Mike the Boilerman portratit photograph

Mike the Boilerman 

Your independent Potterton 

Netaheat specialist covering Berkshire, Wiltshire & surrounding counties

For more information or to book your breakdown repair, call or text me on 

07866 766364 

*Coronavirus* - Back at work now, policy here

Netaheat MkII and MkIIF by Potterton


If you need me to visit and repair your Netaheat MkII or MkIIF, calling or texting my mobile is usually quickest. My number is 07866 766364. (Please note that it is not always possible to repair one particular fault on the MkII or MkIIF - where the pilot flame ignites but not the main burner.)

The original Netaheat was superseded after about five years by The MkII, which looked identical, used the same chassis, heat exchanger and fan but had a completely redesigned and simplified control system. The various functions of the solenoid gas valve, pressure regulator and mercury vapour valve switch are all combined into two components, the combined gas valve and the mercury vapour switch. The pilot light and the main burners are both now controlled electrically, vastly simplifying fault diagnosis.

Here are the common problems:

1) Flue Fan Failure

As with most fanned-draught boilers, the most common failure on all versions of the Netaheat is failure of the fan. Combustion air is blown with an electric fan through the combustion chamber so it is a safety-critical component. If the fan fails to start, the safety device which tests to see if the fan is running before allowing the pilot to light will detect the failure and stop the boiler from lighting. To the user the boiler will stay silent (or possibly hum quietly) instead of bursting into life as normal when a demand for heat is turned on. The fan is still freely available as a spare part and is fairly straightforward to install, but as this boiler is a 'positive case pressure' boiler and replacement of the fan involves removing and replacing the case, it is important to carry out the case seal checks on reassembly as described in Gas Safe Register Technical Bulletin TB006, "Industry guidance for the checking of case seals and the general integrity of  room-sealed fan-assisted positive pressure gas appliances".

2) Air Pressure Switch failure

The safety device which tests the fan for correct operation is the air pressure switch. These are know to fail occasionally and the result is the fan runs, the boiler sounds for all the world as though it is about to light, but no ignition ever occurs, not even the tiny pilot flame in the very back of the combustion chamber. The correct air pressure switch is long obsolete and for a while Potterton used to sell an upgrade kit containing a generic Honeywell air pressure switch. I'm now told this is no longer available either. I see no technical reason not to fit a generic air pressure switch of the same specification. Doing so may not be strictly in accordance with the Gas Regulations though. Occasionally I find a different the reason for the air pressure switch not switching. Inside the boiler there are two aluminium tubes running from the fan at the top, down to the bottom of the boiler, to transmit the air pressure difference from the fan to the air pressure switch in the base of the boiler. These pipes suffer from corrosion and sometimes they corrode right through (particularly behind the fixing clips), preventing the air pressure switch from correctly sensing fan pressure. New tubes can be fitted to rectify this. 

3) Mercury Vapour Switch failure.

When the mercury vapour switch begins to fail, the pilot flame lights perfectly normally but the main burner fails to light. If giving the bottom of the boiler a thump makes the main burners light (or lightly tapping any of the gas control components underneath), then the diagnosis is confirmed and my best advice with this fault is to replace the boiler rather than try to mend it. The mercury vapour switch is a sensor in the pilot flame which, when warm, closes a switch and allows the combined gas valve to send gas to the main burners. It does essentially the same job as the thermocouple in a permanent pilot light boiler except that when it fails in the MkII and Mk IIF the pilot light lights, but the main burners never come on. The pilot flame normally takes about 45 seconds to warm up the mercury vapour phial and operate the switch. As the mercury vapour switch grows old it takes longer and longer to close the switch, the customer normally reporting the fault when this delay stretches out to half an hour or more. The MVS assembly is a long-discontinued part, and even those occasionally appearing on eBay tend not to work when unpacked and tested. They may be notionally “brand new” i.e. never fitted, but most have been sitting on a shelf in a merchant somewhere for 25 years! If the MVS on yours has failed then the chances of repairing the boiler are consequently slim. 

4) Combined Gas Valve failure

The combined gas valve turns the gas on and off to the pilot light and to the main burner in response to electrical signals from the other controls, and it regulates the gas pressure to the main burner. The solenoids which turn the gas on and of eventually fail and a new combined gas valve is the usual fix. Unfortunately the gas valve is no longer available and the only source is eBay. There are usually three or four second-hand ones for sale at any given time, along with the occasional new one.

5) Spark generator failure.

