What Cost Geothermal Under floor Heating?
Reducing the environmental impact of buildings is going to be fundamental to future changes in Building Regulations. We are already witnessing the impact of recent changes to boiler requirements and insulation levels. Media attention has turned to how geothermal heat sources, coupled with under floor heating, can help reduce the operating cost (emissions and financial) of heating systems. So what opportunity does this present for heating installers?
There is no doubt that the use of 'alternative' heat sources for domestic and commercial buildings is set to become more mainstream, no longer will they be considered as alternative. Increasingly heat pumps are being considered as boilers; a packaged unit that just needs to be connected up to the heating system (such as under floor heating) and the fuel source. Heat pumps do not require a gas or oil supply, just a connection to the ground array pipework, or if the heat pump is an air sourced one, no further connections are required.
New technology can be daunting and there's no doubt that a lack of understanding inhibits take up, which is perhaps why interest in heat pumps with under floor heating is still somewhat limited. The technology behind heat pumps has been around for years and is well proven. It is the application that has changed as heat pumps are now used for domestic heating.
In order to make the heat pump cost and carbon emission effective the manner in which they are installed needs to be considered. There are a number of factors to consider including the type of heat pump and how it is to be installed:
- Is the building and its environs suitable for a heat pump, for example is the building sufficiently insulated, is there room for a ground array?
- How is the heat going to be introduced into the building - radiators or under floor heating?
- What type of heat pump is going to be used, air source, ground source or from a river or lake?
- Does the heat pump need to be used in conjunction with a buffer vessel or heat store?
When using a heat pump the levels of insulation should really exceed the insulative requirements of building regulations to ensure the flow temperature from the heat pump becomes as low as possible, which means that efficiency of the pump is increased. For example if a heat pump is delivering water at 50°C with a return temperature of 45°C and the flow rate of water is at 0.5kg/s, the amount of energy that is released in to the building would be 10.45Kw. The typical Coefficient of Performance (COP) would be say 3, depending on source temperature. If the delivery water is at 35°C with a return at 30°C and the flow rate of water is at 0.5kg/s, the amount of energy released in to the building would again be 10.45Kw yet the COP would be over 4, again depending on the source temperature.
Keeping temperatures as low as possible is essential to the efficiency of the system. If radiators are going to be used this means that they need to be carefully sized, with the temperature difference between average heating water temperature and the air temperature being considerably reduced, when compared with a boiler fired heating system. This means that radiators will be typically 2 or 3 times the size.
Under floor heating is the obvious choice in this case, since the low temperatures are entirely suitable. These low temperatures can only be accommodated if the system has been designed to take these factors into account. The type of floor finish also has an effect on the flow temperature, since the more insulative the floor finish, the greater the temperature differential between the water temperature and the floor surface.
The type of heat pump to be used will be determined by the building's location and the type of heat source - air, ground or water. Typically this decision should be made with advice from the heat pump manufacturer: ground source pumps tend to be more efficient, since the ground temperature tends to be more stable during the heating season. However air-sourced heat pumps have a big advantage in that their location is far more flexible, and they don't require the usual array of pipework associated with a ground source heat pump.
The use of buffer vessels also needs to be considered, some manufacturers advocate their use while others do not.
A buffer vessel is to stop the heat pump 'hunting on and off'. The turn down on a heat pump is generally for a single compressor unit about 50%, this means that to prevent the system from hunting on and off there has to be an excess of water to dissipate heat. This can be done either by using a buffer vessel, the size of which is determined by the heat pump supplier, or by using open circuits. Using open circuits means that room temperature control is compromised although it is argued that not using a buffer vessel is slightly more efficient.
At Continental when designing an underfloor heating systems, the heat source is taken into account so that the efficiencies of the heat pump are maximised.
