Saunier Duval boiler repair across London
Repairs where the part can be had, or a new boiler. Ring us, day or night.
- Saunier Duval Thema and Isofast repaired
- All our engineers are Gas Safe registered
Makes we repair
How does booking a Saunier Duval repair work, and what does it cost?
Booking: ring us any time, day or night, or send your details and we ring you back. We'll ask what the boiler is doing, and its model and any code on the display if you can see them. You don't need to know what's wrong. Finding the fault is the engineer's job. Then we tell you when an engineer can come, anywhere in Greater London.
What it costs: a repair's price depends on the part it needs, how long the job takes and when it's done. Our rates are higher in the evening, overnight and at weekends. Once the engineer has found the fault, the rate for the repair is agreed in writing before any work starts.
What happens on a Saunier Duval repair visit?
A repair visit has three stages.
1. Before anything is touched
The engineer covers the floor around the boiler and asks what you have seen and heard since you rang.
2. Finding the fault
The engineer tells you which part has failed, whether they have it with them or must order it, and how long the work should take. No work is done until you say yes.
3. The repair
The engineer fits the part and checks the boiler runs as it should before leaving. The repair is guaranteed for 12 months, covering the parts we fit, the labour and any return visit.
What goes wrong on Saunier Duval boilers most often?
Saunier Duval's own user guides come back to the same problems:
- Low pressure. Most ranges have a light, a lamp or a code for it. Topping up again and again points to a leak.
- Not lighting. An ignition lockout, from the gas supply, the gas valve or the ignition parts.
- Overheating. Air in the system, or water not moving round, from the pump or closed valves.
- Water from the pipe outside. It comes from the safety valve. Leave the valve alone, switch the boiler off and ring us.
- Radiators heating unevenly. Saunier Duval's own answer, on its UK questions page of 2011, is balancing the radiators, which it leaves to the installer.
- Frost. The boiler's frost protection runs with the gas and the electricity on, and protects the boiler, not the whole system.
Saunier Duval fault codes and what they mean
Saunier Duval fault codes depend on the range, and the same number can mean different things on different boilers.
- 05 is an overheat on an Isofast or a Semia Condens. A Thema Condens shows an overheat as F5.
- 21 is low pressure on a Semia Condens or an Isofast Condens.
- 12 is low pressure on an Ecosy, but a control panel fault on an Isofast.
Each panel below names its ranges, and we'll ask for the model when you ring.
Low pressure on a Saunier Duval: a flashing light, 21, 12 or lamp H
What it means: The water in the sealed heating system has lost pressure, so the boiler stops. How it shows depends on the range:
- the pressure light flashing below 0.5 bar on a Thema Condens, ThemaPlus Condens or Isotwin Condens
- 21 on a Semia Condens, at 0.5 bar or less, and on the room unit of an Isofast Condens
- the light flashing red on a Thema Classic
- F with the pressure figure on a Thema Classic Plus
- a reading under 0.7 bar on an EnviroPlus
- 12 flashing on an Ecosy
- lamp H on an SD 30e
- a flashing pressure warning on an Isofast
Possible causes:
- water lost from the system
- a radiator bleed valve left open, which the Semia Condens guide adds
Topping up again and again points to a leak.
What you can do: top up where your user guide shows how. These ranges' guides do, each to its own pressure:
- the Thema Condens, ThemaPlus Condens and Isotwin Condens
- the Semia Condens and Isofast Condens
- the EnviroPlus F24e and F28e
- the Thema Classic and Thema Classic Plus
Each range's pressure is in the reset and pressure section below.
On these ranges, the guide says a competent person fills the system, so ring us:
- the Isofast, Ecosy and Xeon FF
- the SD 30e and EnviroPlus F28e SB
- the Combitek, Thelia and Thema C 23
If water runs from the safety valve's pipe outside, leave the valve alone. Switch the boiler off and ring us. The Thema Classic, Isofast, Combitek, Thelia, Thema C 23 and SD 30e guides all say so. We'll ask what the gauge reads.
