My Efficient Electric Home Handbook is an essential first-of-its kind resource for homeowners and renters. It provides tips and strategies on how to convert your home into an efficient, healthy and comfortable space suitable for our all-electric future.
Sharing insights from working with thousands of Australians in their homes, as well as from hands-on experience modifying his own home, TIM FORCEY explains best-practice heating and cooling, hot water heat pumps, induction cooktops, draught-proofing, insulation, solar energy and much more.
Tim tells us the important things we need to know about heating and cooling …
Heating and cooling
For most Australian homes, the biggest energy cost is for heating and cooling your living spaces. This chapter discusses the cheapest, cleanest and healthiest ways to actively heat and cool (i.e. using heating and cooling appliances). But don’t forget about ways you can improve your home to passively keep it warm or cool, without using any heating and cooling appliances at all.
You may live in a cooler Australian climate zone and be fortunate enough to have living areas with windows that face north and allow the winter sun to heat your home. A home’s orientation to the sun is important to consider when buying a new home, choosing a home to rent, designing a new home or renovation, and when re-configuring living areas, windows and eaves. It’s vital to consider how to make the most of the winter sun – this is known as passive solar heating. Conversely, you also need to keep the summer sun from cooking your home. But despite one’s best passive efforts, most Australian households will at some point need to turn on a heater or cooler.
The cheapest way to actively heat … is with an air conditioner!
In 2015 at the University of Melbourne, we were the first to work out how Australians could save a lot of money by using their reverse-cycle air conditioners for heating instead of burning gas. The savings added up to hundreds of dollars per year for a single household and billions of dollars of saved across the country. I came to realise this was a very big cost-of-living win, but no one else was talking about it!2 A reverse-cycle air conditioner can heat a home at around onethird the cost of burning gas and as little as one-fifth of the cost of using an electric-resistive heater, such as an oil column heater, fan heater or electric panel heater. As reported in 2015 in The Age, our research found that a large Melbourne household could save $658 per year by switching off the ducted gas heating and switching on the air cons.3 The saving is probably even larger today. I meet people all the time who are still unaware of this. One reason is that air conditioners have had a bad reputation.
Demonising air conditioners
Over the years, we’ve done a good job of demonising the use of air conditioners for cooling in summer. ‘Don’t use an air conditioner, it’s expensive, you’ll get a huge power bill’, ‘It will crash the electricity grid!’, ‘Use a fan, put a wet rag on the back of your neck, sleep in the nude!’ However, you may recognise that those comments pertain to using an air conditioner in summer. Indeed, if you can stay healthy and get by without using an air conditioner in the warmer months, I’m fine with that. But what about winter? A reverse-cycle air conditioner is an almost magical device that does two things: heat and cool. Amazing! I’m still amazed.
How often do you buy something for one purpose, and it turns out it’s really good at doing something else? It’s almost like you bought a car that is also very good at washing clothes. However, reverse-cycle air conditioners aren’t magic, they’re actually heat pumps.
How heat pumps work
Heat pumps aren’t magic, but they can be mysterious.4 So how do they work? Read on for a detailed explanation. You have at least one heat pump in your home already. It’s your refrigerator. It uses its refrigerant cycle to move or ‘pump’ heat from one place to another, collecting heat from around your food and pumping it back into your kitchen. A fridge has to work almost continuously because heat from your kitchen is continuously trying to get through your refrigerator’s insulation and into your food. But hopefully your fridge can keep up and your food and drinks remain nicely chilled.
A refrigerative air conditioner used for cooling works similarly to a refrigerator and is another type of heat pump. In summer, an air conditioner collects unwanted heat (not air) from within your home and pumps it outside. That’s why in summer, if you ever walk near the outside unit of a functioning air conditioner, the air being blown around out there will be quite hot. The air conditioner has pumped heat out of your home and added it to the environment outdoors. Note that in this process, no air moves from the inside of your home to outside, nor from the outside to inside. It’s only the refrigerant fluid, travelling through small insulated pipes, that cycles between your living spaces and outside, carrying heat out of your home in summer.
