ABOVE : Hydronic heating systems produce radiant warmth that many people (and critters) prefer.
Heating can be a large proportion of energy use in the home. Lance Turner looks at what efficient options are available, including hydronic and reverse-cycle air conditioners.
OUR previous heating buyers guide looked at heat pumps (commonly called reverse-cycle air conditioners) due to their high efficiency, low cost and simple installation. Later in this guide we take another look at reverse-cycle air conditioners and their advantages, and list the most efficient units currently available.
However, there is another form of heating that not only lets you choose a heat pump as the heat source, but other energy sources as well if they are more appropriate. That system is hydronics.
Hydronic systems consist of a heat source (commonly called the boiler) to heat water, and one or more pipe circuits which have the heated water flowing through them. Each circuit incorporates one or more radiators, which emit warmth into the room.
Most hydronic systems have multiple circuits, so you can heat all or only part of a home, allowing you to leave unused, closed-off rooms unheated to reduce energy use. Water is circulated through the system using low-pressure pumps, and circuits are turned on/off by electrically operated valves, usually controlled by an electronic controller.
The controller enables a system to be programmed to heat certain parts of a home at particular times-for example, heating the living areas during the evening and the bedrooms just before bedtime.
Hydronic systems are recognised to have a number of advantages over other forms of heating. The heat being either underfoot or close to it (through the use ofskirting radiators or panel radiators mounted at floor level) meansthat you get the feeling of warmth with lower ambient room temperatures than with space heating. Also, there is generally very little air movement with hydronic heating, reducing the potential cooling effect of airflows produced by convective heating such as reverse-cycle air conditioners or ducted gas systems.
Depending on the boiler used, some hydronic systems, such as heat pump systems, can also provide cooling in summer. Another advantage isthat some hydronic boilers also provide domestic hot water, eliminating the need for a separate water heater.
Hydronic systems also have some disadvantages. First isthe cost. A complete system can easily cost $10,000 or more, depending on the boiler, the number of circuits and type of radiator. The cost islikely to increase if fitting a hydronic system as a retrofit to an existing home if pipe runs are difficult to install due to lack of space.
However, prices have dropped over time due to increased competition as hydronics have become more popular. Indeed, we have seen complete hydronic heating packages for under $7000, but larger heat-pump systems can exceed $20,000, and geothermal heat pump systems can be considerably more than this.
The next disadvantage is complexity. Hydronic systems can end up using a maze of pipes and valves, so you need a space for all this equipment. This is usually housed in a cupboard or the laundry, out of view most of the time, so the thought of complex piping really shouldn't be a deterrent. Further, 'partage' boxes, such as those from Immergas, are designed to take most of the control pipework from a multi-zoned system and place it all inside a neat box, hiding it from view, giving a much cleaner installation.
Another issue, that only applies to in-slab systems, isthermal lag-the time it takes from when you turn on the heating until you start to feel the warmth underfoot. This occurs because of the high thermal mass of a concrete slab, which takes quite a while to heat up. So if you are away from the house for most of the day and only need heating for an hour or two in the morning and evening, an in-slab system may not be a good choice.
People with in-slab systems tend to leave them running all the time due to the lag. so often use heating more than they need to. A system using wall or skirting radiators allows you to turn the heating on and off at will and get heat within a minute or so from these types of radiators.
Hydronic radiators are not limited to the plain versions of old. These three fixtures are all hydronic radiators from the Hunt Heat designer range of radiators.
Aside from the afore-mentioned in-slab systems, there's a vast array of radiators available, from the traditional standing radiators through to towel rails, bench seats, skirting boards (such as ThermaSkirt), mirror surrounds, in-floor trench convectors (radiators embedded in steel boxes fitted into the floor, covered by a grille, used where above-floor radiators are not suitable), ceiling radiators, and even decorative wall art. Which you choose will depend on your needs, but there's certainly no shortage of variety.
