Housing Retrofit: Radiators & Underfloor heating (ufh)

System selection

• A well-insulated refurbished house, particularly if it achieves performance standards approaching Passivhaus, will need only a fraction of its previous space heating energy input to achieve the same or improved levels of comfort.

• Though other systems, such as blown air, may be considered, the more conventional choice will be between a radiator system and an underfloor heating system (ufh).

• Apart from their different locations in a room, radiators and underfloor heating systems work in slightly different ways. Both systems radiate heat, but each to a different effect. Radiators are comparatively small radiant emitters and they heat space more through convection than direct radiation – this difference accounts for a radiator’s higher operating temperature (circa 80oC). Underfloor heating operates at a temperature that is usually only a few degrees warmer than the room air temperature (typically 21 – 25oC) – but the heat is directly radiated, through effectively a large radiator in the form of the floor, directly to the occupants of the room.

antique radiator



In housing refurbishment, the simplest, cheapest and more straight-forward option is to install radiators. In a house that experiences sporadic occupancy (eg the family is out during the day), a radiator system will meet the need for quick responses.

On the other hand, radiators work at greater temperatures than an underfloor equivalent – this higher operating water temperature will restrict the opportunity to provide low-temperature ancillary space heating sources.


Under-floor heating (ufh)

Though of a higher capital cost and slightly more complex to install, underfloor heating should be of serious consideration as an alternative to radiators. Underfloor’s main two advantages are that it provides better quality heating without draughts or dry air, but also that, because of its low operating temperature, it can be easily linked in with alternative heating sources that output at the same low temperatures. This makes underfloor heating ideal to integrate with solar thermal or ground source / air-air heat pumps.

The downsides to underfloor heating are, conventionally, that it is a system that is relatively unresponsive – it takes a long time to warm up and an equally long time to cool down – this is ideal for a building that is near-continually occupied, but difficult for a home that is more sporadically lived-in; In a refurbishment situation too, the installation of underfloor heating, necessitating either the grubbing up and replacement of existing concrete floors or an additional thickness of screed as well as works to suspended timber floors, might inhibit the designer looking for a cheaper and quicker space-heating solution.


Combined systems

A compromise that is often implemented is to combine the two systems with radiators installed upstairs and underfloor heating to the ground floor. This combination will respond to the different heating response requirements of living and sleeping spaces.

radiators and underfloor heating compared


System selection criteria

• Capital cost
• Operating cost
• Speed of implementation
• Extent of works
• Pattern of heating useage / occupancy
• Operating temperatures
• Option to add alternative sources
• Comfort levels
• Air quality




radiator surface in detail


Re-using the existing system

• If there is an existing system fitted, there is a possibility that it could be re-used – avoiding the need to scrap often quite viable radiators. If this strategy is chosen, it will be highly likely that the new space heating requirements of a refurbished house will be considerably diminished. Simply reusing the existing system might well result in energy inefficiencies resulting from (now) oversized radiators.

• Another drawback to reusing an existing system might be that the radiator feed system originally installed will be of the ‘Single pipe loop’ type. In this system a single pipe runs out from and returns to the boiler. The pipe feeds each radiator in succession with hot water flowing from the pipe into the radiator and then the cooled water returned back to the pipe. Though a simple system, the consequence of this form of series connection of radiators is that the first radiator in line is always the hottest and subsequent radiators connected to the loop become progressively cooler in turn.

• The more recent and largely prevalent ‘feed and return pipe’ systems are more efficient and suitable for re-use.


A new system

• There will be a wealth of choice between types and styles available to the specifier when selecting new radiators – including low surface temperature, compact, towel rail and ‘ladder’ radiators. The most important factor in selecting a radiator though will be its size – this will be calculated from the required output.

• The required output will be calculated by the services engineer. In simple terms, the output will match the overall heat loss from a space, including that lost through the building fabric and ventilation, whilst achieving a comfortable temperature in each space that the radiator serves.

• A lesser, but still important factor in selection will be the radiator’s function. Radiators heat through means of convection and radiation. Convection heating is more efficient and produces a more even heat distribution within the room, though the response is slower to achieve comfort levels. Radiators that have twin panels and fins welded between are optimised to provide convection heating.
Radiators that are of the simple single panel type are more efficient radiators of heat directly towards the occupants. Though this is a more responsive system, the effects tend to be more localised – resulting in possible cold spots within the room.

• The feed system is also likely to be different from that previously installed. Modern feed systems are of the ‘microbore’ type. A mircrobore systems feature a number of manifolds which are fed directly from the boiler through normal-size pipework. Each manifold will serve, in turn, a number of radiators via microbore pipes (usually 8mm).
The return from each radiator is also through microbore pipes back into a manifold and back to the boiler through normal-size pipes.

