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Heat Pump Buyers Guide :: Which Heat Pump To Buy

Heat Pump Buyers Guide

The most efficient electric heating

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A heat pump buyers guide

Electric heating is often the only option for many people with no access to natural gas. But not all electric heating systems are the same. Lance Turner examines why heat pumps come out on top. WHEN most people think of electric heating, the images that come to mind are bar heaters or fan. heaters. No matter what type of electric heating you have, they all have the same efficiency in turning electricity into heat— 100% (i.e. 1kWh of electricity produces 1kWh of heat). All except one, that is.

Unlike other forms of electric heating, which use resistive elements of one form or another to turn the electricity into heat directly, there's one type of electric heating that is much more than 100% efficient-heat pumps.

Heat pumps are all around us. Your fridge is a heat pump, but it only works in one direction. True heat pumps are bidirectional, and when used for space heating and cooling they are more commonly known as reverse-cycle air conditioners. Traditionally, air conditioners were used to cool homes, but some bright spark realised that they could be used backwards and so the reverse cycle system was born.

So what exactly is a heat pump and how can it be more than 100% efficient? As its name suggests, a heat pump pumps heat from one place to another. Instead of turning energy from one form (electricity) into another (heat), it uses electric energy to move heat from one place to another. Because heat is relatively easy to collect and move, heat pumps can move a lot more heat energy than the electric energy they use. For a brief explanation of how heat pumps work, see the 'Heat pump basics' box at right.

Efficiency: COP and EER

The beauty of these systems is that a great deal of heat can be moved from one place to another with a relatively small amount of energy input. This means that you can heat several rooms for the same energy input that would be needed to heat just one room using a resistive-type electric heater. In effect, the system has an efficiency greater than 100%.

Another advantage of using heat pumps is that your greenhouse gas emissions from heating will be much lower than from using electric resistive heating. Further, if you use 100% GreenPower, your heating will effectively have zero emissions.

The efficiency of heat pump systems is given by a coefficient of performance (COP). This is a ratio of the heat moved to the electrical energy input. As an example, if your heat pump uses 1kWh of electricity to move 4kWh of heat from outdoors to inside your home, then it has a COP of 4. There are actually two terms that mean the same thing, but are used in different situations to avoid confusion. When a system is cooling a home, its cooling efficiency is referred to as its energy efficiency rating (EER), while when heating it is called the COP. For this guide, we are only looking at heat pump systems that have COPs of 4 or greater (well, 3.9 or greater to be exact). There are a surprising number of those, as can be seen in the tables.

Other terminology

While we are talking technical, there are a few more terms we should clarify. All of the systems in the tables are what's known as split systems. This means that, unlike the older style air conditioners, where evaporator, condenser, compressor and all the other bits were inside one large box, split systems have them separated and linked by flexible or rigid high-pressure hoses or pipes.

Split systems have the compressor and one set of coilsin a box outside, often mounted against a wall. The part that fits inside the home is called the air handling unit and consists of the other set of coils, a fan to force air over them and the electronic controls for the system.

Air handling units are usually 'wall hung' but there are other types, including floor mounted and 'cassette' types, which are mounted in the ceiling.

Virtually all high-efficiency splitsystem heat pumps are of the inverter type. What this means is that instead of the compressor motor simply being on and off (remember the clunk when your old box air conditioner switched its compressor on and off?), the compressor is controlled by a variable-speed drive or inverter. This allows the compressor to only run as hard asrequired, making the system more efficient and reducing electricity use.

The split system has several advantages over older all-in-one systems. Firstly, the bit inside the home is more compact. Secondly, you don't have to cut a hole in a wall or sacrifice a window to install them. All they need is a couple ofsmall holes for piping and cabling.

Also, because the heating and cooling sections of the system are well separated, there is no leakage of heat from the hot side to the cool side, thus improving system efficiency.

And lastly, because the compressor is outside, they are less noisy as the only noise is from the circulating fan.

Refrigerants

We should take a quick look at one of the critical components of a heat pump-the fluid that does all the work, the refrigerant.

