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Powermax Repairs by Mike the Boilerman - Gas Safe Registered technician covering areas within driving distance of Hungerford, Berkshire. Call or text me on: 07866 766364



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Last updated 2/12/20.

Locked out Powermax 155x boiler in dark cupboard in flat in central London, about to be repaired.

A Powermax 155x lurking in a gloomy cupboard in a bright and modern flat in central London, in locked-out condition and about to get fixed. :)

Powermax 155x

Powermax boilers by Range and by Potterton - Information for owners and technicians

First of all, thanks for visiting Powermax Repairs.

I'm Mike Bryant, also known as Mike the Boilerman. This site specifically to help owners of Powermax boilers and/or technicians repairing them. Feedback I get from owners of Powermax boilers suggests they find the Powermax fiendishly difficult to get serviced and/or repaired. Heating engineers in general are very wary of them due to their chequered history and would rather avoid working on them whenever possible :-/

Anyway, this  page discusses with the Range Powermax. (If you have a Potterton Powermax HE then there is a separate page for this boiler, see the link at the top.) I'll start with a brief description of the Powermax, why it is different from ordinary boilers, then more about the chequered history.... 

The Range Powermax was based on an excellent concept in my view - it's a shame the implementation was so flawed. The idea was conceived back in the days when mains pressure hot water cylinders were not permitted under the Building Regulations. The only way to get high performance showers was by installing pumps. The Range Powermax idea was to combine a boiler and hot water cylinder into one integrated device capable of delivering both central heating and mains pressure HOT water, by using the 'thermal store' principle, so high pressure showers could be fitted in flats especially without the use of expensive and noisy pumps.

A tank full of water was heated not by a separate gas boiler, but by an integrated gas burner that squirts flames down a number of tubes running straight through the water tank. The tubes heated the stored water which was then circulated around the radiators for central heating. But the best bit was the way mains pressure hot water was produced. Cold mains water was fed into a coil of tube suspended inside the heated water tank, and the cold water was heated through the wall of the coiled pipe in real time as it flowed through the coil on its way to a hot tap or the shower. Mains pressure hot water at high flow rates with no pump, in full compliance with the Building Regulations of the day!

Now the problems...

1) The gas valve and burner gasket. The type of gas valve does not take kindly to being fiddled with by technicians (or users) without the use of a flue gas analyser to measure the effect of the adjustments being made. Carbon monoxide can be produced in vast quantities. This problem perversely combines with the fact that the original type of burner gasket is prone to leakage. A new gasket MUST be used every time the burner is removed for servicing according to Potterton-Baxi technical support. Few service engineers carry these burner gaskets in the van so I suspect few actually get replaced on servicing, and CO from poor combustion leaking from a failing original burner gasket has potential to lead to fatality. This is one reason for the dark reputation of the Powermax amongst gas technicians many of whom simply refuse to work on them.

2) The aluminium flue. The Powermax twin-tube flue is very compact and is often installed running long distances across ceiling voids to outside. A poor standard of workmanship installing these flues can lead to the flues coming apart behind panels or boxing and flue gas discharging directly into living spaces. Should this happen on a conventional concentric boiler flue the outer air duct tends to suck the flue gases back into the boiler but not on the Powermax with its pair of separate tubes. Several users have lost their lives as a result of this fault so now there is a mandatory safety notice from Gas Safe Register requiring access to concealed flues, or CO monitors connected to an interlock which shuts down the boiler in the event of a CO leak where a flue is concealed. More details here.  

The Range Powermax is also very thermally efficient but not designed to be a condensing boiler. This high efficiency can lead to  condensation sometimes occurring  inside the flue duct. Not normally a problem except that the condensate generated is corrosive and eats through the aluminium wall of the flue duct in the long term. This is especially a problem on horizontal flue runs, and even more of a problem when the horizontal flue runs are concealed behind ceilings or boxing. The flue duct perforates and leaks products of combustion into the living space. Should poor combustion then occur, possibly due to incompetent adjustment of the gas valve, the user is at serious risk of CO poisoning. 

