Efficiency is using the least possible energy to deliver heating and cooling via mechanical heating and cooling. Efficiency is implemented by installing high performance equipment, and the proper maintenance of this equipment, including the associated air ducts or hot water pipes/radiators. Efficiency measures include: seal ducts, high efficiency air conditioner (or heat pump), and a high efficiency furnace.
In older homes the air ducts that carry warm or cool are usually leaky. It is common to have losses of 25 percent or more. Leaks are often located where ducts run through the attic, basement or garage.
Leaks can be detected by means of professional duct leakage tests. Remedies for duct leakage include application of metallic tape or mastic and duct replacement.
Central air conditioning is the 6,000 kilowatt-hour gorilla of warm climate home energy use. It uses the most electricity of any device (except for a pool heater). It typically consumes one- third of home energy. Taming its energy consumption is central to achieving a Zero Energy Home.
The efficiency of an air conditioner is measured by its SEER rating. The SEER (seasonal energy efficiency ratio) is defined as the AC’s cooling power per unit of electricity used. For example, an AC with a SEER of 10 would deliver 10,000 BTU of cooling per kWh of electricity used, enough to make 50 pounds of ice from room temperature water. A SEER 20 AC would deliver 20,000 BTU of cooling per kWh.
The SEER of AC units has risen over 300 percent between 1970 and 2010. The improvements, driven by government regulation and rising electrical costs, are due to improved engineering of AC units.
What is a ton of AC anyway?
AC cooling power is rated by the “ton”– holdover from the “good-old” days of extensive ice refrigeration. An AC “ton” is the cooling power resulting from one ton of ice melting in one day. One “ton” is 12,000 BTU/hour of cooling power or 288,000 BTU/day.
A 3-ton AC could, in theory, freeze 3 tons of ice in one day. In practice, a 3-ton AC in Florida actually delivers an average of a ton of cooling power: it runs 8 hours per day–one third of the time. Suppose it was a hot day and your AC broke down. As an alternative you could heft one ton of ice (which conveniently fits in 60 five-gallon buckets) indoors. It the ice melted in 24 hours, you would enjoy as much cooling as your 3-ton AC delivers during the same time period.
According to the Florida Solar Energy Center, dehumidification accounts for 30% of your cooling load, the equivalent of 600 pounds of ice per day ( 0.3 tons of AC power). The evidence of this costly dehumidification is the 6 gallons of liquid water per day which falls into your drip pan. A SEER 10 AC requires 6.5 kWh to chill-extract this volume of water from the warm air.
Before you purchase a new AC…
Just before purchasing a new AC is the right time to make sure that certain cooling frugal cooling fundamentals are in place. First reduce penetration of hot humid air by air sealing and/or attic sealing. Then use attic and wall insulation in combination with a high reflective roof and a radiant energy barrier. Finally, upgrade you windows with low-e window film or by installing efficient low-e and impact resistant windows.
Design with climate features such ceiling fans and shade trees also help a lot. And don’t forget to seal your AC ducts.
These frugal cooling fundamentals will reduce your cooling load. So it is crucial to do an energy audit to have your new AC properly sized. If your old AC is rated at 3 tons, you may be able to replace it with a 1.5- or 2-ton model, and save substantial cash. Also, an oversized AC will not run enough hours per day to efficiently dehumidify your home.
How efficient should your new AC be?
If you air conditioner is over 15 years old, it is time to replace it. The chart below shows typical AC operating costs in South Florida. Over a 20-year lifespan, it is economical to purchase the most efficient SEER 23 model. This is true despite the fact that a SEER 13
model may cost $4,000 installed while the SEER 23 model may cost twice as much. The difference is initial purchase cost is more than made up by annual electricity savings of over $300.
The brainy SEER 23 models also have the great advantage of quieter, variable cooling power operation. Because they are programmed to run over longer periods, they offer superior humidity control. This is a great advantage for preventing mold and wood rot and thus improving interior air quality.
It makes no sense to heat or cool your home uniformly when you are only using part of it at a time. Bedrooms are mostly used by night…living rooms in the evening… kitchens in late afternoon or evening…offices anytime. The energy conservation goal is to heat or cool a space only when needed.
Zone controls deliver a custom, thermostatically controlled level of heating and cooling to different parts of your home, at the times you specify . For example heating or cooling is set back in bedrooms during the day. At night they deliver the temperature you prefer for sleep.
A zone is defined as an area of a house with independent heating and/or cooling control. Zoning segments the house into particular areas that can be individually controlled efficiently using a Multi-Zone Controller.