In decades past, all sorts ofscary and environmentally damaging fluids were used, including CFCs (chlorofluorocarbons, the ozone destroyers), HCFCs (hydrochlorofluorocarbons-better for the
ozone layer, but strong greenhouse gases) and many others. For a full list of the many refrigerants, see en.wikipedia.org/wiki/Listof_refrigerants. The most common refrigerant in domestic
heat pumps seems to be R410A, which is a mixture of difluoromethane (CH2F2, called R-32) and pentafluoroethane (CHF2CF3, called R-125). While it is ozone-layer friendly, it has a greenhouse-gas potential estimated at around 1725 timesthat of CO2.

System compromises

To keep costs down, some manufacturers compromise designs to some degree, which can reduce system efficiency.

The most easily seen example of this is the use of the same size air handling unit on compressor units of differentsizes. If you look at the specifications of the different models in some manufacturers' ranges you will see that the air handling units of systems with progressively larger compressors are the same. The manufacturer increases compressor size and therefore heating and cooling capacity, but uses the same sized air handling unit for all models. This means the larger capacity models in the range will be lese cffieicnt, and you can actually see this
when looking at specifications side by side.

A manufacturer might compromise efficiency like tins to save manufacturing costs - it's a lot cheaper to produce one size of air handling unit rather than a different one for each model in the range. I won't single out individual manufacturers who do this, but tear it in mind when checking specifications.

Heat pump basics Heat pumps use a closed system that contains a liquid with a low boiling point, called the refrigerant. A compressor adds energy to the refrigerant as well as increasing the pressure, forming a superheated vapour. This enters a set of coils known as the condenser where the vapour forms back into a liquid, giving up some of its heat energy in the process. It then flowsthrough an expansion valve where the pressure is abruptly reduced, causing some of the refrigerant to form a vapour. It then flows into another coil called the evaporator where it absorbs heat and flows back to the compressor and the cycle repeats.

In a cooling-only air conditioner, or a fridge or freezer, the evaporator is inside the house or fridge cabinet and the condenser is outside. Thisis why the back of the fridge gets warm. In a reverse-cycle system, the system uses a reversible expansion valve and so the inside cooling coils can be either evaporator for cooling or condenser for heating. Of course, the same applies for the outdoor coils.

Improving system efficiency

So, is there anything you can do yourself to improve system efficiency? There are actually a number of things. The lower the temperature differential between the condenser and evaporator, the more efficient the system will run and the less energy it will use to move a specified amount of heat.

The first thing to consider is the placement of the compressor unit. It should be placed outside in full winter sun if possible, but should be shaded with a deciduous tree or shrub during summer. This allows it to be heated by winter sun and so collect heat more efficiently, thus improving system efficiency when heating in winter.

In summer, the compressor will be shaded by the vegetation and so will be more effective in expelling heat. This simple trick can improve efficiency and reduce running costs. This lowering of temperature differential is also the trick used by a special type of heat pump which rather than using the air as a source or sink for heat, uses the ground or a body of water.

The temperature of the earth even just a couple of metres below the surface is quite stable throughout the year, varying far less than ambient air temperature. By dumping excess heat into the ground in summer and extracting it from there in winter, greater efficiencies in heat pump systems can be achieved.

Such systems are known as ground-source heat pumps (sometimes less accurately called geothermal heat pumps) and although they can achieve quite higli COPs, they can be expensive to install. Rather than having a compact external compressor and coil unit, they dissipate or extract heat using a series of coils or pipes buried in the ground, either horizontally (when there is adequate yard space available) or vertically (where space is limited). The big disadvantage with such systems istheir cost-they can cost many times what a conventional air-source heat pump will cost, and at current installation and energy prices they may not pay back the difference in many cases. However, if reduced energy consumption is your primary concern, then a ground-source heat pump should be considered.

A compromise between ground-source and air-source heat pumps is the water-source heat pump. These use a body of water for heat extraction and dissipation, so are only applicable to sites where a large volume of water, such as a dam, lake or large water storage tank is available. They cost more to install than an air-source heat pump but less than a ground-source system, as coils are simply placed in the body of water, so there are no drilling or trenching costs.

"Don't get too excited about all the built-in gadgetry—after ail, how much of it will you actually use. and how much of it will you forget about after the first week."