We should mention here that the operating system temperature will vary depending on the type of radiator. For example, in-floor heating usually uses water temperatures up to 40 °C or so, whereas wall-mounted radiators may run at 70 °C or more. It is possible to have a mixed system with both water temperatures running in the same system, but not all boilers and system designs can accommodate this. Your hydronic system installer will be able to provide more information on mixed temperature systems.
So, you've decided to install a hydronic heating system, but which fuel is right for your situation? If you have mains gas (LPG is simply too expensive to use for heating) then you might consider a gas boiler. The options here are either a storage system, much like a large gas hot water system, or an instantaneous boiler. The latter will be more efficient asit has no standing losses like a storage tank does. It also takes up a lot less room, requiring no tank other than a small expansion tank.
But even if you have mains gas, is it the best option? The price of gas is more closely linked to the international pricing system now so prices have been increasing. Further, as more gas comes from coal seam deposits and by hacking, gas is becoming a dirtier fuel.
For most people, the two main alternatives to a gas boiler are boosted solar and heat pump systems.
Both systems work like large versions of domestic hot water systems of the same technology. Solar systems have roof-mountec collectors that provide a proportion of the water heating, while backup can be done with instantaneous gas, heat pump or even solid fuel heat sources. Bear in mind though, that heating isrequired at times of the year that provide the least solar input, so a solar system really can be thought of as a solar-assisted system, and the 'backup' may well do the majority of the heating.
Heat pumps use a refrigerative system to extract heat from the outside air and concentrate it into the water tank. Even air that feels cold to us contains a lot of usable heat, although the colder the ambient air, the lower the overall heat pump efficiency. Systems that take heat from the air are called air-source heat pumps-, there are also ground-source heat pumps that extract heat from the ground, but these are generally more expensive. See the 'Heat pump basics' box and the 'Efficiency of heat pumps' section for more on how heat pumps work and the efficiencies available.
It should also be noted that the initial upfront cost of even air-source heat pump hydronic boilers is considerably more than gas boilers at the current time, although as more homes go all-electric and more systems enter the market through greater demand, prices should fall.
Solid fuel boilers, such as the Gasogen wood gasification boiler, can also be used to provide heat hydronically, if no other fuel source is available or you have a low cost source of solid fuel, such as fallen timber on a large property. Pellet boilers are also available, although the price of pellets in Australia is still quite high.
Arguably, the most greenhouse-friendly and lowest cost to run system would be a high efficiency heat pump combined with a suitably sized photovoltaic array. If you already have a large solar energy system which generates excess energy during the day, then you would do well to install a heat pump hydronic system, which would be partially or in whole powered from your PV array. In such cases, running costs can be quite low compared to all other fuel options. It should be borne in mind that any PV array is likely to produce far less usable energy in the colder months, but this can be partially mitigated by installing a larger array.
In some areas, winter insolation can be quite high. If you live in one of these zones, a heat pump system is a no-brainer.
While most heat pump hydronic systems use dedicated hydronic heat pump units, some heat pump hydronic system installers use multiple DHW (domestic hot water) heat pumps to provide the required hot water.
Although there are some disadvantages to such a system, such as increased standing losses from having multiple smaller tanks, the advantages include the ability to use CO2 refrigerant systems (see box on refrigerants) as well as having a degree of redundancy built in—if one of several heat pump units fails, you still have the rest to keep you warm-you are not stuck without heating until a repair can be effected, as you are with a single large boiler.
However, multiple smaller heat pumps will cost more for the same heating capacity as a single large system. Note that several DHW heat pump manufacturers specifically state that they don't recommend their systems for hydronic heating use-although none actually gives a reason that we could find!
If you already have a gas hydronic system in place but running costs are too high due to it being an older, inefficient system (or maybe the system is nearing the end of its lifespan), then you should be able to have your system's boiler replaced with a heat pump or other fuel system. Just select a boiler with a heat output similar to what you currently have in the desired fuel type. In most cases, the best option will be to move to a heat pump system.
We should also mention that some suppliers have complete kits ready to install by competent DIYers. If this is your thing and you want to save a considerable amount on installation costs, look out for DIY kits and talk to the supplier to find out what's involved.
