Different types of HVAC systems will carry different designation ratings for energy efficiency. Whether a heating or cooling efficiency rating is most important to you will be determined by whether you live in a predominantly hot or cold climate and based on the severity of your climate. If you live in an area that does not require much summer cooling, focus on the heating system efficiency, and vice-versa for an area with hot summers and warmer winters. If you live in a mixed climate with four true seasons, then you should be considering both heating and cooling efficiency ratings.
Cooling efficiency is expressed as the Seasonal Energy Efficiency Ratio (SEER) for traditional split systems (residential five tons and smaller) and EER, Energy Efficiency Ratio, for other types of systems. The minimum SEER rating for US manufacturers is now 13.0, and high- performance cooling is currently defined as 14.5 SEER and higher, so just getting above 15 SEER is significant. These ratings will certainly go up in years to come, so check the Internet for up-to-date guidance. Many US manufacturers have units on the market achieving 19 and 21 SEER. These units cost a great deal more than those in the 13 to 15 range. Remember, it’s best from a comfort and payback perspective to put your money into a super-efficient envelope before pouring money into a very high-efficiency piece of equipment.
Heating efficiency ratings vary by the type of fuel a model uses. Gas furnaces are rated in terms of their Annual Fuel Utilization Efficiency (AFUE). The minimum efficiency for gas should be 80 percent AFUE for mild winter climates, with high-efficiency equipment rated at 90 percent AFUE or above, which mean that these units can convert 90 percent or more of the fuel that they use to heat for your home. Heat pump efficiency, for systems with capacities of five tons or less, is expressed as the Heating Season Performance Factor (HSPF). High-efficiency heat pumps are rated at 8.2 HSPF, even better ones at 9.0 HSPF. For larger units, heating efficiency is expressed as the Coefficient of Performance (COP), and a high-performance rating would be considered a 4.0 COP unit.
As mentioned previously, is important to note that these ratings are determined under controlled laboratory conditions, which are defined by the Department of Energy (DOE), in order to compare efficiencies across brands and models available on the market. Think of this like miles per gallon ratings (MPG) for your car. If the DOE did not set defined standards for measuring MPG, each auto manufacturer would use methods of testing that would optimize the efficiency of their particular equipment. “Under controlled laboratory conditions” means the units are tested for their rated efficiency over controlled run times, and, of course, the systems have been serviced to assure they are in prime operating condition, and a whole team of engineers is hovering over them like doting parents throughout the tests.
In humid summer climates, be sure that your a/c system includes evaporator and condenser coils with matched tonnages. Do not let your contactor install a mismatched system with two different tonnages. This is a common practice in the industry, but it is counter-productive to good humidity control. By matching a smaller condenser (3.0 ton) with a larger evaporator coil (3.5 or even 4.0 ton), the contractor can often claim a higher SEER efficiency. The problem is that doing this sacrifices dehumidification; the mismatched system will remove less humidity than a matched system, leaving the house uncomfortable and prone to mold growth. The physics of all of this is that the inside coil is oversized and has more surface area to cool as compared to the outside unit, so the inside coil doesn’t get as cold, which results in less condensation on it and less moisture removed from the home. Contractors call it “running a warm coil.” Now, if your climate zone has no summer humidity, this practice is acceptable. Again, how you build and what is right depends on where you are building.
HVAC System Selection
By the time that you have received the Manual J load model from your trusted, qualified and experienced mechanical engineer or ACCA- trained HVAC contractor, you will have already made your decisions on the most efficient building design and construction specifications for your project. Now the question is, what type of heating, ventilation and cooling systems should you invest in? You should look for higher EER or SEER for air conditioning systems, and higher AFUE and HSPF ratings for heating systems. As we mentioned earlier, depending on your climate type, usually at least one of these systems represents the biggest energy user in your home, so you want it to be the most efficient that you can afford.