Ignition lockout on a Saunier Duval: F1, F4, 04, 01 or lamp F
What it means: The boiler tried to light, found no flame, and has locked out. How it shows depends on the range:
- F1 or F4 on the Thema Condens family and the EnviroPlus. On the EnviroPlus, F1 means it failed to light, and F4 that the flame went out once lit.
- 04 on an Isofast or an Isomax
- 01 with a steady red light on a Semia Condens
- lamp F, the red lockout lamp, on an SD 30e
- a spark symbol on an Isofast, for no gas, or on a Thema Classic Plus
- the light flashing red on a Thema Classic
Possible causes, as the manual lists them:
- no gas or too little gas
- the gas valve set wrongly
- a fault in the ignition electrode or its lead
- a faulty igniter
- the air inlet duct and the igniter's connections (the EnviroPlus manual, the maker's own)
What you can do: see that the gas is on, then reset once, as your user guide shows. Each range's steps are in the reset and pressure section below. For the spark symbol on an Isofast, its guide says switch off, see to the gas supply and switch on again.
If you smell gas, don't reset anything. Ring the National Gas Emergency Service on 0800 111 999.
When you ring, we'll ask which light or code shows, and whether your other gas appliances work.
Overheat on a Saunier Duval: F5, 05, 18, 19 or a boiler that stops
What it means: The water in the boiler got too hot, and a safety device shut the boiler down. How it shows depends on the range:
- F5 on the Thema Condens family and the EnviroPlus
- 05 on an Isofast or a Semia Condens. On a Semia Condens, 05 can also mean the flue overheated.
- 18, the flow above 100°C, or 19, the return above 95°C, on an Ecosy
- the light flashing red on a Thema Classic
- a boiler that has stopped, on a Thelia, Thema C 23, Combitek or Xeon FF
Possible causes:
- the overheat thermostat has operated
- the top temperature was passed
- air in the system, or poor circulation from the pump or closed valves
- the sensors' connections
What you can do: on most ranges, nothing at home, as their guides say to ring. Four guides allow a reset:
- the Semia Condens, once, by its selector
- the Ecosy, by its reset button
- the SD 30e, after cooling for at least four minutes
- the Xeon FF, by the overheat cut-off's button under its controls cover
On an Isofast, the overheat thermostat is reset by an engineer.
If it trips again, ring us. Every guide that allows a reset says so. We'll ask what the gauge reads.
Circulation fault on a Saunier Duval: F23, F26, 11, 23 or E 25
What it means: The water isn't moving through the boiler and the heating as it should. How it shows depends on the range:
- F23 or F26 on the Thema Condens family
- F14, F24, F25 or F26 on an EnviroPlus
- 11, the pump blocked, on an Isofast or an Isomax
- 23, a flow rate fault, on an Isofast Condens
- 9 beside E 25 or E 30 on an Ecosy. E 25 means the water heated too fast, and E 30 that the flow and return are too far apart.
Possible causes:
- the pump
- the heating flow and return
- air holding the water back, for the EnviroPlus's F25
- a system that holds the water back too much, for F26
What you can do: look at the pressure gauge, as for low pressure above, and nothing else at home. The Ecosy guide's own checks, the pump running and the valves open, are written for its after-sales engineer.
Ring us at once. We'll ask for the code and what the pressure reads.
Air flow fault on a Saunier Duval: 02, F2 or a fan that keeps running
What it means: The boiler has found too little air moving through its flue and fan, so it stops for safety. It shows as 02 on an Isofast or an Isomax, and F2 on a Thema Condens. On a Thelia, a Thema Classic or an Isofast, the boiler stops while the fan keeps running.
Possible causes:
- a flue blocked, even in part
- the fan
- the air pressure switch or its tubes
What you can do: see that nothing is blocking the flue's end on the outside wall, as the Thelia and Ecosy guides ask. Do nothing else at home.
Ring us at once. An air flow fault involves the burner's fumes. We'll ask for the code, and whether the fan is still running.
If a carbon monoxide alarm sounds, ring the National Gas Emergency Service on 0800 111 999.
