N.B. This page gives an overview and history of the Powermax. I have a I
am developing a separate website dealing with specific faults and breakdowns that occur
with Powermax boilers- www.powermax-repairs.co.uk
The PowerMax falls firmly into the category of 'unusual and interesting'. This may or may not be a Good Thing according to your point of view. The PowerMax has a short but chequered history with many design changes and I'm
writing this page to summarise my own understanding as well as help visitors to
this page. I may be wrong on some of the details in which case if you know
better than me, please let me know.
The basic concept of the PowerMax was to combine both a boiler and a hot
water cylinder into a single appliance. Instead of a gas boiler heating water,
subsequently used to heat a separate hot water cylinder or thermal store, a gas
burner would directly heat the water in a thermal store, meaning the
boiler and hot water storage become completely integrated. The resulting product
would be an easy-to-install appliance that is fitted instead of a separate gas
boiler and hot water cylinder. The PowerMax units are floor-standing, about four
or five feet tall and two feet square. they appeals to builders and developers
mainly, because they are as simple to install as a combi and provide first class
gas-fired central heating and mains-pressure hot water performance for the occupier.
The product was originally designed, developed and manufactured by Range
(Range, I believe, were connected to, or were the same company as IMI Range, the
hot water cylinder manufacturers). The product was very successful but design
flaws meant they turned out to be pretty disastrous from a reliability and
maintenance point of view. Around 2002 I believe, the product name and
manufacturing rights etc were acquired from Range by Potterton-Baxi resulting in
the PowerMax name, along with spares and technical support, being transferred
and re-branded as the "Potterton PowerMax".
Potterton substantially re-designed the product before re-introducing it to
the market. The Range PowerMax came in several different versions but all were
thermal stores heated by gas, were the thermal store itself doubled as the
boiler heat exchanger. Potterton abandoned this principle and the New Potterton
PowerMax models have a small but otherwise conventional mains pressure hot water
cylinder built into them with a separate gas boiler, all in the same
The PowerMax models:
I've always found the model numbers and ranges confusing and I'm not certain
of my facts in this section, but this is my understanding of the history of the
Range PowerMax 155
I think the 155 was the first PowerMax to be launched.
The unit is a 'thermal store', heated by combustion gases from a pre-mix
gas/air burner at the top which pass downwards through eight tubes, or ducts, in
the centre of the water store, to be collected at the base of the unit then
exhausted to outside. The ducts are about 30mm in diameter and each contains a
twisted stainless steel 'turbulator' to slow the passage of flue gas and
increase thermal efficiency. These turbulators have a habit of overheating,
partially melting and blocking the passage of flue gas through the unit, and are
fiendishly awkward (read 'expensive') to remove and replace. This problem is associated with lack of proper
maintenance. There is a gauze filter in the unit sump at the bottom that needs
to be cleaned during servicing, according to the service schedule. The problem
however, is that access to the gauze is appalling. It is not possible in my
experience to remove the gauze for inspection and cleaning without first
squashing it flat, and once removed, it is virtually impossible to re-shape and
replace correctly. I can't do it to my own satisfaction (so I decline to attempt
it these days) and few engineers can either, I suspect. The only way I can think
of to do it is to drain the system, disconnect and and remove the boiler, then
lie it down flat on it's back to get proper access to the underneath. (I haven't
tried this yet!)
The hot water in the thermal store is pumped around the radiators to provide
central heating. Mains pressure hot tap water is provided by passing the
incoming cold mains water through a coiled pipe inside the thermal store where
it picks up heat from the hot stored water. This works very well initially, but
in hard water areas can be badly affected by water scale deposits. The effect of
water scale is to reduce the temperature of the hot water to begin with, then as
the problem worsens the flow rate becomes progressively worse too. These units
need either an effective water conditioner/softener or periodic chemical
descaling. When not suffering from water scale deposits the hot water delivered
can be dangerously hot so a thermostatic blender valve is fitted in the unit to
limit the hot water temperature to a safe level.
Oddly, the 155 has two pumps. One operates the central heating circuit, the
other is described in the instruction book as a "heat Ex. Pump". The
only purpose for this I can imagine is to agitate/circulate water within the
thermal store to ensure uniform store temperature and prevent stratification.
Range PowerMax 140 and 155x
These two models appear to be
essentially the same appliance, and appear to be a development of the original
155. They both share the same Manufacturer's Installation
and Service document and the difference between them appears to be the height of the
units. The 155x is about 5" taller than the 140 so probably has a larger
The 140 and 155x are similar to the 155, and are built is the the same format
with a sump/flue collector at the bottom containing a gauze which is very prone
to blocking and virtually impossible to remove for cleaning and replace
correctly/safely. The only way I can envisage is to disconnect and remove
the boiler then lie it down on it's back to get proper access to underneath.
The main difference between the 155 and the later 140 and 155x is the method
of hot tap water provision. The internal heating coil of the 155 has been
abandoned due to the water scale problems and replaced with an external 'plate
heat exchanger', believed at the time to be more resistant to water scaling.
