Energy:Underfloor heating

From Sustainable Building Resource

Underfloor Heating (UFH)

The basic principle of underfloor heating (UFH) is to create a low temperature heat source with a large surface area. As the heat rises, the room is warmed to a comfortable uniform level, resulting in warm feet and a cool head, which suits our body's natural requirements.

It is the most popular form of heating in Europe. The addition of UFH to a property could increase its value by between10/15%, and work towards reaching low or even zero carbon emissions.

The two main types are warm water or electric systems. The electric systems, which can be heated cables or mats, have the advantage of being easier for retrofit installations. Warm water systems work by circulating warm water through a continuous loop of pipes between two chambers. Different rooms can be zoned with individual thermostats, and when room temperatures fall, hot water from the boiler is blended, via a mixing valve, with the warm water returning from the underfloor heating system. This water is then pumped around the zone until the air temperature reaches the desired level.

The water system can be one of the most efficient ways to heat your home as energy is used to heat water only to the required (low) temperature and then maintain it at that level. Any type of fuel can power the warm water system with a conventional boiler. A condensing boiler is most efficient at maintaining the lower temperatures required by underfloor heating. Alternative power sources such as solar panels, GSHP, heat recovery system, or an Aga can be used in any combination to supplement the output from your boiler and further reduce energy consumption.

UFH can be up to 40% faster to install than radiators and the actual installation is cheaper. However the cost of the component parts can be more expensive. Underfloor heating can heat a room much more efficiently than radiators do, so the running costs are much lower. The underfloor heating system itself requires no maintenance unlike radiators, which require painting. Only the boiler itself requires maintenance.

Conventional convective forms of heating, such as radiators, use air to move energy from a small surface that is comparatively much hotter than the air. Hot air rises so that the hottest air is at ceiling level and the coldest is at floor level. Underfloor heating uses radiant heat to gently heat surfaces around the room, creating an even comfortable temperature. Due to gentle nature of the heating system, air maintains an even healthy humidity level of about 40 – 60%. In conventionally heated homes air can become dry and cause dry eyes and stuffy noses. Convection circulation can also pick up dust from the floors and surfaces. Furthermore, the heat from radiators on poorly insulated external walls is then mostly lost through radiation into and through walls. Radiant heating systems are very efficient as they provide a lower ambient air temperature allowing you to feel warmer at a lower temperature because the radiant heat is lowering the heat loss from your body. Radiant energy from the floor is absorbed by the other surfaces in a room, which warm up and become secondary radiant emitters, filling the room with gently warmed surfaces. Underfloor heating systems with insulated areas of large thermal mass (concrete of limecrete for example) operate throughout the heating season maintaining a comfortable even heat all the time. This actually saves energy and costs less to operate because once the floor has become warm it takes a very small amount of energy to keep it warm; whereas with a radiator system, the radiators have to be warmed up from cold each time the system comes on. Dry installed panel systems use no large thermal mass which in turn leads to faster installation and quicker heating up times.

A possible drawback of underfloor heating is that it is usually installed at a early stage of the building process. To add it to a home that already has its floor laid can incur considerable time delays, labour costs and mess. However, this can be mitigated through the use of electric floor heating, underfloor heating cables are much smaller than water pipes and do not need to be installed as deep into the floor. Therefore, floor-heating installation is less intrusive with fewer installation problems and damage. It is advisable to devise a plan when installing underfloor heating of any kind, showing exactly where the pipes are located; this can be kept as reference for future work to avoid damaging either pipes or cables and incurring considerable costs and inconvenience.

Energy savings from underfloor heating vary depending on the insulation, usage patterns and personal requirements of the occupants. Other factors to bear in mind include the cost of the power source. In the UK, electricity is more expensive per unit of power than mains gas. Underfloor heating is ideal for use with micro generation alternative energy sources such as solar or geothermal energy. Depending on the conductivity of the floor, the water needs to be between 40 and 60°C to reach the design floor temperature. Good design incorporating passive solar gain, such as large south facing windows allowing sun into a tile or polished screed floor will minimise the energy required from the boiler. Linking solar panels into the feed will further reduce the need to burn fuel to particularly grey cold winter days.

Installation

As already discussed it is ideal to lay underfloor heating at as early a stage of the building process as possible. However retrofit is possible. The best choice for you will depend on a number of factors: How large an area, a whole floor, or one room, the use of the room, the floor construction, the floor finish. The first floor is supplemented by heat from the ground floor. UFH can be installed in most types of flooring, over a soild concrete or limecrete slab, under floor boards or between joists, and even in walls.

Solid Floor Pipes are laid within the concrete or screed layer of a solid concrete floor. Insulation is built into the floor either above or below the concrete slab. The insulation should surround the perimeter (external) walls, to a height to include the depth of floor insulation and screed to prevent heat escaping through the walls, and joints should be taped to prevent the ingress of screed between the insulation boards. The heating pipe is fixed to the top of the concrete or insulation, just before the screed is laid. The heating system relies on the conductivity of the screed or concrete to conduct the heat from the pipes to the underside of the floor finish, it also acts as a heat store, releasing heat steadily into the room.

Suspended Floor

The heating pipes are placed over insulation, above or between the joists of a suspended floor. It is possible to use a sand and cement mix to conduct the heat to the floor, or pre-formed panels with or without aluminium/aluminum heat transfer plates to conduct the heat from pipe to floor. Timber suspended floors do not conduct heat as efficiently as screed floors. This means that the heat output is less. However if suspended floors are used as the first floor the heat is supplemented by heat from the ground floor. The timber should be very dry (moisture content between 8% and 10%) or it may shrink and crack with the heat. Alternatively, you could lay it loose for the first year, so that adjustments can be made for any movement. Floating floor UFH is installed in floating floors above an existing solid or timber floor. Pipes can be installed over insulation, or panels with pre-cut pipe channels and factory-fitted aluminium heat diffusers are available. These purpose-built panels have a higher conductivity and help minimise downward heat loss. Whilst warm water underfloor heating is easiest to install in a new build or complete renovation situations, kits exist which allow the addition of a warm water underfloor heating system in for example a conservatory or new extension. These kits are designed to link up and add to an existing central heating system.

Electric heating This involves a flexible heating element shrouded with insulation and mechanical protection. Heat is generated as current is passed through it. The length of an electric heating element has a design calculation to provide a specific number of Watts over a given area when installed to a layout plan. An electric programmable controller energises and de-energises the heating element/s in conjunction with a thermostat that allows you to regulate the floor/room temperature.

A common application for electrically heated floors is 'Undertile Heating'. This is slightly different to underfloor heating. Undertile heating is designed to warm the tile to a comfortable temperature and provide a cosy sensation to the touch. Whilst it will have a warming effect on the room, it is not the same as true underfloor heating and should not be relied upon to heat the living space.