No power on a Saunier Duval: a blank display or no lights
What it means: The boiler has no electricity, so nothing shows on it. On the Thema Condens family the display is blank, and on a Semia Condens the green light is off. On an SD 30e, lamp E is off.
Possible causes:
- a power cut
- the switch at the fused spur or socket turned off
- the fuse on the boiler's board
What you can do: see that the fused spur or plug and the boiler's own switch are on. Saunier Duval's guides say the boiler starts again by itself once the power is back.
On a Xeon FF that won't relight after a power cut, its guide says the overheat cut-off may need its reset button pressed. The steps are in the reset and pressure section below.
MoreEvery other Saunier Duval code and light, in one list
Every code and light in Saunier Duval's papers is set out below, range by range, as each range's own manual gives it. Some manuals survive as library copies, with their text partly jumbled. Where a cause couldn't be matched to its code with certainty, the list says so.
Thema Condens F24E, F30E and F18E SB (a library copy of the maker's manual)
F1 or F4The boiler didn't light (its panel is above)
What the manual lists
No gas or too little, the gas valve's setting, the ignition electrode's lead, the igniter, or the air inlet duct and the igniter's connections
F2An air flow failure, in the user guide's table
What the manual lists
The safety system has shut the boiler down. It needs an engineer
F5The boiler overheated (its panel is above)
What the manual lists
The overheat thermostat, the top temperature passed, the sensors' connections, or air in the system with the heating at its highest
F6The heating flow sensor has a fault
What the manual lists
Not clear in the library copy
F7The hot water sensor has a fault
What the manual lists
Not clear in the library copy
F9The water pressure sensor has a fault
What the manual lists
Not clear in the library copy
F10The heating return sensor has a fault
What the manual lists
The sensor or its cable, or its fixing to the pipe
F11The link to the control panel is lost
What the manual lists
The wiring between the main board and the control panel
F13A fault in the main board's connections
What the manual lists
The connections, or the main board
F16A flame is still sensed more than 5 seconds after the burner stopped
What the manual lists
The gas valve
F18The control panel has a fault
What the manual lists
The control panel
F19The heating sensor is unplugged
What the manual lists
The sensor's connection
F20The software doesn't match
What the manual lists
The wrong control panel or main board, or the wrong product code
F23The water isn't circulating (its panel is above)
What the manual lists
Possibly the pump, or the heating flow and return
F26The flow and return temperatures are too far apart (its panel is above)
What the manual lists
Not clear in the library copy
F28The flow and return sensors keep disagreeing
What the manual lists
Both sensors' connections, or a faulty sensor
F29The thermal fuse
What the manual lists
Not clear in the library copy
Pressure light flashingThe pressure is below 0.5 bar (its panel is above)
What the manual lists
The user guide's step is to top up. Topping up often points to a leak
Blank displayNo power (its panel is above)
What the manual lists
The mains, the spur or the socket. The boiler restarts by itself when the power returns
ThemaPlus Condens F30E and Isotwin Condens F35E (library copies of the maker's manuals)
The ThemaPlus Condens shows the Thema Condens codes above, and F8. The Isotwin Condens shows them without F28 or F29, and adds the codes below.
