Efficient underfloor heating: wet or dry?

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When specifying heating in domestic properties, underfloor heating (UFH) is increasingly seen as the system that offers the levels of energy efficiency and comfort required. In design terms, UFH provides the added benefit that, unlike radiators, it does not have an impact on interior aesthetics and layouts. This leaves just one perplexing issue; whether to opt for a wet (hydronic) system or an electric system, Steven Rooney evaluates.

Both offer advantages and disadvantages. Wet UFH systems circulate water heated to around 45-50ºC through loops of pipe laid into the screed of solid floors or the joists of suspended timber floors. The low temperature of the water used makes these systems energy efficient and ideally suited to use with air or ground source heat pumps or district heating systems. Electric systems use a continuous cable, either laid within the screed or supplied as a mat which is laid above the subfloor.

A key issue when selecting either wet or electric UFH is the need to satisfy the requirements of building regulations. Gas - which frequently fuels the boilers used in conjunction with wet UFH - is still often favoured when it comes to carbon and energy calculations. Cleaner and more environmentally-friendly methods of producing electricity may change this.

Supply and install costs for both wet and electric systems are much the same from £13-15/sqm, including the relevant thermostats. When it comes to running costs, electric systems are marginally more expensive but maintenance is inevitably greater with wet systems, especially if the boiler or heat pump is included in the equation. There are also moving parts in the manifold and annual safety checks are required. Taking these factors into account, the comparative running costs for heating a two bedroom, new build, semi-detached house over 20 years are around £17,804 for a wet UFH system running off a gas boiler and £12,623 for electric UFH. Although electric UFH is invariably promoted as being maintenance free, these figures include replacement thermostats.

One of the very real benefits of electrical systems is that they are self-contained, require little maintenance, except possibly for replacement thermostats, and are ideal for occasional use. Wet systems require connection, via a manifold, to a boiler, heat pump or other source of hot water which must be run every time heat is required. These units take up space, need maintaining and are an added capital cost.

Whether wet or electric, UFH systems offer extraordinary levels of comfort to users and perform equally well, although electric heating mats offer the bonus of fast reaction times so, in locations such as hotel bathrooms, can be useful in providing ‘instant' heat. For these reasons, and the fact that modern control systems are remarkably efficient and flexible, especially when linked to a building management system, wet and electric systems are frequently combined in a single project.

Both wet and electric UFH systems are likely to last the life of the building. 10-20 year insurance backed warranties are the norm and the pipe in wet systems often has a 30-50 year manufacturer's warranty. The associated controls are generally warrantied for two years.

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