Upgrading to underfloor heating

With the development of underfloor heating (UFH) floor constructions for retrofit into existing properties, the market for replacing radiator systems is set to grow. This article explores how best to approach the installation and technical aspects that have to be considered.

The first thing to do is to speak to a reputable UFH supplier to make sure that the property is suitable for UFH. They will be able to discuss potential floor constructions based on insulation levels, the building’s structure, the heat source and how the occupants will use the system. The system designer will be able to recommend the ideal combination of floor construction and controls to suit the project and calculate the flow rates that will be needed. Nu-Heat, for example, looks at all building elements (i.e. walls, ceilings, windows) to ensure that not only the desired amount of heat is provided but that the system is also economical and efficient.

Boilers need to be sized in line with upgrades in insulation and, to ensure that maximum efficiency is achieved, this should be an integral part of the design element for a new build or complete retrofit UFH system. UFH will work effectively and efficiently with combi, system, conventional boilers fired by gas, oil, or LPG in an existing property. Underfloor heating is particularly noted for its ability to enable modern boilers to work in condensing mode due to the low flow temperatures. This will, of course, improve the system efficiency. Where insulation levels have been increased in line with (or in excess of) current building regulations standards and heat losses reduced, a heat pump can also be used.

Different UFH floor constructions have different characteristics; for instance, when first switched on a screed floor can take a while to heat up from cold but it will retain the heat for a long time and conduct heat really well, whereas a floating floor with metal diffuser plates will heat up quickly, conduct heat well and then cool down fairly quickly. In either case thermostats can be set to suit the performance characteristics of the floor construction and deliver heat exactly when it is needed.

LoPro10 with screed



UFH is often considered unsuitable for retrofit due to the issue of floor height build up. However, products such as Nu-Heat’s LoPro10® gypsum-based floor construction offer a practical retrofit solution. LoPro10® has a height of just 15mm and can be installed without needing to remove an existing deck. The floor construction’s low thermal resistance and high-density properties provide a high thermal output compared to similar low height plywood and chipboard based systems.   



System configuration

The existing pipework must be checked to determine the system configuration. For new boiler/cylinder upgrades the ideal is the industry-standard S-Plan configuration based on 2-port zone valves and a 12 way terminal strip. For installations with existing W-Plan or Y-Plan layouts the 3-port diverter valve controlling the heating and domestic hot water can be retained as part of the overall system control where valves, pumps and connections for the UFH circuit pipework are conveniently situated; although, in this case, an additional 2-port ‘blocker’ valve should be fitted before the 3-port valve. Alternatively, the system can be upgraded to a standard S-plan configuration, which will also be necessary if independently zoned and timed towel-rail circuits are specified.

Existing radiator/hot water time clocks can be left in place but a new control system will be needed for the UFH. Cabling layouts depend on the type of boiler, cylinder and room thermostats installed and a good supplier will provide electrical schematics to illustrate how the UFH control fits into the overall system.  

As a general rule existing flow and return central heating pipework can be used to supply UFH manifolds where radiators are being replaced (if correctly sized). If some radiators are being retained supply pipes must go to UFH areas and radiator areas separately. UFH manifolds should always be connected to minimum 22mm flow and return heating pipe, never to 15mm supply pipework. If the overall heating system heat load is to be increased, for example due to the addition of a large extension, then it is important to recalculate the overall heating and hot water load, changing the existing boiler and upsizing the flow and return supply pipework as required. An additional 30% should generally be added to the calculated increase in heating load to help reduce response times between ON/OFF periods.

Underfloor heating vs. radiators

UFH in situ


Logically, UFH has the potential to be the most efficient type of emitter. The surface area of the floor is far greater than that of a radiator, allowing the flow temperature to be lower to achieve the same room temperature. Where a radiator may be heated to between 60˚C and 75˚C, UFH can often achieve the same performance running at just 50˚C, which will allow the boiler to operate in condensing mode. UFH is radiant heat rather than the convective heat associated with radiators, so it gives a very different temperature profile; heat is emitted upwards from the floor rather than being carried by convection currents and heating the room from the top down, meaning UFH offers the ideal comfort profile of warm feet and cool head.

Nu-Heat has successfully integrated its UFH systems into domestic and commercial new build, renovations and increasingly in retrofit through the use of low profile floor constructions. It is used in a diverse range of properties, from family homes, barn conversions and apartments to offices, hospitals – even castles and tree houses. There are, of course, certain properties that will not benefit from UFH, such as older, poorly insulated properties with high ceilings that will require more than 110W/m², but so long as refurbishment projects undergo an upgrade to their insulation or the property was built within the last 10 years to the insulation requirements of the building regulations, warm water underfloor heating is an ideal option.

Smart controls with underfloor heating – from Nu-Heat


Consumers are increasingly looking to smart controls to give them a simple way to manage their heating system. For example, a householder going on holiday may want to remotely control their heating system to go off when they have left and come on again before their return.