F8The hot water store's sensor has a fault (the ThemaPlus Condens too)
What the manual lists
Not given
F12, F13A fault in the main board's connection
What the manual lists
The connections
F14The heating flow is above 95°C
What the manual lists
A system fault, possibly the pump, or the flow sensor
F17The supply voltage is below 170 volts
What the manual lists
The electricity supply and its polarity
EnviroPlus F24e, F28e and F28e SB (the maker's manual, 2000225294A.04.04)
F1The boiler failed to light, and locked out (its panel is above)
What the manual lists
No gas or too little, the gas valve's setting, the electrode or ignition lead, the igniter, or the air inlet duct and the igniter's connections
F4The flame went out once lit, and the boiler locked out (its panel is above)
What the manual lists
As F1
F5Overheated (its panel is above)
What the manual lists
The overheat thermostat has operated, or the top temperature was passed. It may also be the sensors' connections, air in the system with the heating at its highest, or the overheat thermostat's connection
F6The heating flow sensor has a fault
What the manual lists
Its cable, the sensor, or the sensor not fitted correctly
F7The hot water sensor has a fault
What the manual lists
Its cable, its connections, or the sensor
F9The water pressure sensor has a fault
What the manual lists
Its connection or wiring
F10The heating return sensor has a fault
What the manual lists
Its cable, the sensor, or its fixing to the pipe
F11The main board is disconnected
What the manual lists
The board's connections
F12The control panel is disconnected
What the manual lists
The board's connections
F13The main board has a fault
What the manual lists
The board's connections
F14The heating flow is above 95°C (its panel is above)
What the manual lists
A system fault, possibly a pump outside the boiler, or the flow sensor
F16A flame is still sensed more than 5 seconds after the gas valve closed
What the manual lists
The gas valve
F17The supply voltage is below 170 volts
What the manual lists
The electricity supply, or its polarity
F18The control panel has a fault
What the manual lists
The control panel, replaced
F19The heating sensor is unplugged
What the manual lists
Its connection
F20A software failure
What the manual lists
The control panel's board, replaced
F24The heating return is above 90°C (its panel is above)
What the manual lists
A system fault, possibly a pump outside the boiler, or the return sensor
F25The temperature is rising too fast (its panel is above)
What the manual lists
Air in the system, holding the water back
F26The flow and return are too far apart (its panel is above)
What the manual lists
The return sensor, or a system that holds the water back too much
Semia Condens F24E and F30E (a library copy of the maker's manual, 982790002/000)
Green light offNo power, so the frost and anti-seize protection can't run
What the guide lists
The switch outside the boiler
Green light, slow flashOn, with the selector at 0 (off). The protection still works
What the guide lists
The gas stays on
Green light, fast flashThe installer's test mode, switched on by mistake
What the guide lists
Turn the selector to 0 until the light flashes slowly
Red light offWorking normally
What the guide lists
Nothing to do
Red light flashing, with a codeA fault
What the guide lists
The code, below
Red light steady, with a flashing codeA lockout
What the guide lists
As 01
01The boiler didn't light, and locked out (its panel is above)
What the guide lists
After work on the gas pipes, it may take several tries to light. Otherwise, reset it. If it locks out again, an engineer checks the combustion and the flue
05An overheat, at the boiler's safety thermostat or the flue's (its panel is above)
What the guide lists
Reset. If it locks out again, ring us. A tripped flue thermostat is replaced by an engineer
06The heating temperature sensor is damaged
What the guide lists
An engineer
21The pressure is 0.5 bar or less (its panel is above)
What the guide lists
Top up to 1 to 1.5 bar, cold. If it keeps falling, it points to a leak, or a radiator bleed valve left open
Isofast Condens F35E (a library copy of the maker's manual, in part)
The Isofast Condens is run from a wireless room unit. On a fault, the red lights on the boiler and the room unit flash, and the boiler's screen shows a code. For some faults the room unit shows the steps. For others, it asks you to call and give the flashing code. 21 means the system pressure is too low, to be filled to 1.5 bar, and 23 a flow rate fault, to call about. Its full code table didn't come through in the library copy.
The older Saunier Duval ranges: every code and light
Isofast F28E and F35E (the maker's manual, 115919 B 05/99), and Isomax F28E (a library copy)
The warning light comes on, and the display shows the code on the left with FF and a telephone symbol. A spark symbol means no gas, and a flashing pressure warning low pressure. The Isomax's manual prints the same list.
- 00No fault
- 02An air flow fault, from the flue or the fan (its panel is above)
- 04An ignition fault, with no flame detected (its panel is above)
- 05Overheated. The overheat thermostat on the flow pipe needs resetting by an engineer (its panel is above)
- 06The heating sensor or its wiring
- 07The hot water sensor or its wiring
- 08The hot water store's sensor or its wiring
- 09The system pressure sensor has a fault
- 10A fan fault
- 11The pump is blocked (its panel is above)
- 12The control panel has a fault
- 13The main board has a fault
- 14The water is over 97°C, and the pump runs on
- 15A fault in the gas valve's mechanism
- 20A communication fault
Ecosy 24E, 28E, SB24E and SB28E (the maker's manual, 104605 E 07/99)
The left figure shows what the boiler is doing, and the right figures the temperature. A lockout is cleared with the reset button.
