The Baxi 800 is a good combi boiler for a home with one main hot water outlet at a time. Choose the 825, 830 or 836 by output, then compare Baxi’s flow figures with a property-level test. Published flow assumes a stated temperature rise, so it cannot promise the same result at every home.
On this page
- Quick recap
- Check the model output before you compare prices
- Measure the supply with a repeatable installer test
- Compare Baxi’s flow figures with your test
- Check Baxi’s warranty conditions in writing
- Check the regional model and warranty details
- Make the installation quote show every important task
- Use service evidence instead of guessing about failure parts
- Decide if the Baxi 800 fits your household
- Quick questions
Quick facts
- Models
- 825, 830 and 836 combi boilers
- Rated outputs
- 25kW, 30kW and 36kW respectively. See Baxi’s 800 Combi data sheet: https://www.baxi.co.uk/products/boilers/800-combi
- Published hot water flow
- Baxi’s UK data sheet lists 10.2, 12.2 and 15.0L/min at a 35°C temperature rise for the 825, 830 and 836. Check the current table before ordering.
- Best fit
- A home with one main shower or tap demand at a time
- Warranty
- Up to 10 years only under Baxi’s stated registration, installer and service conditions: https://www.baxi.co.uk/support/warranty
- Efficiency
- Check the current product data sheet for the exact seasonal efficiency rating
Quick recap
- Match the 825, 830 or 836 to your hot water demand and heating load.
- Baxi lists the models at 25kW, 30kW and 36kW outputs. Check the current data sheet before ordering.
- Ask the installer to record static pressure, working pressure, measured flow, pipe size and test location.
- Baxi advertises warranty terms only when its registration, installer and service conditions are met.
- Use a Gas Safe registered engineer and get the installation scope in writing.
Check the model output before you compare prices
The Baxi 800 is a wall-hung gas combi range. The 825 is rated at 25kW, the 830 at 30kW and the 836 at 36kW, according to Baxi’s UK product information. Check the current data sheet before purchase: https://www.baxi.co.uk/products/boilers/800-combi.
A combi heats water as it passes through the boiler. It needs no hot water cylinder or loft tank, but it cannot store extra hot water for overlapping demand.
The 825 suits lower demand. The 830 offers more hot water capacity, while the 836 offers the highest output in this range. A higher output does not remove restrictions in the incoming pipework.
Check the case dimensions on Baxi’s current specification sheet before planning the cupboard. Allow for clearances, flue position and service access. If that sounds like your situation, read up on verifying warranty coverage for boilers next.
- 825: 25kW output for lower hot water and heating demand.
- 830: 30kW output for a typical single-shower household.
- 836: 36kW output where a stronger single outlet is useful.
- System boiler with a cylinder: consider this for regular simultaneous shower use.
- compact design
- no cylinder or loft tank
- three output choices
- widely available through heating engineers
- a combi cannot store hot water
- two simultaneous showers can exceed available hot water capacity
- higher output does not cure restricted pipework
Measure the supply with a repeatable installer test
Ask the installer for one documented supply test before choosing the model. The record should show static pressure, working pressure, measured flow in litres per minute, incoming pipe size and test location.
The installer should identify an accessible cold mains point, such as an outside tap or washing-machine valve. They should fit a suitable pressure gauge and flow adaptor without compromising the fitting or causing a leak.
With every outlet closed, record static pressure. Open the test outlet fully and record working pressure while water flows. Measure the discharge into a calibrated container for a timed interval, then convert it to litres per minute. The other half of this decision is water pump garden low pressure.
Repeat the flow test with other significant outlets isolated. Note the stopcock position, meter arrangement, pipe material, pipe size and time of day. The engineer should compare the readings with Baxi’s installation requirements and technical table.
Do not treat around 1 bar as a universal pass or fail. Ask the installer to compare the recorded readings with Baxi’s current operating requirements for the selected model. UK results differ between properties and water suppliers.
| Data point | How it is recorded | Why it matters |
|---|---|---|
| Static pressure | Gauge reading with all outlets closed | Shows available pressure without demand |
| Working pressure | Gauge reading while the test outlet runs fully open | Shows pressure during use |
| Measured flow | Timed discharge converted to L/min | Shows how much water reaches the test point |
| Pipe size and route | Record visible size and restrictions where known | Helps explain pressure loss |
| Test location | Name the tap, valve or adaptor point | Makes the result repeatable |
Fieldpiece - Job Link System Dual Port Manometer Probe Kit - JL3KM2 - Long
Lets the installer measure and record multiple static and draft pressures on-site with its wireless dual-port probe and records up to 8 readings at once (1000' range).
Compare Baxi’s flow figures with your test
Baxi’s UK 800 Combi data sheet lists domestic hot water flow rates at a 35°C temperature rise. The published figures are 10.2L/min for the 825, 12.2L/min for the 830 and 15.0L/min for the 836. Source: https://www.baxi.co.uk/products/boilers/800-combi. Check the current technical table because specifications can change. The other half of this decision is enphase system installation.
A 35°C rise means the water leaves the boiler 35°C warmer than it entered. The figure assumes the boiler reaches its stated output and the water supply can provide the required flow. Colder inlet water needs more energy for the same outlet temperature.
The relationship is Q = P ÷ (4.186 × temperature rise), with Q in litres per second and P in kilowatts. For a 30kW boiler at a 35°C rise, the calculation is 30 ÷ (4.186 × 35) = 0.205L/s, or about 12.3L/min. That matches Baxi’s rounded 12.2L/min figure.