The most common types of heating, cooling and ventilation systems are radiant hydronic systems and forced air systems. The more traditional forced air systems come as either packaged systems or split systems, both types consisting of the same basic components. The difference is that all components are in the same unit in a packaged system (i.e., a rooftop or through-the-wall unit), while a split system is made up of a furnace/cooling coil/air handler (blower) unit inside and a separate outdoor condenser and fan. These types of systems are available as gas, oil or electric. Gas furnaces can be fueled by natural gas or propane. The cooling components of all of these systems are electric. Two-speed, multi-dual stage or variable speed compressors are more efficient than the old kind of blower motor because they can manage average loads while running on lower speeds, producing a colder coil and using less energy. Two-speed condensers or variable speed indoor air handler/furnace fans should be the specified blower motors in the air handlers. These are referred to as Electronically Commutated Motors (ECMs), also known as Integrated Control Modules (ICMs). They do a better job of managing humidity, especially when combined with thermidistat control units. These units achieve greater efficiency by being able to better match the load on the house under a wide range of weather conditions. Most high-rated systems include these upgrades, because they achieve their high-efficiency ratings by running like a small unit most of the time when it is not too hot, but then ramp up in capacity when it is very, very hot in the late afternoon.
In climates with long winters, the key to efficiency is in the heating system, with the air conditioner being of secondary importance. The most efficient choices are high-efficiency boilers or dual-fuel heat pumps. Here again sizing is a critical consideration. As with air conditioning, most residential heating systems are oversized by a considerable amount. This leads to short cycling, more noise and inefficient operation. Most boilers are most efficient when sized to run in longer cycles, allowing them to achieve their full rated efficiency, although the efficiency penalty for oversized heating equipment is generally smaller than for oversized cooling equipment. Other systems for consideration include hydronic heating, using pipes laid into the floors of the house, or the use of heat exchange coils in the furnace air handler. Either of these can deliver both great comfort and high-efficiency operations.
When considering a combustion-based ducted air heating system, it is of critical importance to remember that, in a tightly built envelope like yours should be, open combustion furnaces and water heaters cannot operate without the danger of backdrafting deadly, odorless, carbon monoxide; high levels of particulates and other noxious substances into your home. It is important to keep indoor air quality and safety in mind when selecting your heating equipment. Never install an open combustion appliance within the thermal envelope of a tightly built home! In tightly sealed homes like the one you will be building, only sealed combustion appliances can operate safely. This means units with 90 percent efficiency rating or higher. Even if you live in a warm climate, health and safety trump cost and efficiency in every case. The most common types of non-gas systems are straight electric resistance systems, oil-fired boilers and heat pumps. Straight electric resistance systems include electric furnaces, wall heaters and baseboard heaters. Think wires that glow red like electric stove tops. Electric baseboard heaters provide zoned heat controlled by thermostats located within each room. The quality varies considerably depending upon the model. Cheaper units often give poor temperature control and can be noisy. Overall electric resistance unit inefficiency, coupled with power generation and transmission losses, results in electric heat as the least efficient and most expensive method of heating your home. If you must use this type of heater, look for units carrying Underwriters Laboratories (UL) and the National Electrical Manufacturer’s Association (NEMA) labels.
Although straight electric systems convert one hundred percent of the energy consumed into heat and hence they have a COP of 1.0, they pale in comparison to heat pumps, which are much more efficient. A heat pump with a COP of, say, 3.5 delivers three and one-half times more heat for each watt of electricity used than a resistance unit will. This is because heat pumps don’t produce heat by burning fuel, they work by exchanging already existing heat from an external source. Moving heat from one place to another is far less costly than producing it. The most common type is an air-source heat pump, which pulls heat from the outdoor air (yes, even 40-degree Fahrenheit temperature has a great deal of heat that can be extracted) and transfers it inside to heat the home. If electricity is your only fuel choice, heat pumps are a clear winner over resistance heat and easily reduce electricity use by fifty percent. In extreme cold, though, heat pumps lose their ability to extract sufficient heat from low outdoor temperatures, so these units typically have backup electric resistance heat strips or, better yet, a backup gas furnace to supplement them when needed. A hybrid unit, called a dual-fuel heat pump, combines air-source heat pump efficiency with a gas furnace backup, which greatly reduces energy consumption over the traditional electric-resistance backup heat strips. If you have a source for both types of fuel and live in a climate with a long heating season, this upgrade is something that you want to consider. These units heat the house using the compressor during milder weather, but then switch over to a high-efficiency gas furnace during spells of extremely cold weather. This means that you never have to rely on the most costly and expensive method of producing heat still in use, electric resistance heating elements.
