Ground source heat pumps and air to water heat pumps work in a very similar way, only one draws its heat from the ground and the other from the air.
A common question for both technologies is how it is possible to heat a property from ground temperatures of around 2°C and air temperatures as low as -15°C? In both instances the heat pump is able to extract low grade heat from the source, ground or air, and pass it through the heat pump compressor which concentrates it into a higher temperature capable of heating water for the heating and hot water systems.
Both forms of heat pump service a wet central heating system, i.e. radiators or underfloor heating. Heat pumps work much more efficiently at lower temperatures than those generated by gas or oil fired heating systems. This makes heat pumps more suited to underfloor heating systems or larger radiators, as both operate at lower temperatures over longer periods of time (radiators will never feel very hot to the touch as they do off a conventional boiler). For this reason they do not often make an easy retrofit in older properties as fitting underfloor heating onto existing solid floors or even suspended timber floors is expensive and disruptive, whilst oversized radiators are not very practical.
Furthermore, as heat pumps work best when producing heat at a lower temperature it is essential that the property is well insulated. For older, poorly insulated properties a heat pump will struggle to meet the heat requirements. Heat pumps are really at their most efficient when installed as part of a new build. For homeowners in older properties looking to install a renewable energy system, a biomass boiler would be more suitable, particularly where no mains gas is present. Click here for more on our biomass boiler installations.
Which is best for your home?
If you have a modern, well insulated house with a small garden then the air to water heat pump is the obvious option, but you may need our radiators resized. If you have plenty of land then your decision is likely to be governed by other factors, but principally budget. Ground source heat pumps do out perform air to water, but the ground works make them considerably more expensive. Of course if money is not an obstacle but your garden is small, a borehole is an option.
For those who can do both, the table below gives some indicative figures for a 7kW system ground source heat pump and a 7kW air to water heat pump to assist you in your decision making.
Key is performance: ground source heat pumps achieve a higher optimum COP or “co efficient of performance” (this is the number of Watts generated by the heat pump for every Watt of electricity consumed) of 3.9 compared with air to water’s 3.4. On the surface this is not a huge difference, but the ground source achieves this COP across a wider range of temperatures: 0 – 35°C. For the air to water, 3.4 is only achieved between 7 - 35°C. Drop the temperature range to -7 - 35°C (as can be expected during the winter) and the number of Watts generated for every Watt of electricity consumed drops to 2.3.
Furthermore, as the efficiency of air to water heat pumps reduces significantly at lower temperatures, they are less efficient during the winter months when you use them most, thus increasing the running cost. In short, if you have enough land and money then the ground source is a better performing, more efficient option.
| Ground Source (not boreholes) | Air to water | |
|---|---|---|
| Cost | £10,000 - £14,000 | £7,000 - £10,000 |
| Optimum Coefficient Performance (COP) – number of Watts output for every Watt of electricity consumed. | 3.9 (0/35°C) | 3.4 (7/35°C) |
| Worst COP | 2.9 (0/50°C) | 2.3 (-7/35°C) |
| Typical electrical running cost (varies according to region and insulation values) | £740 | £860 |
More About: Ground Source Heat Pumps
Ground source heat pumps require a ‘collector’ buried in the ground to absorb the heat stored there. Collectors come in three main types: pipe loop(s), compact collectors and borehole (where the collector is set vertically in the ground).
A pipe loop collector is by far the most economical to install, but it does require a significant amount of land to lay it. Where there is not enough land to fit a pipe loop, then compact collectors – large panels laid in up to three trenches – are a good alternative that will not make the installation economically unviable. Boreholes are used where there is not enough land for even the compact collectors, but there are extremely expensive.
The heat pump itself comes either as a ‘combi’, which has an internal hot water tank, or a ‘system’ heat pump, with an external tank. The ‘system’ heat pump is commonly used with solar installations as a twin coil tank is needed. It is also used if you wish to install a different tank to that supplied by the heat pump manufacturer. We highly recommend installing a high performing ACV ‘tank in tank’, which is a fast recovery, high efficiency tank that out performs most others.
More About: Air to Water Heat Pumps
Air to water heat pumps comprises an external collector unit a little larger than a washing machine and as such they are a good option for properties without a great deal of land. For modern, well insulated properties and new builds, with average sized gardens, they are an excellent option and a straightforward installation.
Renewable Heat Incentive
On 10 March 2011, the Government announced the details of the Renewable Heat Incentive; a policy aimed at increasing the number of homes using renewable heat sources by offering a long-term tariff payment for heat generated.
The first phase, currently active, covers only non-domestic properties. The second phase of the RHI scheme, from October 2012, will see domestic households moved to the same form of long-term tariff support offered to the non-domestic sector in the first phase.
In the interim, up to 25,000 installations from July will be supported by a “RHI Premium Payment” to help people cover the purchase price of green heating systems (if fitted before 31 March 2012). For Heat Pumps it will be L1250, but only for properties where no mains gas is present.
Those taking up the Premium will then be eligible for a RHI tariff from October next year when the Green Deal begins, as will anyone else who has had eligible equipment installed from July 2009.
Conditions
In return for the payments, participants will be asked to agree to monitor and record performance to provide some feedback on how the equipment works in practice.
Criteria
To qualify for the RHI Premium Payment, applicants must:
- Install a qualifying system – this includes Ground Source and Air to Water Heat Pumps
- Have a well-insulated home based on its energy performance certificate
What is the long term tariff?
No tariffs have been published for the domestic RHI. For non domestic installations, the tariff is currently 4.3p per kW hour for ground source heat pumps. Within the Government’s ‘Renewable Heat Incentive Consultation on the proposed RHI financial support scheme’ document, published in February 2010, the proposed tariff for domestic Ground Source Heat Pump installations up to 45kW was 7p per kW hour and for air to water 7.5p per kW hour.
Calculating the tariff
According to the same consultation document, each installation’s tariff entitlement will be established by multiplying the proposed tariff per kWh with a deemed heat requirement (number of kWh per year). This is established through an assessment process based on SAP (or variant) at the beginning of the project, rather than measuring production via a meter, which might encourage a household to generate a deliberate surplus of heat or hot water to increase the tariff payable.
For each individual property, the SAP calculation will establish the deemed heat load for that property based on the average heat load of a property of the same type with reasonable energy efficiency measures installed (for instance cavity wall and basic loft insulation). Multiplying the amount of ‘useful’ heat contributed by the renewable energy system (in kW) by the tariff will give the householder the amount of long term tariff payable.
Renewable Energies: Investment returns
The Government incentives for installing renewable energy systems provide compensation to home owners for the cost of installation and pay an investment return over a fixed number of years (yet to be confirmed). According to the ‘Renewable Heat Incentive Consultation on the proposed RHI financial support scheme’ document, this investment is proposed at 12% across all technologies (6% for solar thermal). For ground source the cost of the installation can vary according to the collector type, so for a pipe loop 12% is a comfortable return (based on the consultation document figures), for compact collectors this drops to around 10%.
The Government recognises that, as with any other investment, householders and businesses will be looking for a return on this investment that reflects the opportunity cost of capital and the level of risk and effort involved.
The proposed tariff structure also allows generators to retain the benefit of any future rises in fossil fuel prices. If fossil fuel prices rise, renewable energy generators will save more money compared with a situation in which they had stayed with fossil fuel heating. Conversely, if fuel prices fall, they will save less.
