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To provide central heating for our new barn conversion at Old Cragg Hall we decided to use a ground-source heat-pump. A gas-fired system would doubtless have been a cheaper install but the higher running costs would soon out-weigh that initial saving. The relatively low output temperature from a heat-pump is a poor match for a heating system based on radiators but is ideally suited to an under-floor heating system - difficult to retro-fit but perfect for a new-build. It's also much easier to achieve the highest standard of insulation in a new build as this is the real key to keeping heating costs (and carbon footprint) to a minimum. The barn should require only about 8KW of heat to keep it warm on most winter days.
Our Waterkotte Ai1 heat-pump can produce 8.9KW of heat at typical output temperatures of 25-30C - ideal for an under-floor heating system. A wood-burning stove can be lit on days when the heat-pump can't quite cope. The heat-pump is also capable of producing the higher temperatures needed for a domestic hot water supply albeit at a lower rate and efficiency.
Since we have access to a large field directly in front of the barn we elected to use a ground loop collector, as opposed to a bore-hole, for the ground heat source as this is by far the cheaper option. It still required three 50m trenches each 1.2m deep and 1.2m wide - a fair amount of space. But the excavations were no challenge for a JCB and the trenches were dug in a just couple of days.
We used a company called GeoWarmth (based in Newcastle) to supply, fit and commission the ground loops, heat-pump and under-floor heating system. They may not be the cheapest supplier but they did do a good job. Their quote did not include any electrical work or the actual digging of the trenches. A £1200 grant from the DTI low carbon buildings program is available for ground-source heating installations which also attracts VAT at the reduced rate of 5%.
One further consideration for well-insulated low-energy houses is that they must also be draught-free and essentially sealed. This has a negative impact on air quality, the air in such a house would soon get stale, unpleasant and unhealthy. The answer is more technology - a heat-recovery ventilation system in which the warm stale out-going air is used to warm up cool fresh air as it is pumped into the house. Such a system can recover significant amounts of thermal energy while providing plenty of nice fresh air.
But how does a heat-pump work? Well think of how a refrigerator works - it sucks heat out of the things placed inside it and thus cools them down. It does this by using a compressor to pump a refrigerant fluid around a closed circuit at the back of the fridge, the fluid is forced under pressure through a constriction in the circuit at which point its pressure reduces considerably, causing it to cool. The cool part of the circuit passes through the well-insulated inside of the fridge taking heat away from the contents as it warms before being compressed once again to complete the circuit. Compressing the fluid causes it to heat up and this heat is dissipated through the large black radiator structure at the back of every fridge. The waste heat released at the back is from both the electrical power used in the pump and the heat removed from the inside of the fridge. Now imagine the fridge turned inside-out the 'waste' heat is now what's heating the well-insulated house and the cold part of the circuit is routed under the ground outside which gets cooled down. This is how low-temperature heat is "pumped" from the ground to make high-temperature heat for the central heating system and how 2KW of electrical power manages to produce more than 8KW of heat.
Facts and figures: Heat-pump make & model: Waterkotte Ai1 Electrical power input: 2KW Heat output: 8.9KW Ground loop trenches: 3 x 50m x 1.2m x 1.2m Hot water tank capacity: 250l Hot water temperature: up to 70C Heat recovery system: Vaillant RecoVAIR var275
angela@oldcragghallbarn.co.uk | 01422885581
Old Cragg Hall Barn, Cragg Vale, Hebden Bridge, West Yorks. HX7 5TB
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