- Display: 0No call for heat or hot water
- Display: 1Clearing the air before lighting
- Display: 2Lighting
- Display: 3Heating
- Display: 4Hot water
- Display: 5Waiting for the air pressure switch
- Display: 6Off, as the temperature is reached
- Display: 7, 8The pump running on, after heating or after hot water
- Display: 9A safety shutdown
- Display: 9 with E 25The water heated too fast, over 4°C a second (its panel is above). The boiler stops for 10 minutes and tries again. After 5 tries it locks out, showing 9 25
- Display: 9 with E 30The flow and return are more than 40°C apart (its panel is above). The boiler stops for two and a half minutes, and after 20 tries locks out
- Display: 18The flow is above 100°C, and the boiler locks out (its panel is above)
- Display: 19The return is above 95°C, and the boiler locks out (its panel is above)
- Display: 12, flashingLow pressure, or the gauge below 1 bar (its panel is above). The guide says the system needs filling by the after-sales engineer
Thema Classic and Thema Classic Plus (the maker's manuals, 4000124050-2 and 4000121771-2)
The Thema Classic has one light. It is green when on, and flashes red for any fault, water loss and a pressure of 2.5 bar or more among them. Its fault-finding charts name these codes:
- F1: the burner not lighting, or lighting and going out with no flame sensed
- F1 or F4: the burner lighting, then going out
The Thema Classic Plus's running light turns from green to flashing red. Its display flashes F with a symbol for the fault:
- a spark for ignition
- the pressure figure for low pressure, such as 0.4 bar
- a high pressure fault at 2.5 bar or more
Neither manual gives a full numbered list. Both have an overheat thermostat with a reset button inside the case, for an engineer.
SD 30e and SB 30e (the maker's manual, 221873B.07.01)
The lamps on the front, as the manual's key gives them. The red lockout lamp F lights when the burner goes out during a call for heat or hot water, not when there is none.
- Lamp: F (red)The burner went out during a call for heat or hot water, and the boiler locked out
- Lamp: HLow water pressure
- Lamp: LSummer setting
- Lamp: KWinter setting
- Lamp: DThe burner is lit
- Lamp: EThe power is on
- Lamp: PA call for hot water
- Lamp: QA call for heating
Xeon FF (the maker's manual, 221659B.02.00)
The Xeon FF has no lights for you. Four neon lamps on its circuit board are an engineer's fault-finding aid, and the manual says a failed neon alone doesn't justify a new board. The overheat cut-off has a reset button under the controls cover.
Thelia 23, 23 E and SB 23, Thema C 23 and Combitek F23E (the maker's manuals, 1996 to 1998)
These boilers have no codes or lights. On a Thelia, an air flow device shuts the boiler off while the fan keeps running, and lets it restart once the fault clears. An overheat device shuts it down. On a Thema C 23 and a Combitek, an overheat shuts the boiler down. Their faults go by what the boiler is doing: no heating, or no hot water.
Not found
No fault list was found in the maker's papers for these:
- the Xeon Condens
- the Thema Classic F30E, F35E and system models, beyond what is above
- the Envirotek F28E
- the models made before 1996
The fault codes page explains how fault codes work on every make, and what each kind of code means.
Sources: Saunier Duval's own manuals for the EnviroPlus, Thema Classic, Thema Classic Plus, Isofast, Ecosy, SD 30e, Xeon FF, Thelia, Thema C 23 and Combitek, from archived copies of its old UK website. Library copies of the maker's manuals for the Thema Condens, ThemaPlus Condens, Isotwin Condens, Isofast Condens, Semia Condens and Isomax. All were read on 11 October 2026. The causes are in the order the manuals give them, not ranked by how often they happen.
Which Saunier Duval boiler do you have, and how does it show a fault?
Saunier Duval boilers show a fault in different ways by range. Some show F and a number, some a number of two figures, some a light or lamp, and some no code at all. The model name is on the front of the boiler, and we'll ask for it and any code when you ring.