Zone controls are implemented by dampers on air ducts, or thermostatically controlled hot water valves for hydronic units and radiant floors. Another method is the installation of supplementary heating such as baseboard heaters and pellet stoves. Cooling can also be zoned by the ductless, mini-split air conditioning systems. In these systems, a single air conditioner sends refrigerant to two or more separate air handlers installed in different rooms.
To summarize, the main benefits of zone heating/cooling are:
- Comfort: meets the specific temperature and airflow requirements of one area, without affecting other areas.
- Efficiency: a well-designed zoning system can save you hundreds of dollars in energy costs each year.
- Control: divides the house into multiple areas with adjustable comfort levels.
- Quiet Performance: delivers peak performance and efficiency without continually operating at peak capacity; less noise generated at lower speeds.
Evaporative cooling is the Cinderella of the HVAC industry. Its lackluster image as a lowly “swamp cooler” is yesterday. With a contemporary engineering makeover, it has become the darling of “green” websites. It is also courted by Western U.S. electric utilities anxious to shave their peak summer loads. Evaporative coolers can deliver comparable cooling power as ordinary heat-pump style air conditioners using a small fraction of the electricity.
Evaporative coolers are simple: They have a water-soaked absorbent material (e.g. aspen fibers (excelsior), treated cellulose, fiberglass, or plastic), a fan to blow air across the material, and air channels such as ducts and pipes. Air moving across the wet material evaporates water. The evaporating water absorbs energy from the air. As, a result the air becomes cooler and more humid.
Water deserves praise as a low cost, powerful cooling “fuel.” When a pint of water evaporates it has a cooling power of 1,000 BTU’s. For comparison, a pint of gasoline has the heating power of 15,000 BTU’s.
A moderately active person might generate 600 BTUs of heat an hour. On a hot, dry day a person can perspire one pint of water in an hour to gain a very welcome 1,000 BTUs of cooling. An evaporative cooler on the same day might “sweat” 3 gallons of water per hour and generate 24,000 BTU’s (2 tons) of cooling.
There is a catch: Evaporative coolers are more efficient in dry climates. Water evaporates faster in Arizona than in Florida.
Evaporative coolers increase household water consumption. In Phoenix, evaporative cooler use averaged 16 gallons per day annually, or 5% additional household use. The water use on site was mitigated by the 2,735 gallons less used in generating electricity to cool the home.
Conclusion: The dominance of conventional heat pump air conditioners in the dry west appears to be due to a historic lack of engineering refinements in evaporative cooling systems, and to poor marketing. The image of the lowly swamp cooler persists. The SWEEP report concluded that “a major education and awareness-building effort is needed to convince homeowners and builders that evaporative cooling can be a high-performance alternative to conventional air conditioning systems—it’s potentially much less costly over its lifetime, and can be designed to be as comfortable as the alternative.”
If these cost advantages are to be extended to more humid climates in Eastern United States, the evaporative coolers will need to be hybridized with conventional heat pump systems.
Kinds of Evaporative Coolers: There are a few kinds of evaporative coolers: Widely used are the old-fashioned ‘direct’, the modern ‘indirect’, and the hybrid ‘direct/indirect.’ Very promising, but with little market penetration, is a hybrid which combines an ‘indirect’ evaporative unit and a ‘regular heat pump air conditioner.’
1) Direct: A fan sucks air through panel of water-soaked porous material. Cool humidified air is delivered directly into the house. Direct coolers are most suited to very dry areas where house humidity is not an issue and where low upfront costs are desired.
2) Indirect: A fan blows air through two adjacent sets of pipes. The first set of pipes is lined with water-soaked material; this is where cooling takes place. The dry second set of pipes cool themselves by donating their heat to the cooled wet pipes. Air from the cool dry-pipes is blown into the house. Warm moist air from the wet pipes is exhausted to the outside. Indirect coolers are useful where extra interior humidity is not desired. Indirect M-cycle coolers function well in hot climates with low or moderate humidity.
3) Two-Stage: Direct/Indirect: A Stage 1 indirect cooling module delivers pre-cooled dry air to a direct module. The Stage 2 direct module delivers cool, moist air to the inside. Two-stage coolers deliver colder air with less humidity than the direct type. These are valuable in dry and moderate humidity areas.
4) Hybrid Two Stage: Indirect Evaporative/ Heat Pump Air Conditioner: The Stage 1, M-Cycle indirect evaporative cooler sends pre-cooled air to the Stage 2 AC evaporator. The evaporator further cools the air while dehumidifying it. Stage 1 does most of the cooling. A unit like this won an engineering contest at University California, Davis by providing cooling with 80% less peak demand than an ordinary heat pump AC. The unit is now cooling a library. This hybrid design has promise for efficient cooling in climate zones with moderately humid summers.