If you want the detail, the five key parts of a heat pump refrigerative air conditioner are:
* a heat exchanger located inside your home (with aircirculating fan)
* a heat exchanger located outside your home (with aircirculating fan)
* the refrigerant that circulates from one heat exchanger to the other
* a compressor to pressurise the refrigerant (thereby making it hotter), and to push it around the cycle
* an expansion valve through which the refrigerant loses pressure and cools.
When an air conditioner is cooling a room, refrigerant flows through the expansion valve and gets very cold. This cold refrigerant next passes through the heat exchanger in your home’s inside unit where it collects heat from the room with the aid of the fan blowing air through the inside heat exchanger. This warms the refrigerant. The warmed refrigerant then goes through the compressor, raising the pressure and temperature of the refrigerant. It’s now hot enough that when it next goes through the heat exchanger outside your home, the fan there blows outdoor air over the refrigerant, transferring heat to the outdoor environment and cooling the refrigerant. The refrigerant then flows through the expansion valve where it gets very cold, and the cycle begins again.
In winter, when a reverse cycle air conditioner is used for heating, refrigerant moves in the opposite direction. Refrigerant flows through the expansion valve and gets very cold, just as it did in summer. But this time the cold refrigerant passes through the heat exchanger in your outside unit and collects free renewable heat from the air outside your home, with the aid of the outside unit fan.
The heat collected from the winter air outside your home warms the refrigerant. The refrigerant then gets pushed along by the compressor, raising the pressure and temperature of the refrigerant, making it so hot that when the refrigerant next goes through the heat exchanger inside your home, the fan there blows room air over the refrigerant to transfer heat to your indoor living space. Giving up this heat cools the refrigerant. The refrigerant then flows through the expansion valve and the cycle begins again.
That’s how a reverse-cycle air conditioner works in winter. Still people may wonder, ‘If it’s only 4 degrees Celsius outdoors, how can there be heat in the air?’ The answer is there is always heat in our air, as long as the sun shines somewhere on our planet and our air remains above the temperature of absolute zero or minus 273 degrees Celsius.
It’s correct to say that reverse-cycle air conditioners are actually another way to use solar energy. The sun heats our air and heat pumps collect free renewable heat from that air. The temperature of the refrigerant circulating in a reverse-cycle air conditioner in heating mode might be as cold as minus 30 degrees Celsius, therefore that cold refrigerant will easily absorb heat from the air outside your home, even if it’s only 4 degrees out there on a frosty morning. This means that heating with reverse-cycle air conditioners is a perfect fit for Australian homes. They are also used in far colder places such as Canada and Finland.
Why a reverse-cycle air conditioner is so cheap to run
Why are reverse-cycle air conditioners the cheapest way to heat your home? Because as a heat pump, they collect free heat from the air outside your home. Yes, it requires some electricity to run the compressor and the fans, but this electricity use is leveraged several times during the process of transferring heat from outdoors to indoors. Some people say heat pumps don’t make heat; they simply move heat around.
This is true, and importantly it costs a lot less to move heat around than it does to make it. This is like how it might cost you something to grow a tomato in your garden, but it won’t cost you that much to then move that tomato from your garden to your kitchen. In this way a reverse-cycle air conditioner can be said to be operating at, for example, 400 per cent efficiency. A single unit of electrical energy (e.g. 1 kilowatt-hour) going into your air conditioner can result in 4 kilowatt-hours of heat coming out.
That ratio of four to one (or whatever the exact ratio might be for the model of air conditioner you’re thinking of) is called the Coefficient of Performance (COP). For any air conditioner, you can find the COP listed among the device’s technical specifications. My wife says I should express it this way: ‘Buy one, get four.’ She’s never seen a deal for shoes that good.