Older still are the 500 series, the System 400, the SD 623, the Themis 23 and the SD 235C. The government's database lists them until 1992 or earlier. The 500 series c models, the Themis 23 and the SD 235C are open-flued, with no fan.
Not every light is a fault. On an Ecosy, the left figure counts through what the boiler is doing, from 0, no call for heat, to 4, hot water. On a Semia, a slowly flashing green light means the selector is at off, with the frost protection still on.
Are Saunier Duval boilers still sold in the UK?
Saunier Duval boilers are no longer sold in the UK. The brand is still sold in France, Spain and other countries, as part of the Vaillant Group. The group's own page for the brand lists its websites for Austria, France, Hungary, Poland, Romania and Spain, and none for the UK. The group's page says Glow-worm is very popular in Britain.
Saunier Duval in the UK, until about 2011
Saunier Duval boilers were sold in the UK until about 2011, by Hepworth Heating Ltd of Belper, Derbyshire. The UK manuals name that company, which is now Vaillant Group UK Limited. The maker's UK website still listed a range of boilers in August 2011, and had gone by late 2011.
Glow-worm, in the same group since 2001
Glow-worm and Saunier Duval came into the Vaillant Group together in 2001, when it took over Hepworth Heating. Some Glow-worm boilers were made by Saunier Duval, and the other way round. The government's boiler database lists Glow-worm's Xtrafast 96 and 120 under the same GC numbers as the Isofast F28E and F35E. It names Saunier Duval as their maker, and Glow-worm as the first maker of the SD 30e. Glow-worm boilers: their codes and repairs.
The maker hasn't said whether Saunier Duval parts and support carry on in the UK. No page of Vaillant's, Glow-worm's or the Vaillant Group's that we read in October 2026 offers support, parts or manuals for Saunier Duval boilers.
We repair the Saunier Duval boilers still running in London homes. Before any repair is agreed, the engineer tells you whether the part yours needs can be had.
Is a Saunier Duval worth repairing?
A Saunier Duval is often worth repairing. Age alone doesn't decide it. What decides it:
- whether the part can be had
- the rate for the repair against a new boiler
- how often the boiler has failed
The choice is yours.
Parts: Saunier Duval hasn't said, in anything we found, whether its parts are still supplied in the UK. Vaillant pledges parts for up to 15 years after a boiler stops being made. That pledge is written about Vaillant boilers, and doesn't name Saunier Duval. Before any repair is agreed, the engineer tells you whether the part your boiler needs can be had.
Age: the newest Saunier Duval boilers were on the maker's UK list in August 2011. The non-condensing ranges are older, their manuals dated 1996 to 2003.
A new boiler is the other road. We fit new boilers too, quoted from photos of the old boiler or after a free survey visit. Our engineers size each one with a heat loss calculation, and the makes we fit most are Vaillant and Worcester Bosch.
Approved Document L sets the efficiency a new gas boiler in England must reach, with a lower figure in exceptional circumstances alone. A Thema Classic, Isofast, Isomax, Xeon FF, SD 30e, Thelia, Thema C 23 or Combitek is non-condensing. When one is replaced, a condensing boiler goes in, not like for like. The rule is for the new boiler, not for a working old one. See what a new boiler includes, and how the day goes.
How do you reset a Saunier Duval boiler?
Saunier Duval boilers reset in different ways by range, as their user guides show. Reset once, for a fault the guide allows:
- Thema Classic, Thema Classic Plus and EnviroPlus: switch off at the boiler's on/off switch, wait five seconds, and switch on again.
- Thema Condens, ThemaPlus Condens and Isotwin Condens: the same, for F1 or F4. For any other code, ring us.
- Semia Condens: turn the selector to the reset position until the red light goes out, then back to summer or winter.
- Ecosy: press the reset button on the control panel after a safety shutdown.
- SD 30e and SB 30e: let the boiler and the system cool for at least four minutes. Then press the reset button, marked G, under the red lockout lamp. It takes a few seconds.
- Xeon FF: take off the controls cover and press the overheat cut-off's reset button.