This adds complexity to the unit as a second pump is now needed to pump hot
store water through the heat exchanger during hot water demand, and a flow
switch to turn the pump on and off, and a thermostatic blender valve - necessary
to prevent the domestic hot tap water reaching dangerously high
Range PowerMax 135 and 185
The 135 and 185 appear identical in layout and format, the main difference
between them being their physical size (and output).
The design however, has been substantially changed from the preceding
135/155/155x models, with the gas burner but moved to the bottom of the unit so
the combustion gases now flow upwards for collection from the top of the unit,
and a passive internal heat exchanger installed inside the water store for hot
The fan, gas valve and electronics remain at the top of the unit so the unit
appears superficially similar under the top cover, but different problems have
now been introduced. The designers have abandoned the external plate heat
exchangers of the 140 and 155x and reverted to an internal hot water heat exchanger,
but this is still fiendishly
susceptible to blockage with water scale. The air tube to the gas burner
contains a baffle that tends to block with fluff and airborne debris, preventing
the unit from running. The need to clean this baffle is not mentioned in the
service schedule and I've attended several 185 breakdown calls that have turned
out to be caused by this!
Range PowerMax safety concerns:
There is something being kept from us regarding the Range PowerMax. I have
several reasons for saying this.
1) A PowerMax user in Oxford died recently of CO poisoning. This was, according to
initial news reports, the result of a British Gas engineer incorrectly setting the gas
valve on it. This makes no sense to us gas engineers as all PowerMax models are
room-sealed and gas valve adjustments, no matter bad, should never result in
combustion gas entering the room. The technical details of this case seem to be
being kept secret, presumably for legal reasons. I suspect that in addition to
setting the gas valve wrongly he also failed to correctly replace the burner
flange (using a new gasket as prescribed in the manufacturer's Installation and
Service instructions) allowing combustion products to escape into the room, but
I have no way of finding out definitively. Two British Gas engineers have
now been charged with manslaughter and committed for trial. (Link
to local newspaper story).
2) All Range PowerMax spare parts (even non-gas parts) now come with a stern
written warning NOT to adjust the gas valve without "relevant
training", and to replace the burner flange gasket if it is disturbed in
any way. There is no information available (even from Potterton) on what "relevant training"
3) There is a Product Recall Notice on the PowerMax 135. Bizarrely, the only
mention of this Recall Notice is on the website of the London Fire service. (See
mention of this problem on the Potterton website.
4) I'm told by a reliable source that there is a problem with the
factory-fitted gas valve on the 135. It tends to drift out of adjustment,
leading to explosive ignition, and replacement of the original gas valve is
advised by Potterton. Any 135 which has had the gas valve replaced will bear a
label stating this fact.
5) The flue duct on the Range Powermax is made from aluminium. The Range
Powermax is a non-condensing boiler but runs so fuel-efficiently that condensate
sometimes occurs in the flue duct. Condensate is acidic and corrodes the
aluminium duct, leading to perforation and therefore leaking of both liquid
condensate and combustion fumes into the house or flat. The combustion fumes can
be lethal if the gas valve is not correctly adjusted and carbon monoxide (CO) is
being produced by the boiler. For this reason it is crucial that the full length
of the flue is accessible for inspection and safety-checking. (See this page on
the risk associated with concealed flue ducts.)
Potterton PowerMax HE 80, HE 115 and HE 150
Potterton, having acquired the 'PowerMax' brand name, completely revised the
product, abandoning the principle of a thermal store heated directly by the gas
burner in favour of a separate gas boiler and un-vented hot water cylinder, both
fitted inside a single casing and called a 'PowerMax'.
If you'd like me to come and repair your PowerMax I'll be happy to do so.
Ideally the full length of the flue and air intake ducts will be accessible for
inspection. (This is necessary in order to comply with the legal requirement to
assess the effectiveness of the air supply and the flue whenever carrying out
work on a gas appliance.) Unfortunately many PowerMax installations have flue
and air intake ducts concealed behind ceilings etc., in which case a risk
assessment on the flue (as defined by Gas Safe Register Technical Bulletin TB008
- previously CORGI Technical Bulletin TB200)
will have to be carried out. For more info see here.
Replacing a Powermax
I get a regular trickle of emails from people asking me if there is a modern
equivalent boiler that can replace a Powermax, and in particular, connect to the
existing Powermax flue ducts installed in the ceilings. Unfortunately all new
boilers have to be installed with their own specific flue system, so replacing a
Powermax always involves replacing the flue system too. Regarding a replacement
model, there are no 'all-in-one' format boilers like the Powermax currently on
the market that I would recommend. The best option would be a Vaillant EcoTEC
combi boiler. For hot water performance equal to the Powermax, I'd suggest an
EcoTEC non-combi boiler and a MEGAflo hot water cylinder. The MEGAflo cannot
always be fitted due to difficulty installing new safety discharge pipework in
accordance with the regualtions, in which case I'd suggest a thermal store hot
water cylinder, such as this.
Thermal stores are desperately susceptible to water scaling though, so make sure
you install a water softener or some other effective scale inhibitor.
My Powermax Repairs website
I also have a site specifically for owners of and repair
technicians working on Powermax boilers. It's here
if you'd like have a look...