Heating and Cooling
Reverse Cycle air conditioners, also known as heat pumps, can heat your home at less than half the cost of even the most efficient gas heater. For some of you, this is all the information you need to know to take action. But for those who are curious as to the science and economics behind this saving, read on.
For a detailed presentation about reverse cycle air con as a “Renewable Energy Giant” written by our heat pump expert Tim Forcey- that you can share if you wish – read more here.
The basic understanding of heating our homes is that
- You have an energy source: gas, wood or electricity.
- With gas and wood the energy source is literally burned in your heater. With old fashioned electrical heaters – the electricity is run through a resistive cable, which glows like an incandescent light globe
- Heat is transferred into your house
Lets have a look at an example, gas ducted heating, to get an understanding of how inefficient this traditional process is.
In this example, we start with 34MJ of gas energy and a small amount of electrical energy. As with any combustion process, not all of this input energy is able to be converted to useful heat. In fact, most of the energy contained in this gas is wasted and won’t help keep you warm in winter at all. The next figure shows how much heat you will get in your room from this 34MJ of input heat.
You only get 10MJ of heat in our home from 34MJ of gas being burned? That’s less than 30% efficient! Where does the rest of this energy from burning the gas go if it isn’t heating your room?
5 MJ is lost as “waste delivered heat” which means that often, ducted gas heating unavoidably heats rooms that are not being used and also over-pressures and under-pressures different parts of the house, leading to greater heat loss via draughts especially if a house is poorly sealed.
Now the duct losses. Most gas ducted heaters have a central burner where the gas is combusted and turned into heat, and to get this heat into your rooms, it is carried through ducts. These ducts are inefficient at holding heat, meaning that a large amount of heat is simply leaked into your roof cavity or under your floor – more waste!
Then the flue losses – much of the heat goes straight up the flue, heating the outside air instead of your home. Of course just as we can’t have our car exhaust pumped into the cabin of our car, this flue is needed to remove the dangerous exhaust from the gas heater.
So in total, ducted gas heating is less than 30% efficient.
Reverse cycle air conditioners
Looking at the inputs for the Heat Pump, we see that it is only using 2.55 MJ of electricity.
But what about this ‘ambient heat’ component of 9.53MJ? Where does this ambient heat come from?
Well, without going too into the Physics, air that we feel as cold, say 5 degrees C, actually still has lots of heat energy in it. It isn’t until the temperate drops to minus 273 degrees C that all the energy is gone.
So what a heat pump does is extracts this ambient heat from the air and pumps it into your house.
How does it extract the heat? Well it runs refrigerant through a coil, and then uses a fan to move the outside air over these refrigerant gases. As the refrigerant is kept very cold, say -30 degrees C, the heat from the outside air is transferred into the refrigerant at a fast rate.
See the diagram at the bottom of the page for a more detailed explanation of the various steps involved in moving heat into your home using a heat pump.
How much cheaper is a heat pump compared to a gas-ducted heater?
How do we know a heat pump air conditioner can heat your home for less than half the cost of a gas heater?
Here are the economics
As the diagrams above show, both the gas heater and the heat pump deliver 10 MJ of useful heat into your home. To get this heat, you have to either buy,
for the ducted gas system, 33.41 MJ of gas and 0.61 (0.17 kw-hr) of electricity for the fan;
or for the electric heat pump, 2.33 MJ + 0.22 MJ (0.71 kw-hr) of electricity.
Mutiplying by unit costs from recent bills in Melbourne Victoria,
Cost of heating with ducted gas = (33.41 MJ of gas @ $0.0117 / MJ) + (0.17 kw-hr @ $0.16 / kw-hr) = $ 0.42
Cost of heating with electric heat pump = (0.71 kw-hr @ $0.16 / kw-hr) = $ 0.11
So electric heat pump heating operating costs in this example are 26% of the cost of gas heating.
(Electricity price basis: Energy only, excludes fixed supply charges, January 2016 Powershop, Victoria, includes discounts, excludes GST)
(Gas price basis: Energy only, excludes fixed supply charges, November 2015 AGL, Victoria, includes discounts, excludes GST.)
AND if you have solar PV, you might assign an even lower price for electricity. And if your dog has wrecked your underfloor gas ducts, your gas heating might be even less efficient than the above calculations show.