Why are reverse-cycle air conditioners the cheapest way to heat your home? Because they collect free heat …
A COP of four means that if you’d like heat coming into your lounge room at the rate of 2000 watts, you’ll only have to put 500 watts of electrical power into your air conditioner. How does this compare with a simple electric-resistive heater? An electric-resistive heater (e.g. oil column heater, fan heater or electric panel heater) can be 100 per cent efficient: 1 unit of electricity in, equals 1 unit of heat out. While ‘100 per cent efficient’ might sound good, it can’t compete with the 400 per cent efficiency of an air conditioner.
A gas burner has even lower efficiency than any electric heater. The moment you buy gas, the first thing that happens is that you set it on fire. A good part of the heat available is immediately lost up the flue or chimney. Therefore, the heating efficiency of gas is always less than 100 per cent and can be even lower than 50 per cent for older gas heating systems.
It’s very difficult for gas heating to compete with an conditioner that may be at least five times more efficient. To compete, gas would have to be one-fifth the cost of electricity (on a $/MJ or $/kWh basis) and it’s generally not. A common gas price I am seeing on Victorian bills these days may be $0.03/MJ, which is equivalent to $0.10/kWh. Electricity is more expensive than gas on a $/kWh basis, but it’s not five times more expensive. I’m currently paying $0.28/kWh for electricity, which is only 2.8 times more expensive than gas priced at $0.03/MJ ($0.10/kWh).
During sunlight hours, Australian homes with solar PV panels on their roof can heat with an air conditioner for even less money. This may be the cheapest heat in the world. Sometimes it’s actually free. How is the gas industry meant to compete with that?
Avoiding the sunk-cost fallacy
Here’s about the saddest story I’ve found. I was at the home of a young couple, sitting in their lounge room beneath their split-system air conditioner. I asked them how they normally heat their house. They said they had just spent $4000 to have their ducted gas heater replaced.
But then they added, ‘You know, we were lucky because we could use the split system above you there for heating for a few weeks before the furnace was replaced.’
The look on their faces wasn’t great when I told them they could keep running the split system instead of their new gas heater if they wanted to save hundreds of dollars each winter. Their story is not an unusual one. I hear it time and time again. It was $4,000 for the new gas heater down the drain, basically. Never mind. What you’ve spent in the past on gas equipment, it’s in the past! Avoid the ‘sunk-cost fallacy’, cut your losses and stop using the gas equipment. Even if it’s only two years old or two weeks old, it doesn’t matter, it’s costing you. And since you likely already have or will soon be getting a reverse-cycle air conditioner for summer cooling, use this as your cheapest way to heat. Get off the gas! Your now-redundant gas burning equipment can be sent off to the metal recyclers. The valuable metals won’t be wasted.
Your gas heater could kill you, but your air conditioner never will
Safety alert! To ensure poisonous carbon monoxide gas isn’t entering your home, gas heaters must be checked by a qualified technician every two years.5 An air conditioner, on the other hand, won’t kill you with poisonous gas.
Ditching gas-fired heating and using an air conditioner means more savings because there will be less need for frequent servicing. Although air conditioners (and gas heaters also, for that matter) have filters that need to be cleaned regularly, this is something that you can probably do yourself. Beyond this, if an air conditioner has been operating fine for a period of four years or more with no professional servicing, it might be time to call in a technician to give the equipment a professional check and clean.
I suggest you closely observe the professional air conditioner cleaning process. Learn what is found. Was there any mould hiding within? Is it particularly dirty? Discuss with the technician how to reduce the chances of mould growth or dirt build-up happening again. Ask how parts can be DIY cleaned.
Buying a new air conditioner? Should it be ducted or not?
You’re reading this book and so perhaps you’ve decided, yes, you’ll be getting your home off gas heating! Now you are in the market for a new reverse-cycle air conditioner. Should you get a ducted system or individual (ductless) split systems? Generally, the most cost-effective way to set up a home with reverse-cycle air conditioning is to use individual split systems. 70 Part 2: How to create your own efficient electric home
Why? Because ducted systems:
- cost more to install
- cost more to operate
- restrict how you can use your rooms
- cause you to heat more space than you need.
I explore each of these points below.
