Underfloor Heating

Space heating energy use constitutes a major proportion of total UK primary energy consumption – at least 60% for example in the domestic sector. Despite improvements in energy efficiency (better insulation etc) energy use in our homes is increasing. The number of homes is increasing too and we keep our home warmer than we ever used to (average 16oC in 1990 rising to 18oC in 2004). So how do we maintain this drive for more and more comfort whilst reducing our energy burden – is UFH the answer?

UFH comes in two forms – wet and electric

Lets look at wet systems first – fed by the circulation of hot water through pipes under floors. These systems warm the floor structure by conduction causing the surface to radiate heat into the space above and the primary source of heat is typically a boiler – although many other options now apply which i will deal with later. The boiler heats water to 40-50oC and this is distributed in plastic pipes to one or more manifolds each of which comprises a flow and return header from which loops are taken to serve areas of the building to be heated. In a simple two-storey office for example there would be one manifold serving the ground floor and one for the first each feeding UFH loops in the individual rooms on that floor.

Getting good conduction between the UFH loops and the floor structure is the key to success. Different methods of transferring heat from the pipes are applied for different floor structures. The pipes are usually fixed to the floor insulation and a concrete screed poured over them further to which floor tiles, carpets or other surface finishes are applied. Where floor tiles are used heat radiates into the “treated space” at roughly 100W/m2. The design output achievable with suspended timber floors averages slightly lower at 70W/m2. Note the use of the word radiate – UFH is more akin to radiant heating than is for example the use of radiators which are mostly convection. Radiant heat means less rising heat lost to the roof void as convection – indeed with UFH the reverse is probably true – that is temperature inversion where the floor is hotter than the ceiling.

All of this means that the boiler operates at a lower temperature which promotes good combustion increasing efficiency. Typically a condensing boiler serving UFH would be operating at 90% compared to 87% for the same boiler feeding radiators.

These low operating temperatures also means that UFH is the ideal partner for “renewables” such as ground source heat pumps and air source heat pumps (where the heat exchanger carried out an air to water transfer). Why ideal? Well, simply because these systems tend only to heat water to 40-50oC and although some people like to boost this with a small boiler or an electric immersion element it seems a bit daft to me so I prefer to use the water at the temperature the heat pumps produces it in UFH systems – but heck what do I know?

Pro’s

  • Lower running costs and CO2 emissions
  • Optimises thermal comfort
  • Enhances decor by concealing heating
  • Flexible for use with renewables and CHP
  • Ease of access to manifolds for maintenance
  • Concealed heat emitters improve access for cleaning floors and skirtings
  • Enhances property value
  • Common on continent – becoming more popular in UK
  • Extended warranty of up to 25 years available on pipework loops of most UFH systems

Cons

  • Slower response time
  • Heat output limited to 100W/m2
  • Supplementary heating may be required in small rooms & bathrooms with two external walls, small conservatories and corridors with large glazed areas
  • Remains a “novel” technology, despite recent growth which tends to lead to uncompetitive pricing
  • Can restrict floor finish
  • Pumps can be noisy so need sound damping or remote siting to avoid nuisance to occupiers
  • Deep floor required to accommodate pipework and insulation
  • Not really much of an option for existing properties unless flooring is being replaced
  • Risk of damaging pipework if flooring is penetrated, leading to potentially costly repair works

Electric UFH

I have to confess I have some of this in my own home – and very effective it is too.

We installed it in a wee lean-to conservatory underneath slate tiles and have control via an Aube TH132-F floor sensor stat.

Our space is just over 6m2 so we installed a mat rated at 750W.

The thermostatic control and timed management keep running costs down to a bare minimum and I reckon an entire winter costs us £50-ish.