Is the Baxi Duo-tec any good?
It is a solid, unglamorous combi rather than a class leader, and that is how it should be judged. Four output sizes, honest flow rates, and a boiler the trade knows well.
What it does well
- A size for most homes. Baxi's manual covers the Duo-tec 2 Combi 24, 28, 33 and 40 GA, with maximum hot water outputs of 24, 28, 33 and 40kW.
- Genuinely good hot water on the bigger models. At a 35°C rise the 40 delivers 16.4 litres a minute, the 33 gives 13.5 and the 28 gives 11.5.
- Off the gas grid? It converts. Baxi states the boiler is factory set for natural gas (G20) and "can be converted to operate on Propane (G31)", so it is an option for rural homes on an LPG tank.
Where it is weaker, honestly
- It is off Baxi's current UK line-up. The full-size Duo-tec Combi is not on Baxi's combi boiler page, and its manual sits in Baxi's discontinued products library. Buying new today, you are choosing between its replacements.
- The frost protection is narrower than it sounds. Baxi's manual warns in capitals that leaving the boiler powered gives "BOILER frost protection. NOTE: THIS WILL NOT PROTECT THE SYSTEM!" Pipework and radiators still need a system frost thermostat.
- It is a big box. 763 x 450 x 355mm, plus 175mm above and 450mm in front for servicing. The Duo-tec Compact is 700 x 395 x 285mm.
- It is a combi, so hot water is made on demand. Run a shower and a hot tap together and the flow at each drops, so two bathrooms in daily use want a cylinder.
If your Duo-tec is playing up, the fault code is the fastest route to an answer: see our Baxi fault code list or the common faults below.
What size Baxi Duo-tec do I need?
Boiler size is about hot water, not room count. The kW number is the maximum hot water output, but what decides whether a shower feels good is the flow rate in litres per minute. A typical mixer shower wants around 10 to 12 litres a minute.
| Model | Hot water flow at 35°C rise | Realistically suits |
|---|---|---|
| Duo-tec 24 | 9.8 litres/min | Flats and small houses, one bathroom, no second outlet in use at the same time |
| Duo-tec 28 | 11.5 litres/min | Two and three-bedroom houses, one bathroom |
| Duo-tec 33 | 13.5 litres/min | Three and four-bedroom houses, one bathroom plus a cloakroom |
| Duo-tec 40 | 16.4 litres/min | Larger houses wanting a strong shower, or an occasional second outlet |
Two caveats worth more than the table. Those are Baxi's figures at a 35°C rise, and in winter your incoming mains is colder, so real flow is lower. And no combi beats your incoming mains: if the cold supply is weak, a 40kW boiler will not fix it. Baxi also warns that "it is strongly recommended that the DHW flow rate DOES NOT fall below 2.5l/min".
How much does a Baxi Duo-tec cost?
Baxi does not publish a retail price for the Duo-tec, and because it is off the current UK range, what you are quoted depends on remaining stock and the installer. Anybody quoting a single national "Baxi Duo-tec price" is guessing. What moves the price of a fitted combi is this:
- The boiler size. A 40 costs more than a 24 for the same labour.
- Whether the boiler moves. Swapping like-for-like on the same wall is cheapest. Another room means new gas, water, flue and condensate runs.
- Conversion work. Boiler and cylinder to combi means removing tanks, capping pipework and making good.
- Flue route. The Duo-tec uses a 100mm flue. Long runs, roof terminals and scaffolding add cost.
- Boiler Plus compliance in England. A new gas combi in England needs one extra measure, and that is a real line on the quote.
Get three itemised quotes from Gas Safe registered installers and compare like for like, controls included. Weighing a repair against a replacement? Our guide on repairing or replacing a boiler walks through it, and our new boiler page explains how we help you arrange a replacement boiler.
Baxi warranty on a Duo-tec: what actually applies
The "7-year parts and labour warranty" Baxi publishes today is for the Duo-tec Compact Combi, the model carrying the Duo-tec name in its current range. Baxi publishes no warranty length for the discontinued full-size Duo-tec, so yours depends on what was registered at installation. Whichever term applies, Baxi's conditions are the same, and that is where people get caught.