Ducted systems cost more
Ducted heating and cooling is also known as ‘central’ heating and cooling. Most commonly it means using ducts – either installed up in your home’s roof space or down under your floor – to transfer heated or cooled air from a central heater/cooler to individual rooms. Firstly, a ducted air conditioner is likely to cost more to install than an array of ductless individual split systems). (I describe the various types of ‘split systems’ below.) Secondly, ducted heating and cooling systems can be less efficient and more costly to operate than ductless systems.6 A lot of heat will be lost out through the ducts in winter and gained in summer. Why? Because in Australia:
* ducts are never well insulated
* ducts are commonly installed outside of the building’s thermal envelope, either hanging near the ground beneath the floor or placed above the insulation up in the roof space
* duct installers might not do a perfect job given that they are often working in tight and uncomfortable spaces
* ducts are generally not inspected nor maintained and can be damaged by animals or humans crawling under the floor or in the roof space.
When we had ducted gas heating, the flow of warm air to the bathroom was never strong. I only went investigating a few years later and found that, beneath the house, a cat had chosen to sleep on the warm and cosy duct, crushing it. After we had converted our home to heating with individual split systems, I pulled out the old gas-fired heating ducts. Most of them were in good shape, but I was sad to find in one place the installer had not joined two pieces together properly.
This meant that for the past twenty years whenever we were heating with gas, we were constantly losing air and heat through an unsealed joint. Who knew? More sensible building practice would see ducts being placed within the home’s thermal envelope, as is commonly done in countries with more severe climates. However, this arrangement must be established when the home is being built. It’s not easy to retrofit.
Air must return to the return-air inlet
Thirdly, many home occupants are unaware that with a centrally ducted system, heated or cooled air must return to the return-air inlet. In other words, a centrally ducted system is meant to be operated as a closed-loop system.
What does this mean? Once conditioned air comes into a room, it needs to find a clear pathway back to the return-air inlet, which is often located in a central corridor. In other words, when an air conditioning system is being used, bedroom or office doors must be left partially open to allow air to return to the return-air inlet. This means that zoning off sections of the home by closing interior doors cannot be successfully done with a centrally ducted system.
What this means in practice, is if you close the door of a bedroom when the central heating/cooling is on, you have choked off the system. The bedroom becomes over-pressured. The air that has come into the room can’t find a way out other than to leave through a leaky bedroom window. On the other end of the system, there’s not enough air returning to the return-air inlet. The system is now out of balance.
To make up for the missing air, the system will be forced to draw air into the home from under the front door (for example). What was meant to be operating as an efficient closed-loop heating/cooling system is now an inefficient once-through system. Can you imagine the bills! In many homes the occupants don’t want to leave doors open. A person working in a home office may be communicating on Zoom and need to keep the door closed to reduce noise and enhance privacy. A child or shift-worker may need to sleep. A teenager may go into a bedroom, never to come out again. Therefore, doors will be closed. However, be aware that closing the door corrupts the operation of a centrally ducted system.
ABOUT THE AUTHOR
Formerly, I worked as a chemical engineer in industry. Home energy improvement was just a hobby.
Then, 12 years ago, I became a qualified home energy advisor. I have now advised in over 1,000 homes. I serve the Melbourne area primarily, and elsewhere in Victoria by special arrangement.
My advice and experience was recently recommended by the energy company AGL to their 800,000 customers.
With the University of Melbourne, I researched home energy economics. This featured in The Age Domain, the Herald Sun, on Channel 9 television, and on ABC radio in Victoria and interstate.
I have published many articles on the Conversation, Renew Magazine, the Fifth Estate, and elsewhere.
I am a public speaker and often present to community groups.
I have advised energy companies, local councils, the Victorian state government, and have presented to the Victorian Parliament about home energy opportunities.
To help even more people improve their homes, I created and continue to administer the rapidly-growing Facebook group “My Efficient Electric Home”, which now has over 50,000 members
As a result of what I have learned and practiced over many years, today I am one of Melbourne’s top home energy and comfort experts.
Further, I have been qualified by the Victorian Government to assess homes using the Victorian Residential Efficiency Scorecard.
0 Comments