We work with mains gas only (natural gas) not LPG
This Potterton Suprima is under the work top in a kitchen. Lois is removing the old PCB, chassis and wiring loom to replace it with the later, better Siemens PCB. The Suprima was plagued with circuit board problems until Potterton eventually had a completely new board made by Siemens.
Setting up for combustion analysis on a newly installed Baxi Solo HE as part of the commissioning process. New rules in 2014 made combustion analysis mandatory when new boilers are fitted. Both the air intake and the flue gases must be checked.
Typical data from a gas boiler combustion analysis. This analysis was taken after the service and shows very clean combustion. The pressure figure is the gas inlet working pressure. The flue product has been cooled to only 53°C, allowing most of the the water vapour to condense, releasing the latent heat. The gross efficiency of 88.7% in this high efficiency boiler shows that only 11.3% of the energy from the fuel gas was wasted in the flue gases. This also lowers the environmental impact.
High efficiency boilers have a condensate trap fitted to ensure that flue gases can’t pass out along the condensate drain pipe. The traps can become blocked with debris and must be checked and, if necessary, removed and cleaned out. The better designed traps, like this one in a Greenstar Ri, work as auto-syphons. Instead of a continuous dribble of condensate, these auto-syphons release it a cupful at a time, making it much less likely to freeze.
For us, a gas boiler service should be much more than a quick test with a flue gas analyser. We carry out a range of checks to ensure your safety and to maximise the efficiency with which the gas is burnt. We check the location of the boiler, the condition and operation of the flue and that there is sufficient air for combustion and cooling.
We visually check the combustion casing to ensure there has been no breach due to rusting caused by water damage. Then we partially strip the boiler and check and clean the combustion chamber, heat exchanger, burner and injectors unless the manufacturers specify otherwise; different manufacturers may have different requirements. When we re-assemble the boiler we check the condition of the combustion casing seals to ensure that no flue product can enter the room.
Where possible we carry out combustion analysis and if you’re interested we’ll be happy to explain the figures. In the tests we carry out, most older room-sealed gas boilers will achieve a gross efficiency of about 80%. Newer, high efficiency boilers (also known as condensing boilers) usually reach 88% gross efficiency. Combustion analysis also allows us to make adjustments affecting both efficiency and safety.
Sites like uSwitch provide lots of information and give tables of figures showing you what you could save in gas bills for different boiler efficiencies. The problem is that very few of the older gas boilers we come across are desperately inefficient. We carry out flue gas analysis when we have serviced them and most older boilers show a gross efficiency of about 80%. Changing these for new, high efficiency appliances would make a saving but it would be relatively small.
We have another reservation about uSwitch; looking at their web site it appears that they are in partnership with British Gas. We were under the impression that uSwitch were independent. Apparently most of the web sites encouraging us to change energy supplier receive a significant commission when we switch. It may still be very well worth changing but it’s important to know that the advice may not be entirely impartial!
The efficiency page on our lovekin.net web site may provide additional information to help you to decide for yourself. On the site we also deal with general central heating problems like system pressure and radiator balancing. Motorised valves can be troublesome too and we have a page on Honeywell motorised valves.
If you’d like to know more about High Efficiency (HE) boilers and what gross and net efficiency mean you can find our efficiency article on our lovekin.net site. In the article we talk about the savings you could make by changing your boiler. It’s not always as much as you might think because our industry often overstates the possible savings.
The cost of gas is a major factor in deciding whether it is worth keeping your existing boiler or whether you should have it replaced and, as the cost of gas increases, the balance changes. Another factor to consider is the length of warranty available on a new gas boiler and the quality of the company providing it.
Currently (2018) we recommend Worcester Bosch boilers because their quality, backup, spares and service is second to none. Which? have given all the Worcester Bosch Greenstar gas boilers Best Buy status and that’s for the eighth year running.
A lot of the boilers we service are nearly new but we also service some which are very old. We are happy to service new or old gas boilers, provided they work safely. If your old boiler really is very inefficient (wasting too much gas) we’ll tell you but we’ll never try to bounce you into replacing it unnecessarily.
Even with current expensive gas prices it might be years and years before the savings you’d make would balance out the cost of replacement. If, however, it is old and in need of repair, or becoming more expensive to maintain, it may be time to consider replacing it. We’ll always give you totally honest advice.
