Attic Heat Harvester
attic heat harvesting system, 1 floor


Heat your home for the cost of running a light bulb

The Attic Heat Harvesting System

Energy costs are rising rapidly and are expected to continue rising. In the five years since 2008 UK average annual heating costs have risen by 63% to £600 for an average home. In 2012 on average natural gas costs each Scottish household £1,236 per year, 75% (£930) of this is used for space heating!

<supplementary heating system - buy now Solar energy based supplementary heating systems

In a UK three bedroom semi detached well insulated home solar energy based efficient supplementary heating systems typically reduce heating cost annually by from £20 to £160. In larger or poorly insulated homes the annual savings can be £100's.

Solar energy based supplementary heating systems decrease the use of conventional heating systems and heat pumps reducing air pollution and greenhouse gases that result from your use of fossil fuels such as natural gas, oil, propane, and fossil fuel generated electricity.

Harvesting solar heated attic air is by far the most efficient way to heat your home. The Attic Heat Harvester supplementary heating system is typically five to ten times more efficient than ground or air source heat pumps and ten to fifty times more efficient than other home heating systems.

Impact on room temperatures of harvesting solar heated attic air

The air in attics with sun facing sloping roofs can absorb 100's of kWhrs of solar energy conducted through the roof each month. The Attic Heat Harvester System was designed and developed to make the most efficient use of this free solar energy.

Impact on room temperatures of harvesting solar heated attic air solar heated attic air temperatures

This chart shows room temperature, attic air temperature, outdoor temperature and daily average outdoor temperature recorded every 5 minutes. The green line shows the impact on room temperature when not using and using attic heat harvesting. The Attic Heat Harvester Controller calculates and displays system status and performance. During the four day period the fan ran for 19.2 hours transferring 30.9 kWhrs of heat, the fan operating cost was £0.22 so each kWhr cost £0.007.

When solar energy is absorbed by the roof the attic air temperature rises after a delay permitting the heat to conduct through the roof structure. The roof structure also retains heat for some time after the solar energy peak, which occurs soon after midday, extending the time that heat energy can be transferred to living areas, the delays can be seen in the chart.

The Attic Heat Harvesting Controller, continually monitors the attic and room temperature sensors. If the attic air temperature (red line) is greater than the room temperature (green line) plus a user pre-set temperature offset, an air valve is opened and the fan turned on transferring solar heated attic air via an insulated duct and an air valve to the living area increasing the room temperature.

If the attic temperature is not high enough the fan is turned off and the air valve closed. If the room temperature is above a user pre-set maximum, the fan is turned off and the air valve closed preventing the room getting too hot. A closed air valve stops heat loss from the room into a cold attic.

Increasing the user pre-set 'attic minus room temperature offset' increases the Attic Heat Harvester System efficiency as the temperature difference between the attic and room has to be higher before the fan is turned on. Increasing the offset also reduces the amount of solar heated attic air transferred as the fan will run for a shorter period of time.

supplementary heating system graphic
Attic Heat Harvesting System for a 2 floor home
solar heated attic air implementation schematic

Attic Heat Harvester Schematic

In essence the attic heat harvesting system is simple comprising of an electric fan mounted near the attic apex fairly close to the sunniest portion of the roof, a flexible duct, an air valve, and a specially designed Attic Heat Harvesting controller that controls the fan(s) and air valve(s). The system's simplicity makes it incredibly efficient, inexpensive to buy (around £500) and run, easy to install, cheap to maintain, and a very reliable supplementary heating system to reduce heating costs taking advantage of solar heated attic air.

The attic heat harvester controller has a programmable timer enabling weekday and weekend heating start time to be set if required.

An additional motorized air valve can be used to replace the optional ceiling return air vent. This improves system efficiency further as some of the warmed home air is feed back into the attic to be reheated then re-circulated to the living area. An additional motorized air valve was fitted to the Balerno home early in 2013 improving system performance since then.

Using Solar Heated Attic Air as a Supplementary Heating System

In an average size home an insulated 305mm (12 inch) diameter air duct is adequate. In homes having two or more floors the duct can often be routed through cupboards. For maximum air flow the duct should be as straight if possible as bends will reduce airflow and system efficiency.

Backdraught Shutter To blow hot air into a room an air escape route is needed, this is usually through air gaps. Air gaps in external doors and windows should be minimised to ensure that the heated air escapes to heat the rest of the home.

For closed rooms we recommend that automatically closing backdraught shutters be installed close to floor level in internal walls to permit the heated air to easily move through your home to desired areas.

Attic Heat Harvester Controller calculations do not include the additional heating that will be achieved.

The Attic Heat Harvesting Controller can be supplied with an attic cooling fan control option that reduces excessive attic temperatures and home cooling costs at locations where attic temperatures get very high. When the attic temperature exceeds a user pre-set maximum an additional attic fan is turned on exhausting hot attic air. This fan can be mounted low in an attic wall or soffit so that it draws in cool outside air, or at some high point in the attic wall or roof so that it sucks out hot attic air.

Attic heat harvesting system effectiveness and efficiency is dependant on: the local weather; the design and construction of the home; the homes insulation levels; the attic size, type, pitch, and orientation; the duct length and straightness, and on the temperature of the area being heated. Best performance will usually be achieved in single floor homes.

Attic heat harvester systems are incredibly efficient, inexpensive to purchase, relatively easy to install, cheap to operate, simple to maintain, and very reliable, no other heating system comes close in any of these criteria.