The spark generator is a black box that delivers the spark to light the pilot light. These too are a discontinued part but easily repaired by an electronics engineer. I keep a stock of repaired units in the van which I fit on an exchange basis.

6) Pilot light failing to light or being very small.

Easily fixed and associated with lack of proper servicing. A problem common on lots of boilers. The pilot jet has become blocked or partly obstructed with debris and needs poking out with some fusewire. Part of the manufacturer's service procedure!

7) Relay failure.

There is a generic, socket-mounted eleven-pin relay in the control panel of this boiler. These occasionally fail causing the boiler to remain silent and inactive when there is a call for heat. Much like fan failure. A new generic relay of the same specification is freely available from electronics suppliers. The relay occasionally chatters loudly which is usually caused by low mains supply voltage. The relay is energised by voltage coming from the motorised valve, and certain brands of motorised valve are prone to supplying around 210Vac when the switch inside them begins to fail, rather than the full 240Vac. Not a large difference, but changing the valve usually cures the relay chatter.

8) Thermostat failure.

The boiler overheats, basically. The water in the boiler reaches boiling point, a HUGE banging noise (described to me by more than one customer as sounding like someone trying to break into the house using a sledgehammer to smash through the wall) accompanied, if the customer notices, by a loft full of steam. The thermostat is still, as far as I know, freely available.

9) Cracks in combustion chamber insulation

This isn't really a fault at all, but I'm getting a steady trickle of customers calling or emailing to say their boiler has been condemned due to cracks in the heat insulation lining the combustion chamber. This is perfectly normal and most Netaheats have these hairline cracks, but unscrupulous technicians have been seizing on this as a spurious reason to class it as dangerous and try to sell the customer a new boiler. Netaheats are perfectly safe with these cracks, but if yours has them and you are still worried, I can make and fit new insulation panels from the correct ceramic fibreboard material.

10) delayed lighting.

The boiler sounds as though its running normally but there is no heat, and on checking, there is no flame visible though the viewing window in the front. Ten or fifteen minutes later the fault has cleared and boiler is alight and running normally.

There are several different potential reasons for delayed lighting so this could be a long section, and sometimes more than one fault can be present. The first thing to check is the pilot light, a tiny flame which lights in the back of the combustion chamber at the start of the ignition sequence. One needs to peer carefully deep into the viewing window to see it. If there is no pilot flame and you can see or hear a repeating spark, this is probably a failed gas control valve. Easily replaced with a second hand or repaired one recovered from a scrapped but working boiler. If there is no pilot flame and no spark, a new ignition module or air pressure switch is usually needed. If the pilot flame is present but the main burners are not lighting, there is little hope for this boiler as the mercury vapour switch has usually failed. New switches are not available and second hand often do not work. 

The Netaheat MkII and the MkIIF initially appeared identical to me, but I've recently figured out the difference. The main burner assembly on the MkII can be dismantled into its component parts, while the MkIIF burner assembly is factory fabricated and welded together. The MkIIF is FAR more common than the MkII, and is very different from later models. In fact it is totally unique in the way it works.  An engineer accustomed to modern boilers will usually struggle to understand how it works although diagnosis of these common faults should not present too much of a problem to a competent technician. Especially if s/he has found these notes!

If you'd like me to fix your Netaheat for you, call or text my mobile 07866 766364.

I have to add though, that due to the difficulty in obtaining mercury vapour switches, if your MkII or MkIIF turns out to have a failed MVS I cannot guarantee to fix it, but the visit to diagnose will still be chargeable. In some cases I can find a used and working MVS in my van, or persuade the one in yours to work (by cleaning the pilot jet for example, to make the pilot flame slightly bigger) but sadly I have to warn, there will always be an element of doubt and I cannot guarantee any repair involving the mercury vapour switch. My best advice is replace the boiler.

By the way I’m based in Great Bedwyn, Wiltshire (near Hungerford) and these are the areas I cover: Berkshire, Gloucestershire, Hampshire, Oxfordshire, Surrey, Sussex, West London, Wiltshire.

Call or text me on 07866 766364


If your boiler has broken down contact me for free help and advice by phone or text. I can arrange to visit and repair if necessary.


 If you book a visit I shall wear a face covering (and gloves where practical), and observe social distancing.

Details here

Potterton Netaheat MkIIF, circa 1984.
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Copyright Michael Bryant 2021

Site first published 11th November 2010

Last updated 7th June 2021

Gas Safe Register 197499, CIPHE registration number 56207

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