Morning Report rang me this week for comment about overheating homes, in the face of record temperature spikes. You can listen to the short segment here, or there’s a print version also.
For the physics geeks, here is how I worked out the numbers I provided about emissions. I used an average grid emissions factor of 0.12 kgCO2e per kWh of electricity. A typical hot water cylinder with local pipe install uses 80W every hour, on standby to provide hot water. That amounts to 84 kgCO2e/year.
A new heat pump with R32 accounts for 30 kgCO2e/year in emissions for the refrigerant leakage. I’ve used ASHRAE leakage rates for domestic heat pumps of 2% annually.
The average measured New Zealand home uses 6900 kWh of electricity annually. Roughly a third of that is heating, about 2300 kWh. If that is resistance electrical heating (100% efficient) that emits 280 kgCO2e/year. Switching to a heat pump with a COP of 3 (300% efficient) sees emissions drop to 93 kgCO2e/year.
Even if you assumed cooling used the same amount of energy as heating, less energy would be used by a heat pump to heat AND cool compared to heating with resistance electricity alone. But the worst performing house I’ve seen used less than a third of the energy for cooling than it did for heating.
Of course, I concur with the comments Nigel Isaac made about the need for better design. “We fail to recognise that limitless views come with limitless problems,” is a snappy line I’ll probably borrow.