Even spiders can cause problems for combustion! They commonly produce cobwebs in the air intake ducts of fanned flued and natural draught room-sealed appliances. These cobwebs can significantly reduce the combustion air supply and make the boiler dangerous. It’s simple to resolve if you know that you need to check. Following mild winters, 2014, 2015 and 2016 were good years for spiders. Flue gas analysis will show up dangerous combustion but before adjusting the gas/air ratio in the boiler we’ll check whether the combustion problems are caused by cobwebs!
There are lots of confusing terms used in our industry so to help, we have page of this site dealing with terminology.
The inside of this Greenstar Ri looks quite complicated but these boilers work very reliably and typically produce a gross efficiency of over 88% in our tests. Unlike older appliances, the fan mixes gas and air in a precise ratio and blows it into the burner at the top of the heat exchanger where the flames burn downwards in the draught!
Spares availability is very good with Worcester Bosch and the cylindrical stainless steel clad heat exchanger on the right carries a 10 year warranty. These are fan flued appliances and this one has been fitted in a loft.
This type of pilot assembly in an open flued (conventionally flued) appliance is known by several names. It is called an atmospheric sensing device (ASD) or an oxygen depletion sensor (ODS). We prefer the term precision pilot. The horizontal part of the pilot flame heats just the tip of the thermocouple. This generates a small electric current which is used by an electromagnet to hold the gas valve open. If the oxygen available to the pilot burner is reduced, the flame softens and lifts away from thermocouple, causing it to cool and lock off the gas supply, safely shutting down the appliance.
The air intake filter on this precision pilot had become choked with dust and fibre, reducing the oxygen available to the pilot flame. The pilot flame had lifted away from the thermocouple tip, shutting down the appliance.
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We serviced this very old Vulcan Autostat in Alfonso Close in Aldershot for 13 years. It was room-sealed and combustion analysis always returned a gross efficiency figure of over 80%. It was eventually replaced when it was 44 years old and only because the new owners of the house were fitting a new kitchen. The only repair work in 12 years was one replacement thermocouple. The circulating pump (top left) was massive but was called “Junior”! It was also 44 years old.
Old Baxi Bermuda back boiler with the fire front and combustion front panel removed. The flame picture is OK with the red and orange tinges in the flame being caused by metallic dust particles passing through the burner. There had been some water leakage from the left hand side of the heat exchanger above the burner, as the surface rusting of the burner shows. The leakage had stopped but where water leakage onto a burner affects the combustion it makes the appliance dangerous, particularly since it is open-flued (conventionally flued) and draws its combustion air from the room and poisonous combustion products could spill out into the room. Even a small water leak, if it has any impact on combustion, would require the appliance to be marked up as Immediately Dangerous and isolated from the gas supply.
Worcester Bosch are the dominant manufacturer in the market place at the moment and we deal with a lot of their Greenstar Ri boilers. The Ri is a heat only appliance which originally came in a range of heat outputs from 12kW to 24kW and has now been joined by two more powerful boilers at 27kW and 30kW.
These last two are of a different internal design and the service described was on a Greenstar 15Ri.
After a quick visual check of the outside of the boiler we went out to take a look at the flue terminal. Correct combustion depends partly on adequate air supply and the air ducts can become partly blocked with spider webbing. The flue and air ducts are concentric tubes with the flue duct running down the centre of the air duct. We brushed through the air duct (photo).
To check flue duct integrity we used a combustion analyser to check the levels of oxygen, carbon dioxide and carbon monoxide in the air duct. This is done with the sample probe in the air sample point (the upper sample point). The boiler is run with the outer casing left on.
If a flue duct seal is damaged or displaced there is a risk of flue gases passing out of the (central) flue duct into the (outer) air duct and this can make combustion dangerous. Oxygen in the air duct should be very close to 20.9% and there should be no carbon dioxide or carbon monoxide present. The results were fine and we refitted the air sample plug.
We checked the pipes connecting to the boiler for water leakage and found none. We then removed the outer case of the boiler and and checked for any water leaks or damage internally.
Greenstar Ri boilers are usually very clean but if there was visible debris inside or out we’d hoover it up.
The condensate trap on any high efficiency boiler can need cleaning out. Thankfully Worcester were bright enough to use a clear plastic trap so it’s possible to see whether or not there is debris there without removing it. The photo shows debris at the bottom left of the trap.