- Isofast F28E and F35E: for the spark symbol, switch off, see to the gas supply and switch on again. Its overheat thermostat is reset by an engineer.
- Thelia, Thema C 23 and Combitek: their guides give no reset. An overheat shuts them down, so ring us.
If the fault comes back after a reset, ring us, as the guides say.
A pilot light: the Thelia 23, the Thelia SB 23 and the Thema C 23 have a pilot that is lit by hand. Their user guides show how. Lighting a pilot is work on the gas, so if yours has gone out, ring us. Every other Saunier Duval lights itself.
If you smell gas, don't relight anything. Ring the National Gas Emergency Service on 0800 111 999.
The pressure: what the user guide lets you do depends on the range.
- Thema Condens, ThemaPlus Condens and Isotwin Condens: below 0.5 bar the pressure light flashes. The guide shows the blue tap under the boiler, and a pressure of 1 to 2 bar.
- Semia Condens: 21 shows at 0.5 bar or less. The guide says top up to 1 to 1.5 bar, cold.
- Isofast Condens: the room unit's message says fill to 1.5 bar.
- EnviroPlus F24e and F28e: below 0.7 bar, the guide shows the filling taps under the boiler, to 1.0 bar. On the EnviroPlus F28e SB, a competent person fills it.
- Thema Classic and Thema Classic Plus: the guide shows the filling device under the boiler, to 1.0 bar. The Thema Classic F18E SB's filling device is outside the boiler. At 2.5 bar or more the light flashes red, so ring us.
- Isofast, SD 30e, Xeon FF, Ecosy, Combitek, Thelia and Thema C 23: the guides say a competent person fills the system, so ring us.
Topping up again and again points to a leak. If water runs from the safety valve's pipe outside, leave the valve alone and switch the boiler off. Either way, ring us.
In cold weather, leave the gas and the electricity on, as most of the guides say, so the boiler's frost protection can run. It protects the boiler, not the whole heating system.
No Saunier Duval guide we read covers a frozen condensate pipe. The safe checks for any boiler that won't light, a frozen pipe among them, are on the page on a boiler that won't fire up.
Sources: Saunier Duval's user guides, range by range. Those for the Thema Condens, ThemaPlus Condens, Isotwin Condens, Semia Condens and Isofast Condens are library copies.
Is your Saunier Duval boiler still under guarantee?
A Saunier Duval boiler's guarantee papers say who repairs it under guarantee, so look at those first. Saunier Duval's UK website had gone by late 2011. No page of Vaillant's, Glow-worm's or the Vaillant Group's that we read in October 2026 offers support, parts or manuals for Saunier Duval boilers.
The UK manuals name Hepworth Heating Ltd of Belper, now Vaillant Group UK Limited. The phone numbers they print are out of date.
Once the guarantee has ended, or for a fault it doesn't cover, ring us. We are an independent firm, with no connection with Saunier Duval.
What does a Saunier Duval boiler service cover?
A Saunier Duval boiler service by our engineers covers the same checks as any boiler service.
Where a home is let, the law asks for each gas appliance and flue to be checked for safety within 12 months of fitting. After that, the check is every 12 months at most, with a record kept. Saunier Duval's manuals point landlords to that duty.
If an engineer finds a dangerous fault, the gas industry's procedure for unsafe situations applies. A boiler that is Immediately Dangerous is disconnected, with your permission. One that is At Risk is usually turned off, with your permission, and labelled. That is about what is found, never the boiler's age.
The HSE's warning signs of carbon monoxide are:
- yellow or orange flames where they should burn blue
- soot or staining
- a pilot light that often blows out
- more condensation on the windows
If you see them:
- Switch the boiler off.
- Open the doors and windows.
- Ring the National Gas Emergency Service on 0800 111 999.
We service every Saunier Duval model, and give you the rate on the call. What a boiler service includes, check by check.
Sources: the Gas Safety (Installation and Use) Regulations 1998, regulation 36, and the gas industry's procedure for unsafe situations (IGEM/G/11).
Do you cover my part of London?