However, note that it is illegal to install any mains-pressure plumbing system yourself unless you have the appropriate qualifications. Should a DIY system leak and damage your home, your insurance company may also have grounds to reject a damages claim. Licensed installers should be employed to install hydronic systems as hydronic systems fall under the same regulations as other domestic plumbing.
Split system reverse-cycle air conditioners are compact and simple to operate and modern units generally have high operating efficiencies.
Hydronic systems can provide wonderful warmth throughout the home, but there may be reasons why they are not suitable for you, be it a lack of access for pipework, a dislike of their complexity (some people just prefer things to be simple), a limited budget, or perhaps you just prefer a different form of heating. Whatever the reason, an energy-efficient heating alternative is the reverse?cycle air conditioner, which uses a heat pump to heat (or cool) the air in your home.
Heat pumps are all around us; as already mentioned, they can be used to heat the water in a hydronics system, for example. 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 are known as reverse-cycle air conditioners.
People often think of electric air heating as inefficient. And it can be: many forms of electric heating use resistive elements to turn the electricity into heat directly, and can only ever be 100% efficient.
However, reverse-cycle air conditioners use heat pumps and these are much more than 100% efficient, in fact, up to 550% efficient, meaning that they use a lot less energy to produce the same amount of heat.
How can that be? 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.
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 lkWh of electricity to move 4kWh of heat from outdoors to inside your home, then it has a COP of 4. Note that 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-they are effectively the same thing.
Although both reverse-cycle air conditioners and heat pump hydronic systems use heat pumps, hydronic heat pumps usually have a COP of no more than 4 (less at lower ambient outdoor temperatures), whereas reverse-cycle air conditioners can have COPs as high as 5.5. The actual running COP of both systems depends on numerous factors, including the temperature differential between outdoors and indoors (or outdoors and hydronic water temperature for a hydronic system), the refrigerant and compressor type used, and overall system design.
"Some heat pumps can have their COPs reduced to low levels (less than 2 in some cases) as the ambient temperature approaches 0°C. If you live in an area that sees close to zero winter temperatures, make sure you check the efficiency curve."
While talking about temperature effects, some heat pumps can have their COPs reduced to low levels (less than 2 in some cases) as the ambient temperature approaches 0°C. If you live in an area that sees close to zero winter temperatures, make sure you check the efficiency curve (a graph of COP versus ambient temperature for a given output temperature) if available (some manufacturers will just supply COPs for several outdoor temperatures), of your prospective heat pump units, whether they be hydronic or reverse-cycle air conditioner.
SPLIT SYSTEMS AND INVERTERS
All of the reverse-cycle systems in our guide tables are split systems, where the indoor unit and outdoor unit are separated and linked by flexible or rigid high-pressure hoses or pipes.
Split systems have the compressor and one set of coils in a box outside, often mounted against a wall. The part 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 split system 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 as required, making the system more efficient and reducing electricity use.
The split system has several advantageous features-the air handling unit is quite compact, they only need a couple of small holes in the wall for piping and cabling, the separation of indoor and outdoor units eliminates leakage of heat from the hot side to the cool side, thus improving system efficiency, and they are quiet, as the compressor is outside the home.
As reverse-cycle air conditioner technology has improved and manufacturing has become cheaper, a number of features have been added to systems. These include improved filtration (such as long-life filters that only need washing every six months), air ionisation (to disinfect the air), high efficiency fan designs to 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 features if you really don't need them.
However, some features can be worth paying extra for, such as humidity control. Many people find that reverse-cycle air conditioners cause the home's humidity level to sit outside the best comfort range of 40% to 50%. One air conditioner designed to address this issue is the US7 (Ururu Sarara 7) range from Daikin. These units have pretty much every feature you could ask for in a reverse-cycle air conditioner, including humidity control, high efficiency DC motors, very high COPs, two-stage air filtration, and self cleaning. Of course, you also pay extra for these features.