The following photo shows the cleaned trap, refitted. Removing and refitting the trap may require a seal to be changed, which would add slightly to the cost; if the amount of debris is only very small it would be left until the next service.
This trap is also better than a simple U bend; it has an auto-syphon function so the condensate is released one cupful at a time, rather than as a slow continuous dribble which might freeze in winter if the condensate pipe runs outside the building.
It’s important to check visually and listen for failure of the main combustion gasket. Rarely, this can fail and would need to be replaced. The following four photos show a badly damaged gasket, damaged electrodes and damage to the ignition and sensing leads caused by the escaping hot gases. The following four photos were, however, from a different Ri boiler.
Worcester Bosch specify that the Greenstar Ri boiler should only be stripped down further if the fan pressure is outside the parameters they set, with the boiler running on maximum.
We used the pressure gauge on our analyser, connected by a thin hose to the fan pressure test point just below the air/gas duct near the top of the boiler.
The boiler is set to maximum using a small white plastic tool which is located into a rotary switch at the bottom edge of the circuit board.
Fan pressure for the 15Ri must be less than -5.5 mbar. To accommodate us ignorant heating engineers the manufacturers say it must be negative and greater than -5.5 mbar. This is mathematical nonsense.
Whatever the clumsy wording, it needs to be more negative than -5.5, so -5.6 would be fine, whereas -5.4 would not. If the correct negative fan pressure can’t be achieved, the boiler may need a deeper strip-down and clean. The pressure on this boiler was fine.
The analyser sample probe is connected to the flue sample point on the fan turret. This is the lower of the two points.
With the boiler still running on maximum the carbon dioxide and carbon monoxide are checked, and their ratio. These must comply with the manufacturers specified ranges.
If the carbon dioxide level needs to be altered a 2½mm allen key (hex key) is used to adjust the gas valve (1st photo). When the setting is correct on maximum, the boiler is set to minimum and checked again. If carbon dioxide level is out on minimum, a 4mm allen key (hex key) is used to re-set it (2nd photo).
When the level is correct on minimum it is re-checked on maximum and may need to be re-adjusted. When both are OK the flue sample point is closed and the combustion results are printed out.
The boiler was re-set to run at normal power and the outer casing refitted before handing back control of the heating to the customer.
Different boilers have different servicing requirements but the combustion results, and particularly the combustion ratio, are critical.
Combustion analysis on a Baxi Boston floor standing gas boiler. We are also checking the main burner pressure. This is an open-flued appliance, drawing its combustion air from the room. It tends to draw dust, fibre and animal hair towards it and these can block the filter. It’s very important that the wire mesh air intake filter in the picture is kept clean. If there are pets in the house this filter may need to be regularly hoovered clean!
High efficiency Vaillant EcoTec Plus 824 combi boiler opened up for servicing. This boiler returned a gross efficiency of over 89%. With high efficiency boilers it’s important to make sure that the condensate trap is not blocked with debris as this can cause the boiler to lock out.
This burner was from another Baxi Bermuda, the 551, which was quite a lot older. The heat exchanger had been slowly leaking water onto the burner, probably for years and the boiler had to be replaced.
Parts of a Potterton Kingfisher 2CF open-flued, floor standing boiler. Open flued appliances draw combustion air from the room and have an air connection between the combustion chamber and the primary flue and room air. This makes them inherently less safe than room sealed appliances. They are also less efficient as warm room air is drawn in for combustion and flue dilution and passed out of the building via the flue. Open flued boilers are gradually disappearing as nearly all new boilers are now room sealed and fan flued, making them more efficient and safer.
This is a later Baxi Bermuda back boiler. This type of appliance has been around for about 40 years. They are open flued and draw their air for combustion from room air at low level. This draws in dust and fibre which can block the wire mesh air inlet filter. Regular servicing is very important to their safe operation and, as with all open flued appliances, we recommend that an electronic carbon monoxide alarm is fitted in the room.
Sodium yellow/orange flame tips on a Baxi Bermuda main burner. The colours look quite startling and have been caused by dust particles of metals and metal salts passing through the burner after being disturbed during the boiler service. These colours are harmless and are temporary. They’ll settle down within a few minutes.
If the coloured flame tips were permanent and a pale yellow/white like a match flame or candle flame they would be much more worrying and would indicate poor combustion and a risk of carbon monoxide poisoning!
