmer, a roomthermostat, a cylinder thermostat, and either a divertervalve/midposition valve, or two or more zone valves. The one with the two zonevalves is called s-plan, and the one that has one diverter is called y-plan.

Whenever the programmer is set to come on for either domestic hot water or central heating, and the thermostat is at a higher temperature than present, the control system will send a signal to the boiler to come on. If that sounds like the problem could be in the controls in stead of the boiler, you are right.

There is a third reason these boilers get condemned as beyond salvage. For some reason, they can be quite prone to blockage with corrosion residue. Although in reality this is not actually a boiler fault, it does cause it to go in almost constant “lock out” mode. As no real repair will cure this, it has more than once lead to a completely unnecessary replacement.

Apart from a few peculiarities, the Potterton Suprima is a quite sturdy boiler and in virtually all cases worth restoring. There must be quite a few in the Ashburton area, and as there is ample and free parking in that part of CR0, I’ll be quite happy come and have a look with my normal 100% result guarantee if you have any central heating problems.