These, as I understand it, are the two reasons for the poor safety reputation of the Powermax. Each individual model of Powermax also suffers from various other technical faults and failures just like any other boiler, but none of these other problems presents a serious safety risk provided any technicians working on the boiler have not been negligent.


Common faults and breakdowns:

1) Central heating failure 

The radiators stay cold while hot water continues to work as normal. The immediate cause is the central heating pump not running. The two common causes of this are timer/programmer failure and a seized or failed pump. The seized pump can be diagnosed by measuring for mains voltage on the input terminals. If voltage is present and the pump is not spinning, the pump needs freeing or replacing. If voltage is NOT present at the pump, check the programmer. This is notoriously unreliable. Testing for mains voltage on the output terminal of the central heating channel when heating is selected. No voltage means programmer failure. This is quite a perverse fault as it often occurs immediately the electricity supply is restored after being turned OFF for servicing or another repair. This leads to the technician (or the customer!) wondering if they may have misdiagnosed the original fault or caused the programmer failure by doing something wrong... but this is not normally the case. The failed programmer has to be replaced. New programmers are expensive but if you are competent with electricity and capable of diagnosing and replacing your own programmer there is a cheaper alternative. I know of an electronics engineer who repairs Powermax programmers commercially on an exchange basis. Google "Powermax programmer repair" and his eBay listing usually comes up in the results. (By the way I have no commercial connection.)

2) Hot water runs hot initially, then cools down. 

Often caused by a failed thermostatic blender valve. A new valve fixes the problem. Additionally, the cause can be water scale. If the Powermax has an external plate heat exchanger this can be replaced quite easily and hot water is immediately restored. Many Powermax boilers have an internal domestic water heat exchanger and chemical descaling will be necessary. Not especially difficult but time-consuming and invasive. Involves bringing chemicals, a descaling pump, hoses etc into your house.

3) A water leak from the flow switch.

Pretty much as the photo. Water starts dripping from the plastic nut holding the flow switch in place and a new seal is needed. The following video explains in detail, with photos:

Having just exchanged a few texts with someone with this water leaking problem, I’ve just measured one of these ‘O’ ring seals that needs replacing for them to source their own. The ‘O’ ring measures 22.1mm outside diameter x 13.9mm inside diameter, and the material diameter is 4.1mm. Quite probably a nominal size of 22mm x 14mm x 4mm and my measurements are slightly inaccurate! Suppliers come and go but right now, this listing on ebay is selling the right size O ring: https://www.ebay.co.uk/itm/362938520339 … select 14mm ID x 4mm CS in the drop-down list of sizes.

A further problem cropping up with the flow switch is the black plastic nut, or even the whole flow switch becomes embrittled with age and disintegrates when attempting to replace the sealing ‘O’ ring. In this case the only option is a new flow switch. The correct flow switch for this boiler is freely available from Baxi-Potterton but costs £180 approx. An apparently identical flow switch (but without the relay that comes with the Baxi branded part) is used in Grant combi oil boilers for a far lower cost. It requires the cable on your old flow switch to be cut and joined, but this is no great difficulty. Grant part number MPCBS71X. 

3) Boiler refuses to light at all, showing the red lock-out light after three failed attempts to start. 

Several possible reasons for this, but the most common reason in my experience so far is a blocked automatic-air vent (sealed system versions only). The blocked AAV fails to allow accumulated air to escape from the collection chamber on top of the heat store and the water level switch inside the collection chamber disconnects the gas valve to protect the boiler from 'dry running'. Easily diagnosed by loosening the AAV. If air escapes from the thread and the boiler then lights, fit a new AAV! Next reason is very similar. The level switch inside the air collection chamber fails, ceases to detect the (correct) water level in the air collection chamber and disconnects the gas valve as above. A new level switch needs to be fitted. Third reason would be solenoid failure on the gas valve. The solenoid can be replaced on later (Sigma) gas valves but not on the early Ranco valves. A new gas valve will be needed. Other reasons for failure to light are usually control board failure or problems with ignition electrodes/leads. Both reasonably easy to fix. 