When you get into very high-efficiency HVAC equipment, consider upgrading from the standard split system (outside compressor and inside air handler with ducted air) to a really high-COP-rated geothermal system or to an inverter mini-split ductless or multi-zoned ducted system (described below). Because they use fewer mechanical components, and because those components are sheltered from the elements, geothermal heat pumps are durable and highly reliable and typically last 20 years or more with fewer maintenance requirements than most other systems. These systems are becoming more mainstream, so their technologies have reached mass production and after rebates the prices are within consideration of some traditional high-performance systems. A key consideration in the cost of a geothermal heat pump is the fact that 30 percent of the total cost of all equipment, materials and installation will be returned to the buyer by a US federal tax credit through the year 2016. Many utilities also sweeten the deal with substantial rebates based on the high EER of the units.
Geothermal heat pumps come in the form of ground-source and water-source units. These systems require the drilling of vertical or horizontal wells to pipe heat exchangers into the ground or in ponds, lakes or other waterways. With heating COPS of up to 5.0, these types of heat pumps can be one of the most efficient ways to heat and cool a home and can also provide your home’s hot water needs at far less cost than any other available technology including gas or electric resistance. Geothermal heat pumps often achieve cooling efficiencies of over 32 EER. They can achieve such high efficiencies because they have an almost unlimited source to act as a heat source or a heat sink — the earth or a large body of water. The number of ground wells required depends on the size of the system and the depth of drilling that is best for your geography. Spacing on ground wells is critical, as to not overload the earth around the well components in already hot climates. Too-close spacing can build up heat, affecting system performance or even causing well failure.
Inverter and variable refrigerant mini-split and ducted multi-port mini-split systems are new to the US and Canadian markets. These are forced air systems that use a fan to push air through a coil that heats or cools it in each room. These systems move refrigerant to each zone, allowing each room of the home to be controlled independently of other rooms for both heating and cooling. Rooms that are not in use can be turned off without affecting overall system performance, and each room in use can be thermostatically controlled on its own, allowing some rooms (e.g., laundry) to be set to higher cooling temperature in order to manage higher heat loads, and other rooms (e.g., bath) to be set to heating. These are the most common HVAC systems used in Europe, Asia and South America. Although they have only been on the market a few years here in the United States, production has reached levels such that costs are within consideration of other high-performance systems, and some people predict that they may be the dominant units available in the US market in as little as ten years. Therefore, these systems are definitely worth thinking about.
If you live in a hot climate with low humidity, evaporative coolers are an economical option for staying comfortable. These units work with passive ventilation (i.e., opening the windows) to cool incoming air and exhaust warm air. They use about 25 percent of the energy and cost about 50 percent less to operate than conventional central air conditioning systems.
If you are considering options that allow you some independence or backup capabilities, small combined cooling, heating and power (Micro-CHP) systems produce space cooling, heating, hot water and electricity all at the same time. They work much like micro-power plants, using a turbine to make power and then using the waste heat from this process to heat the house and hot water. These systems are good in remote, off-grid locations or provide humidity control and cooling, especially as a backup power source during severe winter storms that cause electric grid failures. The electricity produced can power essential household appliances, including refrigeration and lighting, produce space heating to keep you comfortable, power absorption cooling systems or even heating swimming pools. These systems can be powered by natural gas, propane or even solar photovoltaic or other onsite power generation systems and are extremely efficient, with only about ten percent loss to exhaust, providing more than double the efficiency of electricity from the grid.
Which type of system is right for you? That depends on several factors, including fuel availability in your area, fuel costs, system efficiency and, of course, your budget. There is a great calculator available from the US Energy Information Administration (EIA) that allows you to input fuel costs in your area along with the efficiency rating of the various types of systems you are considering for purchase, in order to compare annual operating costs. However, when using the calculator, you should consider forecasted future costs and availability from your utility provider for each fuel type in the future. Predicted fuel supply and demand scenarios should be a primary consideration when selecting the type of system you install. The EIA also maintains future price projection reports for various fuels on their.