This pilot injector was taken out of a Glow-worm Ultimate for cleaning. The main burner wouldn’t light because the pilot burner wouldn’t light!
Badly burnt Baxi Solo PCB (printed circuit board). Damage this bad is unusual in a manufacturer’s original part but cloned parts are illegal and are less safe.
Glow-worm Micron fan. The Micron is not one of our favourite boilers because we have found the electronics (printed circuit board) to be unreliable. Glow-worm have produced better models!
Clean blue flame picture on the main burner in a Baxi Solo 2PF gas boiler. Red, mauve and orange colour tinges are fine but pale yellow flames (the colour or a match flame or candle flame) would indicate danger and a possible risk of carbon monoxide poisoning. The red and mauve and “street lamp” orange colours produced when burners are disturbed, or dust round the air intakes is disturbed, are caused by metallic dust particles or metallic salt particles passing through the flames. In many boilers there is a pink or red glow where the flames touch the combustion insulation panels and the panels glow.
Scorched Potterton Suprima circuit board (PCB). This was the original style of PCB which was plagued by problems with the Suprima locking out. The scorching on the board is typical. The later PCB was made by Siemens and completely re-designed. It is an expensive replacement but, thankfully, has proved to be much more reliable.
Broken ignition electrode from an Ideal Icos gas boiler. The ignition electrode is mounted horizontally in the flame and glows red hot while the boiler is running. While it is red hot the metal is softened and, over time the ignition electrode sags under the force of gravity, increasing the distance between the ignition electrode and the earthing electrode. As the gap increases it reaches a point where the ignition spark cannot jump consistently and the boiler may light explosively. The electrodes would normally be replaced, though the ignition electrode can be straightened if it is heated with a blow torch first until it is soft. If the metal is cold when straightening is attempted, the metal is brittle and it will snap as this one did.
During servicing we would normally check either main burner pressure, inlet working pressure or measure the gas rate at the meter. It’s important to know that the correct amount of gas is being burnt.
Clean blue pilot flame heating the thermocouple tip
The nearest row of pins show clear dry joints. These are breaks in the solder connection, visible as a gap between the solder and the pin. This is a recognised fault on many circuit boards including this one from a Baxi Solo 2PF
Premix fan on a Greenstar boiler blows a regulated gas/air mix into the burner
Using a bright torch to illuminate the sump on this condensate trap allows us to see the debris through the translucent plastic. There is only a minimal amount here and the trap does not need to be disturbed. This Vaillant trap is clever. Instead of dribbling condensate continuously into the drain and risking blockage by freezing, this trap functions as an auto-syphon, dumping the warm condensate in intermittent loads of about 150 ml, making freezing much less likely.
Lovekin started servicing gas boilers in Ash, Ash Vale and Aldershot in 1986 and we’re committed to excellent work and great customer service! We know most of our customers on first name terms and many have been with us for over 20 years.
We’re still based in GU12 and we cover an area that stretches out to Camberley, Farnborough, Farnham, Guildford, Aldershot and Woking.
Call us 01252 334440
Or you can email Lou
We believe in good work and good business. It must be good for the company and good for the customer too, providing high quality work, excellent service and fair pricing.
We’re happy to explain what we’re doing, and why. We’ll listen to you and, if we can, we’ll answer your questions. Trust matters to us and makes for a much happier working environment and we continue to be surprised at just how nice our customers are.
We are Worcester Accredited Installers. At present we feel that Worcester Bosch is the best boiler manufacturer because their boilers and backup are first class.
Some information provided on this web site relates to safety and is given in good faith. It is only an indication of our understanding of some of the gas safety issues involved. You must make your own judgements and should take the advice of a Gas Safe Registered engineer on site in your home. We can accept no liability or responsibility for any action or decision you take as a result of reading the information on this site, nor can we accept any liability arising out of misleading, incomplete or incorrect information on this site.
Central heating and gas boiler terminology can be confusing and the meaning of some of the terms used has changed over time. We’ve tried to define some of the terms on a terminology page on this site.
We have a separate web site recording information about central heating problems and fault-finding of gas boilers and central heating controls. You can find it at lovekin.net
The site includes articles on central heating system pressure, high efficiency boilers, gross and net efficiency, condensate drain problems, radiator valves and balancing radiators.