In cool sections of the country, heating costs are half of annual energy bills—an alarming $2,000 to $4,000 annually. What to do??
Lowering heating costs is a two-sided coin. The conservation side is reducing the amount of heat needed to maintain a comfortable temperature. The efficiency side is choosing a thrifty heating system and fuel to meet the reduced heating needs. As an example, let’s look at conservation and efficiency choices for an average house in the cold state of Maine.
There are an amazing variety of space heating options, such as a furnace, boiler, heat pump, stove, room heater. A furnace delivers hot air and a boiler hot water. A heat pump delivers warm air or warm water. A stove delivers hot air and radiant heat. Common fuels choices are heating oil, kerosene natural gas, propane, wood, pellets, coal and electricity.
Imagine a gas furnace that generates a third or more of your annual electricity needs, while efficiently heating your home. Sounds too good to be true? But such a device can now be installed in your basement.
This practice of burning fuel to generate electricity, and using the resulting “waste” heat is known as cogeneration or CHP—“combined heat and power.” This principle is well known in New York City where Con Edison sells steam, a byproduct of its huge electric power plants, to commercial buildings. The steam is used to heat buildings, supply domestic hot water, and for air conditioning. The steam rising from New York manholes is a sign of the steam pipes below.
Technological innovation has made it possible to miniaturize CHP into an efficient home system. The most widely available CHP system, the Honda Freewatt system has two basic components – a small natural gas powered electric generator, and an auxiliary, 95% efficient Energy Star two-stage condensing gas furnace.
It works like this: A quiet generator, with a 160 cc motorcycle-sized engine, produces 1,200 watts of AC electricity; this is the average power drawn by a house. Hooked up to the grid it can run your electrical meter backwards. The generator can be equipped with back-up power, allowing it to run when the grid is down.
The generator co-produces 3,600 watts (12,000 BTUh) of usable heat. The heat can be delivered into your hot air ducts. Or it can be directed into a boiler which supplies hot water for space heating and/or domestic hot water. The system converts 20% of the natural gas energy into electricity and 65% into usable heat; the combined efficiency is 85%.
The generator’s 12,000 BTUh heat output is small compared to the 80,000 to 100,000 BTUh output of a typical home furnace. But it is enough to heat a typical home during a mild winter’s day. Once the outside temperature drops below than 40oF to 45oF degrees, the generator’s output is not enough to keep the house warm. When this happens, the first-stage of the auxiliary gas furnace kicks in. On the coldest days the second-stage kicks in.
The ideal location for a Freewatt system is a climate is where the generator runs continuously during a four- to six-month heating season. The main market for a cogenerating furnace is in the northeast United States where you have high electric rates and fairly high number of days when you need to run your furnace.
Four to six months of steady Freewatt generator operation enables the production of 3,500 to 5,300 kWh of electricity annually; at 15 cents per kWh this is $500 to $800 worth. (For comparison, the average U.S.A home uses 11,000 kWh/year.) Steady operation also enables helps the generator to heat the home very efficiently and comfortably with low-fan noise through most of hours of the cooling season.
Pros and Cons: OK this sounds great so far, so what’s the catch? The Freewatt system may cost $25,000 installed compared to $4,000 for a 95% efficiency Energy Star furnace. This means that the ordinary furnace out-performs Freewatt financially. You need to produce a lot of electricity to make up the difference in installation cost. The Freewatt system also costs a little more for annual maintenance.
Another way to look at it is that for the extra $20,000 you pay for a Freewatt, you could do some serious air sealing, insulating, and upgraded storm window installation. The conservation approach would have a higher rate of return than producing electricity with a Freewatt.
So where would a Freewatt shine? In cold regions, prone to winter grid failure, a Freewatt will keep your home running and plumbing from freezing. It can also help you move towards a zero-energy home. It is an efficient way to produce electricity, as compared with a 30%-efficient central coal power plant which just dumps its leftover heat into water or a cooling tower. And it might be teamed with photovoltaic panels: the panels would generate most of their annual power in summer, with the Freewatt taking up the slack in the winter.
Hopefully, with economies of scale, the Freewatt and similar innovative systems, will come down in relative price. Combined heat and power is generation a sound concept for the home. It deserves and will have broader application.