Another useful feature is the inbuilt timer. Most units have these now, and they allow you to turn on heating before you wake up, or turn on cooling before you get home from work. Some units also feature internet connectivity (often via an optional wi-fi adaptor) to enable you to control the unit using a smart phone or tablet in the home, or from almost any device via the internet.
The advantage to this is that you can turn the unit on before you get home, or turn it off remotely if you forget.
Many units also feature adjustable airflow patterns, allowing warm air to flow across walls and ceilings rather than onto occupants. In summer, random airflow changes are supposed to simulate the natural variations of a cooling breeze.
There are actually a number of things you can do to improve the efficiency of your reverse-cycle air conditioner. The smaller 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 improv efficiency and reduce running costs. The same applies to a heat pump hydronic system - siting the outdoor unit in the sun will increase efficiency in winter, but if it is also a cooling unit then it should be shaded in summer.
Air conditioners have filters inside the air handling unit to remove dust from the air.
These should be cleaned regularly when the unit is in use, although some systems are now self cleaning or have filters that only need to be washed every six months. They can usually just be washed with warm soapy water, rinsed and dried.
Multi-head split reverse-cycle air conditioners use a single compressor to drive two or more indoor units, which may be wall-mounted units, console units or ceiling-mounted cassette units. Refrigerant hoses run from the compressor to each unit, limiting the distance between the indoor units and the compressor, often to around 10 metres, although longer runs can be installed if extra refrigerant is added. Of course, the pipes should always be well-insulated, regardless of the pipe length.
Most manufacturers allow air conditioner 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.
Alternatively, for heating multiple rooms, fitting a number of single-room units may allow for better overall efficiency as system COP is usually better for the smaller systems. Multiple smaller units also gives a better level of redundancy—you won't lose all heating capacity should a compressor unit fail.
The downside to multiple small air conditioners isthat you increase the required wiring, as each unit needs to be wired in. Also, you will have one outdoor unit for each air conditioner, which takes up more space and may be harder to hide visually Many units use power for their crankcase heaters (low power heating elements used to prevent refrigerant mixing with crankcase oil when the unit is off, and to prevent condensation of refrigerant in the crankcase of a compressor), at least in the cooler months, thus increasing standby load considerably. The standby load may or may not be listed in brochures and datasheets, so if it is not listed for your preferred model, contact the manufacturer or the installer/salesperson, but the latter may not know about this issue.
To keep costs down, some manufacturers compromise designs to some degree, which can reduce system efficiency.
One such compromise isthe use of the same size air handling unit on compressor units of different sizes. If you look at the specifications of the different models in some manufacturers' ranges you will see that the air handling units ofsystems 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 less efficient; you can see this when looking at specifications.
A manufacturer might compromise efficiency like this 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. So bear this in mind when checking specifications.
There are two sides to the economics equation when choosing a system. First is the upfront cost. Generally, a hydronic system will be more expensive than even a top-level reverse-cycle air conditioner, although you would require more than one of the latter to provide the heating capacity of the typical hydronic system; so the cost to purchase several air conditioners to heat a whole home could be similar.
One advantage with going with reverse-cycle air conditioning isthat you can start small, by installing a single unit, and then adding more as the budget allows. With a hydronic system you really need to buy a boiler and ancillary equipment sized to suit the entire heating requirements of the home.
With a reverse-cycle air conditioner, you also get cooling without having to buy a separate system; then again, some heat pump hydronic boilers can also provide cooling, or an optional chiller can be fitted to the system.
The other part of the economics equation is running costs. It can be difficult to compare the two approaches. A single efficientreverse-cycle air conditioner will certainly be cheaper to run than a whole-of-house hydronic system.
Even if you install several reverse-cycle air conditioners to allow you to heat the whole house, you are probably unlikely to run them all at once as they are separately controlled you may be less likely to heat the bedrooms for much of the day, for example—and so your running costs may also be lower. However, a well-zoned hydronic system can help reduce costs for hydronics. Comfort comes into the equation here as you may be more comfortable heating a larger part of the house, even if that costs more with either type ofsystem.
