Underfloor Heating And Renewables - A Perfect Partnership

Using renewable energy can cut down on carbon emissions and deliver savings on fuel bills; Bill McConnel, Technical Director of Continental Underfloor Heating explains how using underfloor heating (UFH) can reinforce these energy efficiencies.

Renewable energy occurs naturally and is in effect 'perpetual' in the environment. Solar energy is the most common form of renewable being used directly in solar, thermal and photovoltaic systems, and indirectly through the use of heat pumps, wind energy and biomass boilers.

So why is UFH proving so popular for use with renewable energy sources?

Significantly, it compliments the technology. UFH uses less energy for distribution because the flow and return temperatures can be kept considerably lower than those of a traditional radiator system. Also, since the method of heating is primarily radiant - from the floor upward (rather than convective) - the human body feels warmer at a lower air temperature (typically 1 or 2°C) rather than say with a traditional convective radiator heating system. Therefore, in a room that has a heat loss of 2kW with an internal temperature of 21 C degrees and an external temperature of -3 C, then potentially the heat loss can be reduced by up to 8% due to the radiant effect of UFH.

The low flow temperatures in UFH work particularly efficiently when partnered with ground (GSHP) and air (ASHP) heat pumps. The efficiency, or Coefficient of Performance (COP), of these devices is crucial to be able to determine the energy savings and payback period of the installation. The best way of increasing the COP is to try and ensure that the discharge water flow from the heat pump is matched as closely as possible to the source temperature. For instance, with a COP of 4 (400% efficient: heat output/ electrical output) and a ground temperature typically between 0 C and 10 C in the UK during winter, the flow temperature of the heated water produced by the GSHP is between 30 C to 35 C, which is ideal for a UFH system, but too low for radiators without additional electric or boiler heating. Insulation levels of the building should conform to or exceed building regulations. This allows the flow temperature of the UFH system to be kept as low as possible, which increases the efficiency.

There is a direct relationship between the floor surface temperature and the air temperature of the room. So for example, if the heat output of the underfloor heating needs to be 90 watts/m2 and the room air temperature 20°C, then the floor temperature would need to be about 28.5°C in order to meet this demand. In modern buildings typically the heat loss is frequently nearer 50 watts/m2 or less. This means that the surface temperature of the floor is much closer to the air temperature, which gives the underfloor heating a quicker reaction time. This has an added advantage because heat pumps can be used at potentially lower tariff rates overnight, and provide enough energy in the screed to be gently released during the day. During cold snaps the heat pump can be programmed to boost the system during the day.

A growing trend is for the use of a combination of renewable energies to provide both space heating and hot water - for example, using GSHP's and solar thermal systems. It is technically feasible to use the excess heat generated by solar panels or tubes in the summer to 'top up' the heat available in a GSHP borehole by pumping the excess hot water down into the borehole. This helps to ensure that the stabilised temperature in the borehole is higher during the winter than it would otherwise be. However, the extra efficiency is hard to quantify, so my advice is to keep things simple and use the solar system for hot water and the GSHP for space heating, or to link the systems with a thermal store.

Wind power can be used to offset the electrical cost of running the heat pump: for example, if 20kW of heat is supplied by an underfloor heating system via a GSHP that has a COP of 4, then a 5kW wind turbine in theory could supply all the power requirements for the heat pump. Unfortunately this is not really feasible since the wind turbine will rarely work at full power due to fluctuating wind conditions. Also, many heat pumps require significant starting currents that a wind turbine would be unable to provide. However, wind power can be used to considerably reduce the cost of running the heat pump by exporting electricity back into the national grid.

Also growing in popularity are biomass boilers, typically using wood pellets or wood chips. These boilers are becoming increasingly more sophisticated, some offering high levels of modulation. Biomass boilers are usually installed with buffer tanks or heat stores, a simple device that enables them to be used with UFH to offer another efficient heating option.