Nu-Heat’s touchscreen range, the Touchscreen thermostat, Central Touchscreen Console and Smartphone App offer individual room control with the enhanced functionality and flexibility to suit busy modern lifestyles.

NetMonitor+ enables the heating system to be controlled remotely via the web or smartphone App. The iPhone App is free from the Apple AppStore and includes passcode protection, real-time temperature display, a home/away function and temperature override functions. The smartphone App can be used in conjunction with a central console if desired.

To view an animation of the App in action, visit: www.nu-heat.co.uk/netmonitor

LoPro10® enhances retrofit options for UFH with condensing boilers



“Modern gas condensing boilers offer high efficiency heating opportunities when used in partnership with low temperature heating systems like underfloor heating,” says Nu-Heat’s Technical Director Andrew Grimsley. “Although underfloor heating (UFH) has traditionally been considered difficult to retrofit, there is now a new and exciting business opportunity available to the trade, with the introduction of an ultra-slim gypsum based UFH panel that provides excellent response times and high thermal outputs.”



The Green Deal hopes to encourage older homes towards such general insulative upgrades as double-glazing, cavity wall insulation, 300mm of roof insulation and good draught proofing. While insulation upgrades are being made, it is also the perfect opportunity to replace radiators with underfloor heating (UFH) and old gas boilers with new, condensing boilers.

The technology – Condensing boilers reduce carbon dioxide emissions and improve household efficiency, resulting in reduced fuel bills. They work on the principle of recovering as much as possible of the waste heat which is normally rejected to the atmosphere from the flue of a conventional boiler. The combustion process in the boiler combines the fuel with oxygen from the air, but as well as releasing heat there are by-products of carbon dioxide and water. The water is in the form of vapour that emits from the boiler’s flue. At the critical temperature, called the ‘dew point’, this vapour will condense into liquid. The condensation releases ‘latent heat’ from the vapour over the boiler’s heat exchanger increasing the boiler’s overall efficiency even more.

In order for a boiler to operate in condensing mode the return temperature of the heating system must be lower than the dew point. Radiators sized traditionally to a design flow temperature of around 70°C with a return temperature of 60°C run, in general, outside of a condensing boiler’s range. UFH, however, requiring a flow temperature of 50°C flow and 40°C return can obviously take more advantage of the boiler’s condensing range.

Which properties? – “There are, of course, some properties that will not benefit from the combination of a condensing boiler with UFH,” says Andrew. “It’s unlikely to be relevant for Victorian or other older, poorly-insulated properties with high ceilings that will require more than 110W/m². However, it’s a great combination for a refurbishment with suitable insulation upgrades or most properties built within the last 10 years within the higher insulation requirements of Building Regulations.”

It is in these types of property that a system comprising a condensing boiler partnered with a low profile, gypsum based UFH system is ideal. UFH has traditionally had slower response times than radiator systems, but with a low mass, highly conductive gypsum-based floor construction, a very fast response both to warming up and cooling down is achieved.

Nu-Heat’s LoPro10®a composite gypsum construction with one-third less thermal mass than a screed floor will heat up and cool down rapidly and give low thermal resistance and high-density properties that provide high thermal output compared to similar low height plywood and chipboard based systems. Response times in comparable, non-gypsum based products are slower due either to use of a thicker board or a board that is a poor heat conductor.

To help meet Building Regulations Part E acoustic criteria, this type of floor construction should provide airborne acoustic reduction on timber upper floors by adding mass – 15.5kg/m². This also means that it feels more solid underfoot than many lightweight floating floors.

Room thermostats – provide a major benefit when retrofitting UFH systems as they remove the need for potentially disruptive and messy hard wiring. Wireless stats also provide independent time and temperature control for each room or zone, including a set back temperature function to further improve overall running costs when owners are out at work or asleep at night. This automatically drops the room temp by 4°C within set timed periods and returns it to the desired temperature when required.

Running costs – Electric UFH has been used in the retrofit market for many years due to its inherent low floor height build up and until now there has been no equivalent retrofit low profile wet UFH system available.  The new low profile UFH system, when combined with a gas fired condensing boiler, costs approximately 4p per kWh to run. In comparison, direct electric UFH systems cost around 12p per kWh, so are about three times more expensive to run.

Floor coverings – The aesthetic of the finished floor is likely to be high on your customer’s priority list so it’s important to offer advice on the wide variety of floor coverings available that can achieve the best from their heating system.

Contemporary and desirable finishes such as ceramic tiles and limestone, which conduct heat well and warm up quickly, are a good choice and dependent on your customers’ design preference engineered boards, kiln dried hardwood and carpets are also suitable options.

Floor coverings are taken into consideration as part of the design process so whatever the customer’s interior design choices they can enjoy the benefits of underfloor heating with no constraints on floor finishes or room layout.