Supplemental home heating for the cost of running a light bulb
Two prototype Attic Heat Harvester Systems, designed to use attic heat, were installed, tested and enhanced over a period of two years. The first system was installed in Balerno, near Edinburgh, Scotland, and the second in Toledo, Ohio, USA. Over the period system performance data has been logged and proves the huge benefits of using attic heat, especially from the perspectives of reducing the use of fossil fuels and reducing heating costs.
The air in attics with roof surfaces open to solar radiation gets hot even on cloudy days as the sun's energy penetrates most clouds. The attic Heat Harvester System enables you to use attic heat very efficiently and at an extremely low cost.
Home temperatures have been monitored for many years at both the Edinburgh and Toledo homes.
Attic temperatures and Outdoor temperatures 9th May to August 26th 2011
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The red only portions of the chart show where attic temperature exceeds the outside temperature providing the option to use attic heat, the greater the difference the more efficient the heat harvesting is.
The 2011 summer in Balerno, Scotland, was somewhat cooler than normal. Between 9 May and 27 August there were only 3 almost totally sunny days on 3rd June and 3rd and 27th July, indicated by high attic temperature spikes, the majority of days were cloudy or partially cloudy sometimes with periods of rain.
However over the 109 day period in the chart, attic temperature exceeded 21°C for 778 hours (32.4 days) giving the opportunity to use attic heat. Additionally, the average number of hours that the attic temperature exceeded the room temperature by more than 3°C was 654 hours (27.25 days), and the average temperature difference during this time was 6°C.
The amount of heating required can be indicated by the average monthly outdoor temperatures which were:
System testing for 2 years in both the UK and the USA show that peak periods for using attic heat occur when average daily outdoor temperature is between 10 and 15°C (50 to 60°F).
Using attic heat
The following 5 day chart from 5th to 11th August 2012 shows room, outside, attic and roof surface temperatures related to a small portion of the chart above when the harvesting system was active using attic heat. The system controller was pre-set to provide a maximum room temperature of 22°C so only a small portion of the heat available was used.
The peak roof surface temperature shown by the pink line was some 20°C above the attic air temperature. The Edinburgh home roof consists of slates mounted to a wooden planked roof, the high sunny roof surface temperatures show that roof design can evolve to a) increase the free solar heat available, b) increase the use of attic heat, and c) extend peak periods of operation.
The heat and efficiency of the Attic Heat Harvester system can be increased by adding internal reflective insulation to non sunny sides of attic roofs.
Attic heat harvesting systems are incredibly efficient, inexpensive, relatively easy to install, cheap to operate, simple to maintain, and very reliable - no other heating system can compete under any of these headings.