Running costs will depend on many factors including the size of the system and its efficiency,so be sure to look at the COP for heat pumps, whether for a hydronic system or a reverse-cycle air conditioner, and the size of the system. A primary factor determining running costs is the thermal efficiency of the house-the heating system only replaces the heat lost by the housing envelope, and the rated capacity of the heating system isits maximum capacity; it doesn't produce this much heat continuously. For a given level of heat loss in a home, a system with higher COP will use less energy to maintain required temperatures.
When looking at gas system running costs, you are comparing the efficiency of gas systems. Comparing gas running costs to an equivalent-sized heat pump requires that you compare the cost of gas to produce a certain amount of heat to the cost of electricity a heat pump would use to produce that same heat.
Gas is metered and charged in megajoules (MJ), but you can calculate the equivalent kilowatt-hours by dividing by 3.6. Hydronic gas usage will vary depending on climate, operating time, home thermal efficiency and unit efficiency. A gassystem that uses 300MJ per day and has an efficiency of 80% is producing (300x0.8)/3.6 or around 66.6kWh of heat for the home. To quickly compare this to a heat pump, just divide by the heat pump's COP. For example, a system with an average COP of 3.5 would use around 19kWh per day of electricity to do the same job asthe gas unit. Compare them financially by looking at the cost you pay per MJfor gas and kWh for electricity. Note that this quick comparison doesn't take into account different operating modes as discussed previously or that you could eliminate your gas connection fee if you disconnect from the gas network.
If you have excess PV-generated electricity, you could use this to offset some of the running costs for heat pump hydronic systems and reverse-cycle air conditioners.
The systems are running in winter, with lower insolation levels, but an oversized solar system can help to some extent. A hydronic system with a large water tank could mean you could heat the water during the day from PV to give some heating into the night. With a reverse-cycle air conditioner, you could pre-heat the home using excess PV-generated electricity for when you come home from work, but the savings are likely to be small (see 'Pre-cooling your home' in ReNew 130 for our modelling of doing this for cooling; the results are likely to be similar, although there islikely to be less excess solar in winter).
A battery could power the system at night, but currently the bill savings won't offset the battery cost.
Many other factors come into operating costs. Hydronic systems tend to feel more comfortable at lower temperatures than reverse-cycle air conditioners as the heat is at floor level, and there is no cooling effect from air movement. With heat pump hydronics, you might also be able to access a cheaper off-peak tariff to heat the water, at least for part of the day. This will depend on your system's design and your energy company's tariff usage requirements.
As mentioned previously, given that gas prices are now tied to international prices, and that those prices are steadily increasing, the future costs of running a gas boiler, even an instantaneous one, can be difficult to predict. There's also the issue that natural gas is becoming dirtier as more is sourced from tracking and coal seams.
Each home's situation is different,so you need to evaluate the economics of the systems based on your own particular circumstances.
All hydronic boilers and reverse-cycle air conditioners have a rated heating capacity (and cooling capacity, for reverse-cycle air conditioners), so you need to have a basic idea of how much heat is flowing into and out of your home.
Doing such an assessment is beyond the scope of this article, and is really 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.
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 or a few rooms and find that a 2400 watt fan heater can keep up with heat losses in each room, 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 efficient system with a rated heat output of at least that of the fan heater(s).
Once you know the heating requirements then you should also know the cooling requirements, to a reasonable degree, as the temperature differential between the room and outside in the coldest days of winter will be similar to the differential on the hotter summer days.
You might want to size based on cooling requirements, using the BTU calculator below. However, as discussed in ReNew 133, many system size calculations are based on having the temperature higher/lower than many people need, or don't take into account all factors regarding the house's insulation, so consider whether a smaller system could do the job just as well. However, one advantage of a slightly oversized system is that you can heat (or cool) rooms more quicldy-something to consider if you often come home to a hot home after a day out. Also be aware that some installers may tend to oversize based on past experience with less than well-insulated homes. If your home performs well thermally then make sure the person quoting understands this.