Yes: we travel all over Greater London, day and night, to repair Saunier Duval boilers in every borough, north and south of the Thames. Pick your side of the river to see the areas.
North of the river
Every borough north of the Thames: Barnet and Enfield at the top, Hillingdon in the west, Havering in the east, and central London in between.
South of the river
Every borough south of the Thames: Richmond and Kingston in the west, Bexley in the east, and down through Croydon to Sutton and Bromley.
The other makes we repair
Every main make, combi, system and regular. Pick yours for its common faults, its codes and how we repair it.
Questions about Saunier Duval boilers
Do I need to know what's wrong with my Saunier Duval boiler before I ring?
No. Ring and say what the boiler is doing, or what you can see or hear, and we'll ask the rest. Finding the fault is the engineer's job.
Do you charge more in the evening or at weekends?
Yes. Our rates are higher in the evening, overnight and at weekends.
Can you repair an older Saunier Duval boiler?
Usually, yes. It depends on the boiler's condition and whether its parts are still made. We'll ask for the model when you ring.
Is my Saunier Duval boiler worth repairing?
Often, yes. Age alone doesn't decide it. What decides it is the rate for the repair against a new boiler, and how often the boiler has failed. The engineer gives you the rate for the repair, and the choice is yours. When a new boiler makes more sense.
Do you fit Saunier Duval's own parts?
Yes, the maker's own parts wherever they are available. Each repair is guaranteed for 12 months, covering the parts we fit, the labour and any return visit.
How much does a Saunier Duval boiler repair cost?
The cost of a repair depends on the part, how long the job takes and when it's done. The rate is agreed in writing before the repair starts. How our rates work.
Is Saunier Duval still in business?
Saunier Duval is still in business, as a brand of the Vaillant Group sold in France, Spain and other countries. Its boilers are no longer sold in the UK. We repair the Saunier Duval boilers still running in London homes.
Are Saunier Duval and Glow-worm boilers related?
Saunier Duval and Glow-worm are both brands of the Vaillant Group, which brought them in together in 2001, when it took over Hepworth Heating. Some Glow-worm boilers were made by Saunier Duval, and the other way round. Glow-worm boilers: their codes and repairs.
Can you still get Saunier Duval parts?
Saunier Duval parts are a question for each repair. The maker has said nothing we found, either way, about its parts in the UK. Before any repair is agreed, the engineer tells you whether the part your boiler needs can be had. A new boiler is the other road. Vaillant's pledge of parts for up to 15 years is about Vaillant boilers.
Why does my Saunier Duval boiler keep losing pressure?
Pressure that keeps dropping on a Saunier Duval points to a leak. The Semia Condens guide adds a radiator bleed valve left open. Top up where your user guide shows how, and ring us.
Do Saunier Duval codes mean the same on every model?
Saunier Duval codes differ by range, so the same number can mean different things. 05 is an overheat on an Isofast or a Semia Condens, where a Thema Condens shows F5. 21 is low pressure on a Semia Condens or an Isofast Condens. 12 is low pressure on an Ecosy, but a control panel fault on an Isofast. We'll ask for the model when you ring.
Is it illegal to have an old Saunier Duval boiler?
An old Saunier Duval boiler isn't unlawful because of its age. The efficiency rules in Approved Document L apply to a new boiler. The law on gas safety is about danger. A gas appliance known or suspected to be dangerous mustn't be used, under regulation 34 of the Gas Safety (Installation and Use) Regulations 1998. A Gas Safe registered engineer can tell you whether yours is safe.
Who makes Saunier Duval boilers?
Saunier Duval is a French brand, founded in 1907 by Charles Saunier and Maurice Duval, and part of the Vaillant Group since 2001. Its UK boilers were sold by Hepworth Heating Ltd of Belper, now Vaillant Group UK Limited. Saunier Duval's own UK history page says it made the first wall-hung combi boiler, in 1963.
Don't know what has failed? You don't have to.Ring and say what you can see or hear. Finding the fault is our job.
Calls answered 24 hours a day
A real person answers and gives you the rate on the call. No work starts until you say yes, and repairs are guaranteed for 12 months: parts, labour and any return visit.