4) Random locking out. 

The user realises there is no hot water or heating, and the boiler is found to have locked out the with red light on. Re-setting the boiler makes it start again and run apparently perfectly normally but after a random period (hours or days) it locks out again, driving users (and their boiler technicians) to distraction. Firstly, the inlet gas pressure, gas valve settings and combustion settings all need to be checked and verified correct (specialist work) then if the fault persists, speculative parts-changing can begin. Changing the ignition electrode, ignition cable, gas valve and electronic control board in that order usually hits on an answer. These parts may be replaced serially to save money but multiple technician visits can be needed. Alternatively they can all be replaced in one visit at enormous cost for a more probable fix, but as with all intermittent faults, there is no certainty of a repair when the technician cannot reproduce and observe the fault occurring :-( 

5) Pressure gauge falls to zero then boiler locks out and refuses to start. 

Re-pressurising the boiler to 1.0 bar makes it run again but the problem returns after a few hours or days. A complex fault and a bit of a paradox because unlike many boilers, the Powermax does NOT have a pressure switch to turn the boiler OFF when pressure falls to zero! This fault is usually caused by a small volume of air accumulated in the air collection chamber in the top of the boiler. This air would normally be released through the AAV (auto air vent) but the AAV has stopped working. This leads to the level switch in the air collection chamber sensing low water level and disconnecting the gas valve leading to ignition failure and locking out. Re-pressurising the system compresses the trapped air, the water level in the air collection chamber rises slightly and the level switch senses this,  re-connects the gas valve and the boiler works again for a while. If the expansion vessel has also lost it's air charge (another common problem) the system pressure may then rise above 3.0 bar and the PRV (pressure relief valve) will let some water out of the system. The PRV will sometimes fail to fully close again and continue leaking water from the system until the pressure reaches zero, when the level switch will disconnect the gas valve. Re-pressurising makes it all work again and the cycle repeats. A new AAV and PRV and recharging/replacing the expansion vessel usually fixes the problem. The expansion vessel is also prone to diaphragm failure and leaking, often only discovered when trying to re-pressurise it. A vessel with a failed diaphragm will discharge water instead of air from the Schrader valve provided for re-pressurising. Re-pressurising a vessel with diaphragm failure works initially but the pressure charge is rapidly lost over the following days or weeks as the air dissolves into the circulating water. A new vessel will be needed.

6) A high-pitched monotone 'singing' noise when running, so loud it is often audible from outside in the street. 

There seems to be no one single cause of this but my current theory is that it caused by the perforated stainless steel face of the burner assembly resonating. No-one seems to know why a Powermax should spontaneously start singing having run silently for many years, but re-calibrating the combustion settings, replacing the gas valve and/or replacing the burner head all seem to affect the singing behaviour. In most cases one of these remedies (or all together!) will get rid of the problem. There is one customer I have however whose Powermax continues to intermittently sing, whatever I try. Consequently I do not feel able to positively guarantee a fix in every instance of singing until I crack this fault on this particular boiler. (This is the only Powermax that has failed to my attention though, so far!)

7) Disintegrating turbulators

An expensive-to-fix problem is melted turbulators. The turbulators (long twisted stainless steel strips inserted into the vertical gas-ways through the heat store) overheat and fall down into the base of the boiler and are fiendishly difficult to remove. The sump cover underneath the boiler usually has to be removed and tough leather gloves worn to avoid the viciously sharp edges on the turbulators when pulling them out with brute force. The sump then needs to be refitted and resealed correctly (crucial for the boiler to operate safely) and new turbulators are then installed from the top. Then hope the same problem isn't found at the next bi-annual service! The turbulators are fitted in the first place to slow down the hot combustion gas flow rate through the heat exchanger tubes to improve the thermal efficiency of the Powermax. On the Powermax training course run by Baxi-Potterton  at Dartford the trainers say there is no harm in running a Powermax with no turbulators, my current view is that partially eroded turbulators can be ignored provided the combustion readings are good. 