If electrical power is generated from a coal plant, only 33% of the energy of the coal reaches the consumer. The rest is lost in generation and transmission. So the ultimate efficiency of coal-generated electricity is lower than the efficiency stated on the appliance: A labeled electrical furnace efficiency of 99%, translates to a 33% coal-to-home energy efficiency.
Geothermal heating has a very high upfront cost of around $30,000, but low operational costs. Two-thirds of this cost is for a few hundred of feet of water filled pipes buried deep in soil where the temperature is constant (40o to 70o F). The underground pipes have an expected life cycle of 50 or more years; the heat pump 25 years.
The water in the pipes passes in a closed loop between soil and heat pump. In winter heating mode, the heat pump acts like an air-conditioner in reverse. It chills the incoming water and sends warm water to home water/air heat exchanger. The chilled water then recirculates back though the buried piping, where it picks up soil heat before returning to the heat pump. In summer, the heat pump delivers chilled water into the home heat exchanger, and warm water to the buried pipes. Underground, the warm water is cooled by soil heat before its next trip through the heat pump.
The great disadvantage of geothermal heating is the high-front cost. Also, a home may not have room the buried water pipes. The geothermal installation is for a home with a substantial lot, in a climate, with mild winters and hot summers, with access to clean hydro-electric energy. In such a location, a geothermal installation eliminates the need for separate heating and cooling equipment, and makes the best use of a renewable energy source.
Another advantage of a geothermal system is that it is quiet without the noisy outside compressors of air-to-air heat pumps. However, air-to-air heat pumps shine in efficiently providing the modest heat requirements of sub-tropical regions like Florida, as well as, summer cooling.
In cold climates, your hungry furnace eats up one-half of your energy bill. Keeping its appetite for fuel under control is one of the great problems of home management.
Do you reduce your heat demand by air sealing and insulation? …or install a new furnace for fuel efficiency?…or do both?
Before You Install a New Furnace: Before purchasing a new furnace, is the right time to make sure that certain energy conservation fundamentals are in place. First reduce penetration of cold outside air by air sealing leaky places, and install any needed insulation. If necessary, upgrade your existing windows, or install efficient low-e windows. And don’t forget to seal your air ducts, or make sure your hydronic (hot water-based) heaters are in order. These frugal fundamentals will reduce your heating load.
If you do extensive air sealing and insulation you will reduce your heating demand. Thus, it is crucial to do an energy audit to make sure your new furnace is not oversized.
When to Buy a Furnace: To replace, or not to replace? That’s a big question. The American Council for an Energy Efficient Economy (ACEEE) offers the following indicators that it’s time to replace your old furnace or boiler (ref. http://www.aceee.org/consumer/heating).
• Furnace older than 20 years
• Old coal burner that was previously switched over to oil or gas
• Old gas furnace without electronic ignition. If it has a pilot light, it was probably installed prior to 1992 and has an efficiency of about 65% efficient (the least efficient systems today are 80%)
• Old gas furnace without vent dampers or an induced draft fan (which limit the flow of heated air up the chimney when the heating system is off).
New Energy Star furnaces are much more energy efficient and safer than old furnaces. The best ones are distinguished by three modern features which increase their efficiency: Capture of heat from flue gases, sealed combustion, and two-stage variable heating.
1) Flue gases are directed into a secondary heat exchanger which condenses gases (mostly water vapor) and captures the heat of condensation.
2) Efficient furnaces pipe in outside air to a sealed combustion chamber, instead of drawing it from inside your home. This keeps your pre-heated interior air inside, where it belongs. Combustion gases (e.g., dangerous carbon monoxide) are directly vented to the outside and never enter your house. This is a very important safety feature. Sealed combustion is especially important in super-insulated homes. These homes are tightly air sealed which limits the supply of internal air for combustion. Thus, the use of inside air for combustion might cause back-drafting of flue gases into your home.
3) Two-stage furnaces are like having two furnaces in one: a small furnace (first stage/low heat) for routine heating and a larger furnace (second stage/high heat) for very cold days. Variable heating allows efficient heat delivery on mild days with
Wood heating is a renewable energy source which is relatively economical. However, it poses high risks of indoor and outdoor air pollution. Efficient two-stage burning techniques and catalytic convertors have been developed to reduce air emissions. Creosote accumulation in chimneys is a potential fire danger. Ash disposal is messy and labor intensive.
By contrast natural gas burns cleanly and generates minimal air pollution. It is also highly efficient 95% in advanced closed systems which use outside combustion air and recover heat from flue gases. Also, it is relatively inexpensive. Propane is also efficient and clean burning but is relatively expensive.
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