Having said this, there are very few ground? source or water-source systems available for domestic use in Australia. The ground-and water-source heat pump industry is yet to become competitive, which will ultimately drive prices down. Because of this, we are not looking at these systems further in this guide.

Other efficiency measures

Most air conditioners have filters inside the air handling unit to remove dust from the air. These should be cleaned around once a month when the unit isin use. They can usually just be washed with warm soapy water, rinsed and dried.

Like any heating system, a heat pump will use less energy if the home it is trying to heat doesn't leak heat like a sieve. The more efficient your home at preventing thermal transfer, the less energy your system will use and the more comfortable you'll be.

This means that you need to take all the usual efficiency measures, such as insulating roofs and walls (and under floors if possible), sealing draughts, and insulating windows with either double glazing, curtains and pelmets, or both. Remember, the better insulated the home, the less energy needed to heat and cool it, and the smaller, and therefore cheaper, the heat pump system you need to install. In short, spend some money on energy efficiency measures up front and you will save in both the long and short term, especially as energy costs continue to rise.

Heat Pump Features

As technology hasimproved and manufacturing has become cheaper, a number of features have been added to systems. These include improved filtration (such as long life filtersthat only need washing every six months), air ionisation (to disinfect the air), high efficiency fan designsto reduce energy use and fan noise, variable speed compressors (usually using DC motors), remote controls with timer functions, adjustable airflow patterns, economy modes, infrared sensors to reduce operation when rooms are empty, humidity sensors, and many other features, which may or may not be of use to you.

When looking for a system, don't get too excited about all the built-in gadgetry-after all, how much of it will you actually use, and how much of it will you forget about after the first week. There's no point paying for extra featuresif you really don't need them.

Other air conditioner options

Most manufacturers allow system components to be mixed and matched to some degree, or at least provide several options for each system. For instance, if you only want to heat one room then you might buy one standard air handling unit and the appropriately sized compressor, but if you need to heat more than one room then many systems are available as a larger compressor unit that can have two or more air handling units connected.

Sizing a system

So, you've decided to go for a heat pump system but don't know what size you will need. How do you find out?

All heat pumps have a rated heating and cooling capacity, so you need to have a basic idea of how much heat isflowing into and out of your home.

Doing such an assessment is beyond the scope of this article, and isreally something an energy assessor should help you with. There are many assessors available who can provide such services, and a number of online resources available to help you find one in your area, such as the ABSA website (www.absa.net.au) or the NatHERS site at www.nathers.gov.au.

However, if an assessment is not in the budget, then you can make an educated guess with a bit of basic knowledge. For instance, if you are heating just one room and find that a 2400 watt fan heater can keep up with heat losses, then you know the minimum heating capacity required. Indeed, as crude as it sounds, this is actually one of the simplest ways to find out how much heat you need. Set up a fan heater or two on a cold day and see how it goes. If the room is still cold after half an hour then you have some more insulating and/or sealing to do. If it is nice and toasty warm then simply buy the most efficientsystem with a rated heat output of at least the fan heater(s) capacity.

You might want to oversize your system a little if you suffer from heatstress during the summer months, but don't overdo it too far as the system will be oversized for at least half of the year.

What about ducted cooling systems?

Ducted systems sound great in theory-you can keep the entire home at a comfortable temperature. However, there's a large price to pay for this, and that's energy consumption (and therefore energy waste). After all, you can only use one room at a time, and most people, even families, will tend to spend most of theirtime in one room or another, such as the lounge room or study. Heating all the other rooms, whether they are used or not, is a bit pointless. If you need heat in rooms you visit for a few minutes each day, such as the bathroom, then use spot heating such as radiant heaters. A 1kW radiant heater used for 15 minutes usesjust 250 watt-hours of energy? much less than if you were to heat that room continuously, even with a heat pump.

Of course, ducted systems can often be used selectively, to only heat a few rooms at a time, but most users tend to not do that and instead just leave all rooms heated. The very nature of ducted systems encourages owners to waste energy heating seldom used rooms unnecessarily.

If you need to heat more than one room, get either several smaller high-efficiency units or a larger compressor that can take multiple air handling units.