Attic Heat Harvester
attic heat harvesting system, 1 floor

UK Attic Heat Harvesting Performance Example

Attic heat harvester systems provide very cheap heat

A recently developed innovative Attic Heat Harvesting system provides solar powered home heating, cheap heat which reduces heating costs.

The Attic Heat Harvester System harvests and transfers solar heated attic air, when it is both available and required, into the living area using an attic mounted fan, ducting and a powerful microprocessor based control system to provide cheap solar powered home heating. In comparison with all other home heating systems 'cheap heat' is produced because the heat energy transferred is far greater than the energy used to transfer it.

Temperatures (degrees C) in 4 Home Locations from 25 March to 4 April 2012
Balerno temperatures March 25 - April 4

The chart shows temperatures for an eleven day period in 4 separate locations by a data logging system in an Edinburgh, Scotland home.

The harvesting system fan only runs when attic temperature - the red line, exceeds the heated area temperature - the green line by a control system pre-settable value.

When attic temperature exceeded the heated room temperature plus the pre-set offset the system fan was turned on, when the fan runs the heated area temperature rises.The heated room has a number of sun facing windows that provide passive solar heating so the impact of the Attic Heat Harvester System is masked. Passive solar heating also reduces the Attic Heat Harvester System performance figures.

In the last 3 days both attic temperature and outside temperature were low and the fan was not turned on. On the morning of 3rd April a wood burner, located in the living area and rated at a maximum output of 12 kW, was lit and operated at a low setting.

From 25th to 28th March the north 'non sunny' internal slope of the attic roof was insulated to maximize attic heat, this will increase maximum and minimum attic temperatures increasing system efficiency, and also improve the home insulation levels.

The Attic Heat Harvester Power, i.e. the heat energy transferred from the attic to the heated room, was calculated and recorded by the control system as are the Fan running hours, these figures enabled the following chart to be produced.

Attic Heat Harvester Power per day Chart
AHHS Power per day chart

The Daily Cost of using the AHHS is calculated by:

Daily cost = Fan Running hours each day * Fan power * electricity cost per kWh.

The fan power is 90 watts. On 1 April 2012 The UK average electricity cost is £0.137 per kWh.

During the 11 day period the total solar heated attic air energy harvested was 46.5 kWhrs. The fan operated for a total of 35.1 hrs consuming 3.16kW which cost £0.42. In effect the cost per kWh was £0.42/46.5, i.e. 0.9 pence per unit which is 'cheap heat'.

The system efficiency, as calculated by energy out/energy in, was 46.5/3.16 = 14.7.

Solar Powered Home Heating

The difference between the attic temperature and heated room temperature indicated in chart 1 is proportional to the energy transferred. In this example the average temperature difference during the period was relatively small so the overall system efficiency achieved was modest. However the temperature difference each day and hence the daily efficiency figure varied achieved and is shown in the following chart.

Daily efficiency figures
Solar powered home heating, Balerno efficiency figures

When the attic - heated room temperature is high the system efficiency is high, and when low the efficiency is low.

The Attic Heat Harvester system blows hot air via a duct into the living area. In order for the system to operate, air in the living area has to escape or back pressure will reduce or stop the air flow.

The heated room area of the 2005 home where the attic heat harvesting system was fitted and tested, is a large 'open plan' kitchen, dining and lounge area having a partially vaulted ceiling and a volume of some 220cum. The area has 6 internal doors having air gaps around their edges, these allow warm air to escape into other parts of the home.

One of the internal doors leads to another large room which has one small window, the second door from this room leads to a hall area. The purple line in chart 1 shows the temperature in this room. The impact of the harvester system on the adjacent room temperature is clearly shown. From 26 to 28 March the both doors to the adjacent area were opened around midday permitting heat to flow freely through the room. When the attic heat harvesting system was not operating the temperature in the adjacent room dropped slowly, it continued to drop even when the wood burner was operating.

In effect solar heated attic air is reused in cooler areas of the home after it exits the heated room, this further increases the systems efficiency but this is not accounted for in the calculations.

A comparison on the operating costs of a number of different home heating systems can be found elsewhere on the web site.

Attic Heat Harvester System performance is dependant on your homes Latitude. Balerno's Latitude is 55.95°N which is similar to the Latitude at the lower part of Hudsons Bay Canada. In general attics get hotter at lower latitudes.

Attic heat harvesting systems are an incredibly efficient method of utilsing solar powered home heating. Inexpensive, relatively easy to install, provide cheap heat, simple to maintain, very reliable and they ventilate attics - no other heating system can compete.