Baxi's own warranty page is blunt about it: "If the boiler warranty is not registered within 30 days of installation, the Benchmark checklist is not completed by the installer during installation or an annual service of the boiler is not carried out, the warranty will revert to the standard one year."
A Baxi warranty of any length needs all three of these, every year:
- Registration within 30 days of installation, by you or the installer.
- A completed Benchmark commissioning checklist. Baxi's manual states that failing to commission to the instructions and complete the checklist "will invalidate the warranty". It sits at the back of the manual and should have been left with you.
- An annual service, carried out properly and recorded. The Duo-tec even prompts you: after 11 months of operation a "Service Due" message appears on the display.
Moved into a house with a Duo-tec on the wall, or lost the paperwork? Start with our Baxi warranty guide. Where Baxi accepts the warranty, the parts and labour are covered; the same repair outside warranty is charged.
What goes wrong with a Baxi Duo-tec?
The Duo-tec shows a flashing two or three-digit code on the display. These are the ones Baxi's manual singles out.
| Code | What Baxi says it means | What it usually points to |
|---|---|---|
| 118 | Primary system water pressure too low, displayed when pressure is under 0.5 bar | The commonest one by far. Usually a slow leak, a bled radiator, or a failing expansion vessel |
| 117 | Primary system water pressure too high, displayed above 2.7 bar | Over-filling, or a filling loop left open |
| 133, 134, 135 | Gas supply interrupted, ignition failed, or flame not detected | Gas supply, ignition electrode, or gas valve. An engineer's job |
| 128 | Flame failure during normal operation | Combustion or flue-related. An engineer's job |
| 110 | Safety thermostat operated: overheat of the primary system water | Poor circulation, a stuck pump, sludge or a blocked heat exchanger |
| 125 | Circulation fault in the primary circuit | Pump, air lock, or system debris restricting flow |
| 160 | Fan or fan wiring fault | Fan failure, a common wear item on any condensing boiler |
| 20, 28, 40, 321 | NTC sensor faults (heating, flue, return and hot water) | A failed sensor. Usually a straightforward parts replacement |
What you can safely do yourself. If the display shows 118, check the pressure gauge and top the system up using the filling loop: our Baxi boiler pressure guide walks through it. Baxi's manual says that for codes 110, 125, 133, 134, 135, 09, 15, 128 and 384 you can hold the reset button for one to three seconds. Do that once. Baxi's own instruction is that if the codes appear regularly, "further investigation is required", which means a Gas Safe registered engineer.
What you must not do. Never open the casing, and never touch the gas valve, burner or flue. That is for a Gas Safe registered engineer only. If you smell gas or suspect carbon monoxide, stop using the appliance and call the National Gas Emergency Service on 0800 111 999.
Duo-tec throwing a code?
Tell us the number on the display and we will arrange a Gas Safe registered engineer to look at it. Call-outs run Monday to Friday, 08:00 to 18:00, and we aim to get an engineer out to you within two working days.
Baxi Duo-tec specifications
All figures below are from Baxi's Duo-tec 2 Combi GA Range Installation & Servicing Instructions, which notes that "all data in this section are nominal values and subject to normal production tolerances".
| 24 | 28 | 33 | 40 | |
|---|---|---|---|---|
| Max hot water output | 24 kW | 28 kW | 33 kW | 40 kW |
| Max heating output (condensing) | 21.2 kW | 25.3 kW | 29.6 kW | 33.8 kW |
| Hot water flow at 35°C rise | 9.8 l/min | 11.5 l/min | 13.5 l/min | 16.4 l/min |
| Hot water flow at 30°C rise | 10.9 l/min | 12.9 l/min | 15.3 l/min | 18.3 l/min |
| Height | 763mm | 763mm | 763mm | 763mm |
| Width | 450mm | 450mm | 450mm | 450mm |
| Depth | 355mm* | 355mm* | 355mm* | 355mm* |
| Installation lift weight | 36kg | 36kg | 38kg | 39kg |
| Flue diameter | 100mm | 100mm | 100mm | 100mm |
| NOx class | 5 | 5 | 5 | 5 |
| SAP 2009 annual efficiency (natural gas) | 89% | 89% | 89% | 89% |
| SAP 2009 annual efficiency (LPG) | 91% | 91% | 91% | 91% |
*Baxi notes the depth "can be reduced to 345mm by removing the boiler control access flap". Heating pressures: 0.5 bar minimum and 2.5 bar maximum operating, 1 to 2 bar recommended. Clearances are 175mm above, 150mm below, 5mm each side and 450mm in front for servicing.