• The advantage of a microbore system is that it loses less heat through the delivery pipework, though a disadvantage is that the size of the pipes can lead to blockages through transmitted ‘sludge’ and limescale in hard water areas.



Underfloor heating

underfloor heating pips

• Underfloor heating works by circulating warm water through a network of cross-linked plastic or composite plastic and alumium pipes installed integrally either within a structural slab or floor screed on top of a slab. Equally they can be fitted directly beneath a flooring system such as sheeting or t&g floorboarding. 15 or 16mm pipes are the most common diameters of pipes in the UK.

• For an effective and efficient system, design should be for as low a flow temperatrue as possble. This is best achieved through the use of a large surface area of emitter: ie large diameter and closely-spaced pipes.

• Pipes are laid around the house in a series of loops. Although loops can be applied to whole floors, dedicating individual loops to rooms provides more opportunity to control room temperatures. Each loop is brought back to a manifold, usually one on each floor, which is usually where the controls are located.

• Though it is generally recommended that piping is installed over a structural deck, it is perfectly feasible to retrofit underfloor heating between the existing joists in suspended timber ground and upper floors.

• Piping can be, and commonly is, installed within a concrete screed – but this can lead to the slowest response times. If this is a problem, a ‘dry installed’ system provides for a quicker response where the piping is located between the slab and a floating floor above.

• The choice of floor finish effects heating performance. Heat transmission is best through stone, tiles and slate. The moisture content of timber used in a wooden floor should be about 8% for retrofitting and 10% for new build – to avoid warping in close proximity to a heat source. Carpets and rugs are insulators and should be avoided; They will require extra heat and are likely to provide a breading ground for dust mites.


Design considerations

• Loop design to optimise control
• Flooring materials, their u-values and emissivity.
• Control of expansion and avoidance of cracking in concrete and tiled surfaces.
• Selection of wood species when specifying floor boards to ensure dimensional stability.
• Curing times and temperatures for poured floors.
• Screed installed response time
• Pipe location within suspended floors
• Operating temperatures
• Surface temperatures for comfort, safety and material integrity.
• Control optimisation.


Ground floor: pipework within screed

heating pipes in screed


Ground floor: pipework within slab

heating pipes in slab


Timber suspended floor: pipework between joists

Timber suspended floor: pipework between joists





Building Regs

• Building Regulations Approved Document ADL2 sets out the requirements for existing buildings.
• The regulations do not oblige the designer to perform DER/TER calculations, but where a new system is installed or an existing system replaced, the installation must follow the requirements of the ‘Domestic Heating Compliance Guide’ (DCLG, 2006).



Domestic Heating Compliance Guide, DCLG, 2006
Domestic Heating Design Guide, CIBSE, 2007



• BS EN 442-1:1996. Specification for radiators and convectors. Technical specifications and requirements
• BS 7478:1991. Guide to selection and use of thermostatic radiator valves
• BS 6880-1:1988 Code of practice for low temperature hot water heating systems of output greater than 45 kW. Fundamental and design considerations
• BS 6880-3:1988 Code of practice for low temperature hot water heating systems of output greater than 45 kW. Installation, commissioning and maintenance
• BS 6880-2:1988 Code of practice for low temperature hot water heating systems of output greater than 45 kW. Selection of equipment
• BS EN 1264-5:2008. Water based surface embedded heating and cooling systems. Heating and cooling surfaces embedded in floors, ceilings and walls. Determination of the thermal output
• BS EN 15316-1:2007. Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies. General
• BS EN 15316-2-1:2007 (3 parts) Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies.
• BS EN 15316-3:2007 (3 parts) Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies.
• BS EN 15316-4:2007 (6 parts) Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies.
• BS EN 14336:2004. Heating systems in buildings. Installation and commissioning of water based heating systems
• BS EN 12831:2003. Heating systems in buildings. Method for calculation of the design heat load
• BS EN 12828:2003. Heating systems in buildings. Design for water-based heating systems
• BS 8211-1:1988. Energy efficiency in housing. Code of practice for energy efficient refurbishment of housing
• Domestic Heating Compliance Guide, (Compliance with Approved Documents L1A: New Dwellings and L1B: Existing Dwellings), First Edition, Communities and Local Government.


Further information

• Boiler and Radiator Manufacturers Association Ltd (BARMA)
• Underfloor Heating Manufacturers' Association (www.uhma.org.uk)
• Hot Water Association (www.hotwater.org.uk)
• Heating & Hotwater Industry Council (www.centralheating.co.uk)
• Association of Plumbing and Heating Contractors (www.competentpersonsscheme.co.uk)

Underfloor heating systems on GreenSpec



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