8) Boiler gurgles and won’t light

Boiler won't run properly and on checking is heard to be making a quiet gurgling, swooshing noise that sounds rather like a washing machine. This only happens on the 140, the 155 and the 155X. It is Bad News... REALLY BAD NEWS. Terminally bad. One of the heat exchanger tubes inside the copper cylinder has sprung a leak and has filled the flue collector sump at the base of the unit with water, blocking the flue gas path. There is no fix for this as there is no way to get at the interior of the combined copper cylinder and heat exchanger. The combined cylinder and heat exchanger is not available as a spare part, so the only fix for this is a whole new boiler. 

***Update 23/10/13***  I have been contacted by a by an engineering firm in Leeds who can repair this fault by fitting new heat exchanger tubes. Cost is similar to a new boiler but far less disruption is involved as there are no flue or supply pipework changes necessary. You can contact them directly by email at info@powermax140-155-repairs.co.uk . Bob is the guv'nor there, or you can email me to ask for his mobile number.

9) Flues and air inlet ducts: 

Although Range (who made these early Powermax models) went out of business many years ago, technical support and spare boiler parts are still available despite what you may have heard elsewhere. Flue parts unfortunately have been recently discontinued so alteration of a Powermax flue or air inlet duct, replacement of a damaged part, or repair of an incorrectly installed flue or air duct is no longer possible.

10) Relay failure. 

The Powermax runs a pump while the burners are alight to prevent stratification in the heat store. If the pump stops running, stratification occurs and the boiler overheat thermostat trips, turning the boiler OFF. A visiting technician will often spot this tripped thermostat and re-set it, then declare the boiler 'fixed'. The problem will return though unless s/he also addresses the reason for the tripping. The pump which ought to be running is operated by a relay on the upper left hand side of the wiring panel. This relay is very prone to failure and will usually be found to have failed whenever the overheat thermostat has tripped. An easy repair but one which foxes many inexperienced technicians.

Given that many Powermax boilers I attend turn out to have multiple faults, the cost of a repair can turn out to be very high when I attend and find multiple components needing replacement. Several cost well over £100 each and the labour costs mount up too. This leads me to suggest visitors to this site first try the fixed price repair service available from Baxi-Potterton specifically for the Powermax. Their service dept is now called "Heat Team". I understand that whatever is wrong, Heat Team will fix it for a fixed price of (usually) around £350. Sounds like a bargain to me when parts alone can occasionally exceed this when I attend a Powermax! The only problem is that Baxi-Potterton only seem to offer this service intermittently, so you may or may not get lucky. In addition, my initial advice to Powrmax owners is to replace it rather than repair it, as one expensive repair by me does not guarantee the whole boiler against a different, equally expensive failure occurring soon after. Three expensive repairs in, say, a year or two can easily approach the cost of having replaced the boiler in the first place.

I'll finish today by saying I lived in Reading, Berkshire until recently when I moved 25 miles west to a country village near Hungerford. Most of my work is still in Berkshire, Hampshire, Oxfordshire, Surrey and west London but if you are outside this area then I'm perfectly happy to visit. In fact I'll go anywhere! The only trouble with this is, from your point of view, is that I charge for all the time I spend repairing a boiler, and this includes the time spent travelling to and from site. This means the further away you live from me, the less economically viable it is to get me to visit.

Alternatively I'm happy to give email advice to anyone wanting it, but not telephone advice. I had to stop that years ago when the weight of calls grew too great.

Finally, a lot of people seem to have lost the User Instructions that came with their Powermax. Setting the programmer is a virtually impossible task without them, and is still not easy even WITH the instructions! I have a copy of the User Instructions in PDF format. You can view the User Instructions or download a copy by clicking here :-)

For my main site, check out www.miketheboilerman.com

Once again, thanks for visiting.

Mike Bryant, AKA Mike the Boilerman. 


Pwermax flow switch leaking
Powermax flow switch 'O'-ring seal.
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Copyright MICHAEL BRYANT 2020

First created 21st July 2009

Last updated 10th May 2021

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CIPHE registration number 56207

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