The calculation is a guide, not a promise at the shower. It assumes full useful output, water density close to 1kg/L and no allowance for pipework losses, flow controls or outlet restrictions. Compare the Baxi figure with the installer’s measured result.
| Model | Rated output | Baxi flow at 35°C rise | What to check |
|---|---|---|---|
| 825 | 25kW | 10.2L/min | Compare with measured flow and the demand at your main outlet |
| 830 | 30kW | 12.2L/min | Check whether the supply can deliver the published rate |
| 836 | 36kW | 15.0L/min | Confirm the supply and outlet can use the higher output |
Check Baxi’s warranty conditions in writing
Baxi advertises warranty periods of up to 10 years for eligible products, but the term depends on its current offer and conditions. Baxi’s warranty page sets out requirements covering registration, installation, commissioning and servicing. Source: https://www.baxi.co.uk/support/warranty.
Ask the installer for the exact term for your model, the registration deadline, the approved installer requirement and the service interval. Ask for written confirmation before work starts if the promised term exceeds the standard cover.
Keep the Benchmark commissioning record, installer details, serial number, registration confirmation and service records. Check Baxi’s current warranty terms for any exclusions, required parts and evidence needed for a claim.
Gas work must be completed by a competent person. In the UK, domestic gas work normally falls under the Gas Safe Register scheme. Check the engineer here: https://www.gassaferegister.co.uk. Building Regulations guidance explains notification and competent person routes: https://www.gov.uk/government/collections/building-regulations-guidance.
- longer cover may be available
- terms are published by the manufacturer
- registration and commissioning can be checked
- the headline term is conditional
- poor records can complicate a claim
- installer status matters
Check the regional model and warranty details
This guide uses Baxi’s UK 800 Combi information. Baxi model names, outputs, controls, efficiency ratings and warranty offers can differ outside the UK.
Check the Baxi website for the country where the boiler will be installed. Use that market’s technical data sheet and warranty terms, not a UK retailer page.
Ask the installer to quote the exact model code, output, local approval requirements and warranty registration process. Keep the local documents with the commissioning record.
- Confirm the country-specific model number.
- Use the local technical data sheet.
- Check local gas, flue and notification rules.
- Get the local warranty term in writing.
Make the installation quote show every important task
Ask for a written quote with the exact Baxi model and rated output. The price should show the flue route, controls, condensate work, gas pipe checks, system filter, system cleaning, inhibitor and Building Regulations notification.
Include any changes needed for the new boiler. A replacement may need a new flue, condensate route, electrical connection or gas pipe section. The engineer should explain each change rather than hide it inside “supply and fit”.
Check who will register the warranty and notify Building Regulations. Keep the final invoice, commissioning record and product documentation with the warranty details.
Use service evidence instead of guessing about failure parts
A diverter valve or plate heat exchanger can need attention on any combi, but don’t treat either part as a guaranteed Baxi failure. Ask the engineer for service records, repair data or manufacturer guidance supporting any claim about common faults.
Scale risk depends on water hardness, temperature, flow and maintenance. Your local water supplier can provide hardness information. A qualified engineer can advise on treatment compatible with the boiler instructions.
Follow Baxi’s service instructions and warranty terms. Ask what inspection is required, which consumables are used and how the service record will be retained. Service frequency and cost vary by region and engineer, so get local quotes.
- Check local water hardness with the water supplier.
- Ask for evidence before accepting claims about common failures.
- Keep every service record with the boiler documents.
- Use treatment equipment suitable for the manufacturer’s instructions.
Decide if the Baxi 800 fits your household
Choose the Baxi 800 when one main shower or tap normally runs at a time and the measured supply supports the selected model. Compare the installer’s test with Baxi’s published flow at a 35°C rise.
Consider a system boiler and cylinder when two showers regularly run together or several occupants need stored hot water. The right design depends on space, usage, heating load and the proposed cylinder size.
Get two written quotes from Gas Safe registered businesses. Compare the model, output, flue, controls, filter, cleaning, condensate work, notification, commissioning and warranty registration.
| Check | Good sign | Action if uncertain |
|---|---|---|
| Hot water demand | One main outlet normally runs at a time | Discuss a cylinder if demand overlaps |
| Supply test | Static pressure, working pressure and flow are recorded | Ask for the test method and location |
| Boiler choice | Model output matches the Baxi data sheet | Check the current technical document |
| Warranty | Exact term and conditions are written down | Read Baxi’s current warranty terms |
| Installation scope | Flue, controls, filter, cleaning and notification are listed | Ask the engineer to revise the quote |
- good option for suitable smaller households
- three output choices
- manufacturer warranty terms are available
- not ideal for regular simultaneous showers
- installation quality affects long-term results
- published flow depends on stated test conditions
The Baxi 800 can be a good combi for a home with one main hot water demand at a time. Check the model’s published flow, record the property supply and confirm warranty conditions before buying.
Quick questions
Is a Baxi 800 a reliable boiler?
It can be reliable when correctly sized, installed, commissioned and serviced. Check the current warranty terms, service instructions and local engineer support. Don’t assume a particular part will fail without service evidence.
How many showers can a Baxi 830 run at once?
Don’t choose it by shower count alone. Baxi lists 12.2L/min at a 35°C rise for the 830. Inlet temperature, outlet temperature, measured supply and pipework affect the result. Ask for the documented supply test.
Does the Baxi 800 warranty last 10 years?
Baxi advertises up to 10 years for eligible products, subject to current registration, installer, commissioning and servicing conditions. Confirm the exact term and registration deadline at https://www.baxi.co.uk/support/warranty.
What should a Baxi 800 installation quote include?
It should name the model and output, flue route, controls, condensate work, gas pipe checks, filter, cleaning, inhibitor, commissioning, Building Regulations notification and warranty registration.
How much does a Baxi 800 service cost?
Costs vary by region, engineer and service plan. Get local quotes and ask what the service includes. Keep the service record because Baxi’s warranty terms may require maintenance evidence.
