Potterton Promax SL, HE and HE Plus
This Potterton Promax review excludes the Potterton Promax System HE Plus
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
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
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
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.
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
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.
Cheaper to fit a new boiler tan to deal with a snapped-off tap, should it
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
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
But try me. You never know!!!
First created 14th March 2008
Last updated 26th September 2014