Sizing a hydronic system can be a fairly involved task as it usually means sizing a system for the whole home while matching radiators for each room size. Hydronic system sizing is probably best left to the system designer/installer.
If you decide on reverse-cycle air conditioning, you might be tempted to go the whole hog and install a ducted system. 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. After all, you can only use one room at a time, and most people, even families, will tend to spend most of their time in one room or another, such asthe lounge room or study. Heating all the other rooms, whether they are used or not, will add unnecessarily to energy use and cost.
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 uses just 250 watt-hours of energy-much less than if you were to heat that room continuously, even with a heat pump.
Of course, modem ducted systems have zoning controls, just like hydronic systems, so efficiency can be enhanced if the householder uses those controls correctly.
The biggest problem with ducted systems is that they can lose a considerably amount of heat through the ducting, both as radiant heat loss and if the ducts become damaged by critters (or people working in the roof). Ducts are rarely cleaned or checked for damage, and ducted systems usually end up costing more to run overtime as efficiency degrades. If you need to heat more than one room, get either several smaller high-efficiency heat pumps, a high efficiency multi-head split heat pump, or go for a multi-room hydronic system with smart zoning controls.
Of course, any heating system 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 underfloors 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 heating 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 shortterm.
There are four full PDF tables are available here. While they were produced for the Australian market, the values are international.
The reverse-cycle air conditioner table covers many of the systems available internationally with COPs of 4 or better. Note that most of these units are split systems with single wall-hung air handling units-that tells you a lot about the efficiency of such a design. Heating capacities range from just over 2kW to more than 25kW which should be more than enough to heat even the largest well-sealed and insulated home. The hydronics table covers boilers and systems that are aimed at the domestic market, and range up to 50 kW capacity or more.
The BTU Calculator can estimate the amount of BTUs you will need to heat or cool your home, based on the size and the temperature you want for your home.
Please note that this calculator can only gauge a rough estimate. You should also consider factors like efficiency decrease of the heater or air conditioner with time, the shape/type of your home or room, insulation conditions, and other factors. 1 watt is approximately 3.41214 BTU/h.
The British Thermal Unit, or BTU, is an energy unit. It is approximately the energy needed to heat one pound of water for 1 Fahrenheit. 1 BTU = 1,055 joules. 1BTU/hour = 0.293 watt.
When you start shopping for your new system, it would be great if you knew exactly what brand and model you want. Unfortunately, that's very difficult to determine. There are many brands to choose from, such as Trane, Lennox, York, Carrier, Rheem, Ruud, Goodman, and many more. In order for your to work like it's supposed to, all major components (including the refrigerant lines and duct system) have to be matched exactly to each other. That's not so easy to do.
You could come up with literally dozens of combinations within one brand alone! There are all kinds of different models of furnaces, condensing units and evaporator coils as well as different efficiency levels, capacities and sizes. It's mind-boggling to think of the possibilities. And once you start taking bids, you'll see how many different choices you're given. How can you decide what you need?
Most air conditioning contractors will give you an estimate on one or maybe two different brands. That's probably what they warehouse and sell, and there's nothing wrong with that. Just remember this; it doesn't matter what brand you choose if the system is installed improperly!
Of course, the top-quality brands will generally last longer, but nothing impacts the life of your new system like the installation process. Most of the problems that arise after you purchase a system (including shortened equipment life) are caused by the installation and/or improperly sized equipment.
If you find a contractor you can trust, it shouldn't make much difference what kind of equipment they install. The brand issue is secondary since the wholesale price to the dealer is not much different between brands, so that shouldn't make much of a difference in the estimate. A lot of the cost differences are installation-related (it costs more to do it right).
If they can do it right, let them use whatever they want. If you have a personal preference, let them know and they can probably get that brand. Unlike car dealerships, HVAC contractors are pretty much free agents. You're better off with their brand properly installed than your brand improperly installed.