Underfloor heating suits the room if the floor is coming up anyway, in an extension, a new build or a full refurbishment, and the finish will be tile, stone or a thin wood covering. Radiators suit the room if the existing floor is staying, if the space sits under carpet and underlay, or if you heat it for a few hours at a time rather than all day. Both can run from the same boiler, provided a qualified heating engineer separates the circuits and limits the underfloor flow temperature.
Underfloor heating or radiators: what actually differs in a single room
Underfloor heating and radiators both carry heat from the boiler in water, but they run at different temperatures and need different fitting work. The table sets out the differences that decide most rooms.
| Criterion | Underfloor heating (wet) | Radiators |
|---|---|---|
| Flow temperature | Around 30 to 40 °C; Vaillant quotes about 35 °C for a wet underfloor circuit | Up to around 70 °C on conventional sizing, about 50 °C on medium-temperature sizing |
| Fitting into an existing room | The floor covering comes up, pipes go in and the covering is relaid, so it suits new build, extensions and full refurbishments | A radiator can be changed or added as a like-for-like job, with far less disturbance to the room |
| How the room responds | Heats the room more slowly, so it suits spaces held at a steady temperature | Responds more quickly, which suits a room heated for a few hours at a time |
| Floor covering | The design has to allow for the thermal resistance of the covering; a typical 6 mm carpet underlay causes a large temperature drop | Works under any floor finish, including thick carpet and underlay |
| Space in the room | No visible heating surface, so the wall stays free for furniture | Each radiator uses wall space, and a lower flow temperature means a larger radiator |
| Repairs | A fault in a buried circuit means work on the floor itself | A failed radiator is replaced with a same-size unit, with little mess |
The answer changes room by room. A tiled kitchen floor that is being rebuilt points one way. A carpeted bedroom in a 1930s terrace points the other. In both cases the sizing comes from a room-by-room heat loss calculation, and that is the installer's work, not a rule of thumb.
When underfloor heating is the right choice for the room
Underfloor heating makes sense when the floor is being built or lifted anyway. Retrofitting it means taking up the existing covering, laying the pipes and relaying the finish, so it belongs to a bigger job rather than a single upgrade.
- A single-storey extension or a new floor build-up, where the pipes go in before the finish
- A kitchen, utility room or bathroom finished in tile or stone, the coverings usually laid over underfloor heating
- An open-plan room where you want the walls clear of heating surfaces
- A full refurbishment, when the covering comes up across the whole room
- A system planned around a low flow temperature, since an underfloor circuit works at around 30 to 40 °C
Wood floors such as parquet are used over underfloor heating as well as tile and stone. Low flow temperatures also suit heat pump technology, if that is a later consideration for the house. The choice between a wet circuit and electric mats is a separate question, covered in wet or electric underfloor heating. We fit wet systems as part of our underfloor heating work.
When radiators are the better choice for the room
Radiators are the better choice when the existing floor stays where it is. Swapping or adding a radiator keeps the work in the room above floor level, which matters when only the heating is being changed.
- A bedroom or living room in an occupied house, where the floor and its covering are staying
- Rooms with carpet and underlay, because a typical 6 mm underlay causes a large temperature drop over underfloor pipes
- Rooms you heat for a few hours at a time, where faster response matters more than even warmth
- Older or poorly insulated buildings, where underfloor heating is limited in what it can deliver
- A single room that needs one radiator replaced or added, rather than a whole system rebuilt
Radiators do get larger as the flow temperature falls, because output drops when the gap between water and air narrows. Whether the radiators already on your walls can still cover the heat loss comes out of a full assessment of the house by the installer, not from the age of the radiator. Do your radiators need replacing with the boiler goes through that in more detail.
Why the 55 °C flow temperature rule in England shapes the underfloor heating or radiator decision
In England, a new or fully replaced wet heating system in an existing home must be sized to meet the heat loss at a flow temperature of 55 °C or lower. That comes from Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments). Where 55 °C is not feasible, for example because there is no room for larger radiators, the system is designed for the lowest practicable temperature. The requirement covers space heating, not hot water.
An underfloor circuit already works in that range, at around 30 to 40 °C. Radiators meet it by getting bigger, which is why a low-temperature design often changes the radiator schedule rather than the pipework. A 2026 edition of Approved Document L has also been published; which edition applies to a particular job depends on transitional arrangements, so confirm it with your installer or building control before the design is fixed.
Can underfloor heating and radiators run from the same boiler?
Underfloor heating and radiators can share one boiler, and a mixed system is often the practical answer on an extension or a part refurbishment. The underfloor goes in the new area, the radiators stay in the rest of the house. The two then need separate circuits and separate control, because they run at different temperatures.
Manufacturer instructions set out how that separation is built. Worcester Bosch requires the underfloor plastic pipe to be limited by a thermostatic mixing valve to around 50 °C, with at least 1000 mm of copper or steel pipe at the boiler connection, and a spring-return three-port motorised valve so primary circulation stays possible on an underfloor-only demand. Viessmann notes that a combi boiler can serve underfloor heating, with a suitably qualified heating engineer fitting a two-port valve where radiators and underfloor run from the same appliance.
Mixing valves, manifolds, two-port and three-port valves and system balancing are work for a qualified heating engineer, and any work on the gas side is for a Gas Safe registered engineer. Under the Gas Safety (Installation and Use) Regulations 1998, as amended, nobody may carry out gas work in Great Britain unless they are competent, and HSE states that domestic gas work must be done by a Gas Safe registered engineer with the right qualification. What you can safely do yourself stays at the boiler pressure gauge, the thermostat and programmer settings, the reset button and bleeding radiators.
What controls does England require for radiators and underfloor heating?
Approved Document L, Volume 1 requires thermostatic room control in each room when a heating system is new or the heat generator is replaced in an existing home in England. Zone control may be used instead where that is justified. In practice it is met by a room thermostat in the room it serves, plus thermostatic radiator valves (TRVs, the numbered dials on the radiators) everywhere except that room, or by networked control of each heat emitter.
- Underfloor heating must have control of its operating temperature, not just an on and off signal
- New dwellings with a floor area of 150 m² or more need at least two independently controlled heating circuits
- Single-zone exceptions include single-storey open-plan layouts where the living area is more than 70 per cent of the total floor area
- Adjacent rooms with the same function, such as a kitchen and a utility room, may form one zone
- Controls must be wired so the boiler and pump shut down when neither space heating nor hot water is being called for
A mixed system is where this matters most. Underfloor and radiators cannot sensibly sit on the same thermostat, so the control layout has to be planned at the same time as the pipework.
Underfloor heating or radiators: which to choose for the room you are planning
Choose underfloor heating if the floor covering is coming up anyway, the finish will be tile, stone or a thin wood floor, and the room is held at a steady temperature. Choose radiators if the existing floor is staying, the room sits under carpet and underlay, the building is old or poorly insulated, or you heat the space only for part of the day. For an extension next to an existing house, both at once is normal: underfloor in the new area, radiators in the old.
Either way the sizing follows the heat loss of each room and the flow temperature the system is designed around, not the type of emitter you prefer. If the heating in that room is part of a boiler change, a free home survey works out those figures before you get a written fixed price for new boiler installation.
Sources
- Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments)
- Approved Document L: Energy and greenhouse gas emissions (2026) – GOV.UK
- The Gas Safety (Installation and Use) Regulations 1998
- HSE – Check an engineer: are they Gas Safe registered?
- Viessmann – Underfloor heating vs. radiators: which should you choose?
- Vaillant – Do heat pumps work with existing radiators?
- Worcester Bosch – Technical Bulletin TB 0137 B (07/2019)