RETROFITTING 

Home built roof heating: Reduce heating bills with energy from the sun

If your house has the right type of roof and insulation combination, you can use the roof cavity as a huge solar heater. Alan Leenaerts describes his set up.

IF YOU have spare space on your roof (not taken up by solar hot water or solar electricity) then you have a cheap resource available to improve the comfort of your home. If your roof is well insulated (as it should be), none of the rooftop heat will get through the ceiling to warm your home—unless you actively transfer it into the living space.

The average Australian rooftop collects 220,000 kilowatt-hours of heat every year. In Melbourne, the range is from about 1200 watts per square metre in summer to about 600 watts per square metre in winter.

Solar heating systems are available commercially for $4500 to $8000. This is an expensive option if you already have a heater and it does not need replacing. You can easily make the following system for less than $600, and reduce your heating bills by up to 50%. With the system I built, my winter bill was lowered by 75% and payback was less than one winter.

Materials

For a 4x5m room, I used:

• an air transfer kit (about $90), which contains two ducts, one in-line fan and two vent grilles

• Herpetarium thermostat (about $70 online)

• a plastic box ($15)

• a self-closing louvre vent ($10). It prevents loss of warm air from the room by thermal convection at the end of the day

• a ‘DraftStoppa’ — the type designed to go in your ceiling space over a bathroom fan. Again, this prevents thermosiphon losses.

• duct tape ($10)
  

Installation

To install the room’s warm air inlet and ducting, I first cut a hole in the ceiling for a fixed louvre vent. Around this hole, I removed enough insulation to allow for the plastic box. I fitted the louvre vent into the ceiling and then inserted a duct into one side of the plastic box. The self-closing vent goes on the inside of this duct entry and prevents thermal convection losses. The box was then positioned over the vent in the ceiling.

I placed the insulation cut out from the ceiling on top of the box and put a brick on top to hold it in place. I sealed up the edge of the box where it met the ceiling with duct tape.

I then ran the duct to the fan and mounted the fan on a roof joist. From the other side of the fan, I ran a second duct to the apex of the roof, using duct tape to secure the duct in position.

To facilitate the exhaust air, I cut another hole for the second grille in the ceiling of a cupboard in the same room. I fitted the grille and put a ‘DraftStoppa’ in the ceiling space over the top of the grille to prevent recirculation.

Operation

When the cupboard door is closed and the fan is running, cold air from near the floor is sucked under the cupboard door and into the roof to be warmed, and eventually blown back into the room by the fan.

When the fan is running, warm air is drawn from the apex of the roof and blown into the room via the ceiling vent. Cooler air from the room is forced out through the vent in the cupboard into the roof space, where it is warmed and eventually makes its way back into the room.

I set the Herpetarium thermostat switch ‘on’ point at around 22 to 24 °C. There is little value in heat below this temperature so there’s no point running the fan below this. Whenever the roof temperature is above the set point, the fan runs and pumps warm air from the roof into the house. The sensor for the thermostat needs to be located close to the roof so that it can sense the air temperature.

I turn the thermostat off for summer so that I don’t overheat the house! I have a remote switch and turn it on in April and off in October, with the exact timing depending on the weather.

To heat a large room, or a whole house, you would probably need to use more than one fan. My house is 180m2 and I have four fans, operating out of half the available roof surface area.

Suitability and warnings

The system works better if the ceiling is insulated, rather than the roof, as the blanket style roof insulation will prevent heat entering the roof cavity. Tile roofs are not suitable for this retrofit as they have too much air leakage.

You also need to consider the type of insulation in your roof — fibreglass or mineral-fibre insulation are a source of free- floating fibres that would be a health hazard if you were to duct them inside. My insulation is the foil-backed variety so loose fibres are minimal. You could fit a filter to each of the ducts to remove such pollutants, but note that these would then require maintenance.

Result

This system has reduced my heating bill by 75%, with a 50% reduction in my total gas usage for the winter quarter in 2012 compared to 2011, and virtually no gas heating costs for the rest of the year. This system obviously works best in a well-designed and insulated house. Using this system, my 7.5 Star house can stay warm until 2 or 3am. No heater is required at all on a sunny day, even in the middle of winter when the usable sun is gone by 4 pm. On an overcast day you will still get some heat output, but you may need to run a backup heater on low.

My back-of-the-serviette calculations show that my house is performing at 46 MJ/m2 or 8.5 Stars — better than the hypothetical 7.5 Stars it’s rated at.


Reprinted from ReNew issue 123 (renew.org.au) with permission of the author