Home Forums General Discussion Very high energy bill in 8 year old house – no apparent cause Reply To: Very high energy bill in 8 year old house – no apparent cause

Ian McChesney

Hi Sarah
Regarding technical info on ventilation systems, you are right, suppliers are very vague about this info. I did a report for EECA on home ventilation systems about 5 years ago, and the following is what I wrote about typical fan specs and running costs:

“Energy for fan operation – this depends on air flow rate, pressure and resistance in ducts, efficiency of fan and motor, fan speed and operating time. Most fans/motors used in PP-RCHT systems typically draw power in the range 20-100W with suppliers citing running costs typically stated as “8 cents per day” or “no more than an average light bulb to run” etc. These costs typically work out to be around $2.50-$10/month, which with electricity prices of about 20c/kWh equates to some 12-50kWh/month. In fact it is quite difficult to determine how much electricity PP-RCHT systems will use because of the many different assumptions used about fan flow rates, operating hours, pressure and resistance in ducts, pressure within the house, efficiency of fan and motor etc.

AirCare Ltd (suppliers of the Homevent system) is one of the few suppliers to provide a transparent calculation on their website. They cite a maximum fan load of 85 Watts, with a typical load factor of about two-thirds (i.e. 57W). Energy use is thus 1.36kWh/day, or ~41kWh/month ($8/month). This is at the higher end of costs cited by suppliers.”

Newer systems may have improved somewhat, but the above is probably still reasonably applicable for older systems.

The specs for heating units (if installed) – generally they come in size increments of ~1kW.

Regarding the warming of the roof space with the downlight heat – not sure that there would be sufficient heat to raise the cavity temperature that much? I imagine the roof cavity is fairly large, with a large area of exposed roof that will lose heat quite rapidly most evenings. I guess the householder could monitor roof cavity and house temperature differentials by checking the HRV controller regularly for a period?

Hope this helps