Portrait photo of Mike Bryant, AKA Mike the Boilerman

Mike the Boilerman -

Your Gas Safe Registered boiler 

and central heating repair technician in west Berkshire. Willing to travel :)



I am still carrying out house visits. Full details explained


(Last updated 21st April 2021)

Potterton Promax SL, HE and HE Plus

This Potterton Promax review excludes the Potterton Promax System HE Plus / Combi HE Plus which, despite sharing the Promax name, are all a completely different boiler. None of the comments here apply to the “Promax System HE Plus” or the “Promax Combi HE Plus”.

Confusingly, as mentioned above, the 'Promax' name is shared by two completely different boilers. This review deals with most common Promax, which is an early design of condensing boiler. 

Unusually for a condensing boiler, the Promax has a single, one-piece cast iron heat exchanger. (Most modern condensing boilers have single heat exchangers made from either cast aluminium or fabricated stainless steel. Those with a cast iron heat exchanger usually have a second, separate condensing heat exchanger.)

Combustion arrangement is unusual too, with a modulating gas valve supplying a gas/air pre-mixing fan feeding an up-firing ceramic gauze burner. First stage heat exchange occurs above the burner, then the flue gases are routed to the back section of the heat exchanger where the condensing stage of flue gas cooling is carried out.

Ignition is electronic, as with all condensing boilers. This means there is no pilot light running 24/7 wasting gas. 

I'm encountering the Promax more often these days, which is unfortunate because I don't like them. It's worse than that actually, it's one of the few boilers I avoid working on whenever possible. In fact I'm STILL being coy about it. It's on my (very) short list of boilers I am only prepared to maintain on a strict "no guarantee of any sort whatsoever" basis.  In particular, I decline all  requests for me to service the Promax (or any of it's many variants) and I only venture to repair a broken Promax if the customer accepts the repair will not benefit from a parts and labour guarantee, or any other sort of guarantee. I'd prefer really to completely decline all Promax repairs.

Here are the problems with servicing:

Firstly, like many older condensing boiler designs, the combustion chamber seals MUST be replaced every time the combustion chamber front cover is removed. Unfortunately few merchants stock this part and it will probably have to be ordered, with a wait of a day or two before the work can be completed. This part must be replaced every time the boiler receives it's annual service.

Secondly, dismantling a Promax for servicing properly can lead to serious difficulty re-assembling it in a leak-tight manner in my personal experience. To service this boiler properly the aluminium combustion box base must be removed for cleaning. This part collects the acidic condensate and routes it to drain. Sadly, aluminium gets corroded by condensate and the joint seals usually get damaged too on removal too. Either a new combustion box base and seals are needed, or, the old one can be stuck back on with lashings of silicone bath sealant as a stop-gap fix to get the boiler running again. The plaster-it-up-with-silicone method also means the next engineer to service it has virtually no chance of removing the combustion box base for cleaning.

This two factors lead to most street-wise engineers removing neither the combustion chamber front panel nor the combustion box base during a service. This means the boiler hasn't been serviced at all, just safety-checked and given a cosmetic hoover-out. As time goes by a breakdown becomes ever more imminent due to blockage of the condensate drain. To service a Promax properly it is essential to fit new combustion chamber seals and occasionally a new combustion box base and other seals. This is EXPENSIVE, there are usually delays obtaining the parts, and working on a condensate-corroded and possibly leaking boiler is never pleasant and if the corrosion causes more problems later, I tend to get the blame. Hence my reluctance to get involved with the Promax at all.

Common faults:

1) Random and intermittent failure to light. when this happens, the user needs to press the 're-set' button on the front panel of the boiler, then it appears to work again normally for a random amount of time until the next failure. You'll be driven mad by the random nature of the failure where you arrive home from work to a cold house or wake up in the morning to a cold bath or shower. This is a most infuriating fault as calling out a boiler technician won't help. The boiler will behave itself perfectly when the technician arrives, so they can only check over the basics then announce they can find nothing wrong. 

2) An apparently minor leak, were water drips very slowly from the front corner of the boiler casing. This is usually a terminal fault requiring replacement of the boiler. There are aluminium pipe-connection manifolds bolted to the cast iron heat exchanger, sealed with large oval 'O' rings. The aluminium corrodes and the 'O' rings begin to leak. New manifolds and 'O' rings are available but the problem arises removing the bolts fixing the old manifolds in place. They just snap off when turned, instead of unscrewing. Now a resourceful technician might drill out the snapped-off bolts and attempt to enlarge and re-cut new threads in the holes, but the cast iron is of such a nature that the thread-0cutting tap usually binds in the hole and the tap also snaps off. Guess how I know this… A new heat exchanger can be obtained but there is sometimes other damage causes by the leak - water gets into the gas control valve, the burner assembly might need replacing, and the aluminium sump components will also need renewing and I’m not sure they come with the new heat exchanger. Quicker and cheaper to fit a brand new identical boiler than to deal with all this uncertainty in my opinion. If you want another Promax, that is! 

3) All the usual things that go wrong with any other boiler e.g. fan failure, gas valve failure, control board failure.

My personal opinion is that this is a boiler well worth replacing if you have one. Replacing with almost any other make and model of boiler.


Apologies if this review page has turned into a bit of a rant.... ;-)


If you'd like me to attempt to mend your Promax, feel free to call me. I'll probably be extremely negative about it and advise you to just buy a new boiler... 

But try me. You never know.


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Copyright Michael Bryant 2021

Site first published 16th January 2004

Last updated 26th April 2021

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