Our experience with...replacing oil-fired central heating with a sustainable alternative
Our house doesn't have a gas supply and we had oil central heating. Oil was getting very expensive and is a polluting way to heat your home so in 2006 we decided to install an alternative system. The Green Building Bible lists carbon dioxide emissions for various heating sources. Wood is best (if sustainably sourced and efficiently burnt) and next come heat pumps – if correctly used. So we investigated wood-fired boilers. Nowadays these are fairly automatic – pellet-fired boilers completely so - but big! Our preferred option, an Okofen wood pellet boiler with automatic pellet-feeding hopper, would have necessitated building an extension to house it. This would eat up our financial resources and could cause more greenhouse gas emissions than we subsequently saved. Frustratingly too late for us, Okofen have now introduced a prefab wooden shed to house the boiler and hopper.
So we considered the most efficient heat pump. A ground source heat pump can give 4 kW of heat for each kW of electrical input. However we had a problem – our garden is too small! We couldn’t face destroying the entire garden to accommodate the two trenches, each 1metre deep and 40 metre long, for the pipes to extract heat from the ground. The alternative was a borehole 120 metres deep – but the machinery couldn’t access our garden and the cost was prohibitive.
Frustrated again, we investigated air source heat pumps giving 3 kW of heat for each kW of input but cheaper and easier to install, being just a large box fixed to an outside wall (see photo). Microgeneration, who did all the installation. suggested we also add a second solar panel to the one we already installed in 1992, which was still working perfectly, and a Torrent Solar thermal store. This splendid new invention replaces both hot and cold water tanks, accepts input from solar panels, heat pumps and boilers, and supplies hot water for taps and radiators. So the solar contributes to central heating on bright winter days. The cost was around £13,000. We warranted a £2400 Government grant but failed to get the application in and approved before work started – be warned! We took the plunge but had the thermal store equipped to accept input from a log burning stove with back boiler in case the system was inadequate during really cold weather.
The system went live in October 2006 and did pretty well - the heat pump runs intermittently but makes a noise, which might annoy neighbours but fortunately that side of our house overlooks farmland. It doesn’t bother us indoors and isn't needed in the summer, as the solar supplies all our hot water. On dull days with no solar input the heat pump alone could produce hot water at 47°C and keep the house almost comfortable at 15°C! It will work with outside temperatures down to minus 15°C but efficiency drops off with increasing differential between input and
output temperature, which is why they are recommended for use with
underfloor heating which gives a warmer house at a lower output
temperature. We like to have our house a wee bit warmer than 15 especially when we have visitors so we decided to add in the efficient log-burning stove with back
boiler in 2007. This has done the trick nicely and we can now get really comfortably warm in the winter as well as having the cosy fire to watch.
The thermal store is cleverly designed, so hot water is unaffected by
the central heating demands of a cold day and maximum use is made of
input from the solar panels. Our thermal store is in the attic but
could be sited in a garage or airing cupboard where your hot water
cylinder used to be. Our airing cupboard was too small –our usual
problem!
So now our hot water and central heating are fired by sun, sustainably sourced wood and heat extracted from the air by the heat pump. The latter of course uses electricity but we get ours from Good Energy who match your consumption with 100% renewably generated electricity - if funds and energy allow we may install some PV cells to generate our own but for the moment we are very pleased with our much reduced carbon footprint.
Q&A
What products were installed?
Heat King Air Source Heat Pump
Solar Torrent Thermal Store
1 30 tube evacuated heat pipe solar collector to match our original one from Thermomax
Charnwood Country 8B efficient log burner with back boiler
Who was the installation contractor?
Microgeneration (www.microgeneration.com)
Date and cost
September 2006 for heat pump, thermal store and solar collector - £13,900.
October 2007 for Charnwood log burner - £3000
Has it lived up to its emission reduction claims?
Yes - from burning over 1000 litres of oil per year we theoretically emit no emissions as all our electricity comes from renewable sources.
Have you had problems in service and did the contractor fix them promptly?
There were one or two minor teething problems with the heat pump but the contractor came promptly to fix them at no charge.
Overall would you recommend your project to others?
Yes.
More information
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Centre for Alternative Technology Information Sheets. 01654 705989; www.cat.org.uk
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Energy Saving Trust: 0854 727 7200; www.est.org.uk
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Green Building Bible. Third edition. Published 2006 by Green Building Press. www.greenbuilding.co.uk
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Good Energy: 0845 456 1640; www.good-energy.co.uk
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Okofen wood pellet boilers: The organic energy company, 0845 458 4076; www.organicenergy.co.uk
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