Duo-tec, Duo-tec Compact or Baxi 800?
If you are shopping now rather than diagnosing a boiler you own, these are the honest comparisons.
- Baxi Duo-tec Compact Combi. Still in Baxi's range, in a smaller 700 x 395 x 285mm cabinet, with a "7-year parts and labour warranty" and uSense 2 compatibility. Baxi notes it is "Exclusively available from APP Plumbing & Heating", so not every installer offers it.
- Baxi 800 Combi 2. Baxi's flagship, with a "10-year parts and labour warranty", the longest Baxi offers. Note the condition: the magnetic filter supplied must be "fitted and maintained for the length of the warranty, otherwise the warranty will revert to only two years".
- Baxi 600 Combi 2. A "7-year parts and labour warranty", and the closest current equivalent to the Duo-tec.
- A heat-only or system boiler. With two bathrooms in daily simultaneous use, stored hot water beats any combi.
Cover for your Baxi, without the guesswork.
Smart Plan is a service plan, not insurance. You build your own cover and pick only the parts of your home you want covered, so boiler and central heating cover can stand on its own. Parts and labour are included up to the cover limit, which is £500 in a 12-month period if your boiler is under 7 years old and £200 if it is over 7. A £95 call-out fee applies, including in your first 30 days and for excluded issues, and cover still applies during that period. Once you use a service, a 12-month agreement period begins. Repairs are carried out by a national network of thousands of Gas Safe registered engineers.
Baxi Duo-tec questions, answered
Is the Baxi Duo-tec 28 a good boiler?
For a two or three-bedroom house with one bathroom, yes. It gives 11.5 litres a minute at a 35°C rise and 25.3kW of heating output, a sensible match for that property. Buying new you would look at a Baxi 600 or the Duo-tec Compact instead, but there is nothing wrong with a well-serviced Duo-tec 28 already on the wall.
How long does a Baxi Duo-tec last?
Baxi does not publish a design life for the Duo-tec, so anyone quoting an exact number of years is guessing. Baxi does say the boiler needs an annual service to keep its warranty, and it prompts you with a "Service Due" message after 11 months. A boiler serviced every year, on clean inhibited water, lasts considerably longer than one that is not.
Why does my Baxi Duo-tec keep losing pressure?
Error 118 appears when the primary system water pressure drops below 0.5 bar. Topping up puts the number back but does not answer where the water went. The usual causes are a small leak on pipework or a radiator valve, water pushed out through the pressure relief valve, or an expansion vessel that has lost its charge. Topping up more than twice a year needs looking at.
What does Boiler Plus mean for a new Duo-tec in England?
Boiler Plus applies "whenever a boiler is installed in an existing heating system anywhere in England". It sets a minimum of 92% ErP, makes timers and room thermostats an explicit requirement, and adds one further measure for combis: flue gas heat recovery, weather compensation, load compensation, or smart controls with automation and optimisation. The Duo-tec supports an optional outdoor sensor for weather compensation (Baxi's installation manual, section 18). Ask your installer which measure your quote includes.
Is a boiler warranty the same as boiler cover?
No, and it is the most common mix-up we hear. A warranty covers the boiler failing through a manufacturing fault on Baxi's terms, and Baxi reduces it to one year if registration, the Benchmark checklist or the annual service is missed. A cover plan is a service agreement you buy separately for breakdown repairs, and on Smart Plan it can extend to your central heating, not just the boiler.
My Duo-tec has broken down and it is out of warranty. What now?
Get the fault code off the display first, then decide between a one-off repair and cover. If the boiler is old and the repair is a major component, replacement is often the better spend. If you want an engineer booked either way, tell us what the boiler is doing and we will arrange a visit. Call-outs are Monday to Friday, 08:00 to 18:00, and we aim to get an engineer out to you within two working days.
Sources. Specifications come from Baxi's own Duo-tec 2 Combi GA installation manual, the Baxi Duo-tec Compact Combi page and Baxi's warranty information. Boiler Plus is set out in the government's Boiler Plus factsheet, and the HSE explains what to do if you smell gas.
