HEATING AND COOLING SYSTEMS
First, I want to start by saying this. An HVAC system is probably going to be the single most expensive investment you make in updating the energy efficiency of your home...and probably the least understood. It is absolutely imperative that you understand the proper sizing and terminology around this investment. Whomever you hire, make sure they are certified and well referenced.
Sizing Heating and Cooling Systems
Older space conditioning systems (more than 10 years old) are often unreliable and much less efficient than a modern system. When it's time for a new replacement, choosing one of the correct size (heating and/or cooling output) is critical to getting the best efficiency, comfort, and lowest maintenance and operating costs over the life of the new system. Some national surveys have determined that well over half of all HVAC contractors do not size heating and cooling systems correctly.
The most common sizing mistake is in oversizing. This not only makes the new system more expensive to install, but also forces it to operate inefficiently, break down more often, and cost more to operate. Oversized heating equipment also often creates uncomfortable and large temperature swings in the house. Oversized air conditioners (and heat pumps) do not run long enough to dehumidify the air, which results in the "clammy" feeling and unhealthy mold growth in many air-conditioned houses.
Manual J and Manual D: The Correct Way to Size a System
Correct system sizing requires considering many factors other than simply reading the nameplate of the existing unit. Key factors for correctly sizing a heating and cooling system include the following:
- The local climate
- Size, shape, and orientation of the house
- Insulation levels
- Window area, location, and type
- Air infiltration rates
- The number and ages of occupants
- Occupant comfort preferences
- The types and efficiencies of lights and major home appliances (which give off heat).
Homeowners should insist that contractors use a correct sizing calculation before signing a contract. This service is often offered at little or no cost to homeowners by gas and electric utilities, major heating equipment manufacturers, and conscientious heating and air conditioning contractors. Manual J, "Residential Load Calculation," published by the Air Conditioning Contractors of America (ACCA), is the recommended method for use in the United States. There are also many user-friendly computer software packages or worksheets that can simplify the calculation procedure. You should make sure that the procedure used by the contractor follows Manual J.
If ducts are part of the installation, they should be sized using the ACCA's Manual D, "Residential Duct Design." Also if ducts are part of the installation, ensure that the duct joints are sealed with mastic. This will prevent the loss of conditioned air into the walls and floors. The ACCA also offers a comprehensive guide for choosing home heating and cooling systems, called Manual S, "Residential Equipment Selection."
Heat Pumps - An Alternative
While heat pumps aren't new technology, they are gaining momentum as an alternative to traditional the traditional furnace and air conditioner due to their extreme energy efficiency and relatively low operating costs. Like your refrigerator, heat pumps use electricity to move heat from a cool space into a warm, making the cool space cooler and the warm space warmer. During the heating season, heat pumps move heat from the cool outdoors into your warm house; during the cooling season, heat pumps move heat from your cool house into the warm outdoors. Because they move heat rather than generate heat, heat pumps can provide up to 4 times the amount of energy they consume.
The most common type of heat pump is the air-source heat pump, which transfers heat between your house and the outside air. If you heat with electricity, a heat pump can trim the amount of electricity you use for heating by as much as 30%–40%. High-efficiency heat pumps also dehumidify better than standard central air conditioners, resulting in less energy usage and more cooling comfort in summer months. However, the efficiency of most air-source heat pumps as a heat source drops dramatically at low temperatures, generally making them unsuitable for cold climates, although there are systems that can overcome that problem.
Higher efficiencies are achieved with geothermal (ground-source or water-source) heat pumps, which transfer heat between your house and the ground or a nearby water source. Although they cost more to install, geothermal heat pumps have low operating costs because they take advantage of relatively constant ground or water temperatures. However, the installation depends on the size of your lot, the subsoil and landscape. Ground-source or water-source heat pumps can be used in more extreme climatic conditions than air-source heat pumps, and customer satisfaction with the systems is very high.
A new type of heat pump for residential systems is the absorption heat pump, also called a gas-fired heat pump. Absorption heat pumps use heat as their energy source, and can be driven with a wide variety of heat sources.
For more information on these specific types of heat pumps, see these sections:
Steps a Good Contractor Should Take to Size Your System
Many factors affect a home's heating or cooling requirement, or "load." A good estimator will measure walls, ceilings, floor space, and windows to determine the room volumes, and will assess the R-value of the home's insulation, windows, and building materials. A close estimate of the building's air leakage is also necessary. A blower door test is the best way to measure air leakage.
A good estimate will also include an inspection of the size, condition of seals on joints and insulation, and location of the distribution ducts in forced air systems. The placement of supply and return registers should be appropriate for the system type and size.
The orientation of the house also affects heat gain and heat loss through windows. Overhangs can reduce solar gain through windows. Make sure the contractor uses the correct design for the outdoor temperature and humidity in your area. Using a higher summer design temperature results in oversizing air conditioners.
When the contractors are finished, get a copy of their calculations, assumptions, and the computer printout or finished worksheet. This is your only proof that they did the job right. To summarize, when designing your new heating and air conditioning system, the contractor you choose should do the following:
- Use a computer program or written calculation procedure to size the system
- Provide a written contract listing the main points of your installation and includes the results of the heating and cooling load calculation
- Give you a written warranty on equipment and workmanship
- Allow you to hold the final payment until you are satisfied with the new system.
Understanding the Efficiency Rating of Furnaces and Air Conditioners
A central furnace or boiler's efficiency is measured by annual fuel utilization efficiency (AFUE). The Federal Trade Commission requires new furnaces or boilers to display their AFUE so consumers can compare heating efficiencies of various models. AFUE is a measure of how efficient the appliance is in the energy in its fuel over the course of a typical year.
Specifically, AFUE is the ratio of heat output of the furnace or boiler compared to the total energy consumed by a furnace or boiler. An AFUE of 90% means that 90% of the energy in the fuel becomes heat for the home and the other 10% escapes up the chimney and elsewhere. AFUE doesn't include the heat losses of the duct system or piping, which can be as much as 35% of the energy for output of the furnace when ducts are located in the attic. If you are upgrading your system, if possible, aim for a high-efficiency system:
- Condensing flue gases in a second heat exchanger for extra efficiency
- Sealed combustion
- 90%–97% AFUE
Before buying a new furnace or boiler or modifying your existing unit, first make every effort to improve the energy efficiency of your home, then have a heating contractor size your furnace. Energy-efficiency improvements will save money on a new furnace, because you will need a smaller furnace. A properly sized furnace will also operate most efficiently. You'll also want to look for a dependable unit and compare the warranties of each furnace or boiler under consideration.
When shopping for high-efficiency furnaces and boilers, look for the ENERGY STAR® label. If you live in a cold climate, it usually makes sense to invest in the highest-efficiency system. In milder climates with lower annual heating costs, the extra investment required to go from 80% to 90%-95% efficiency may be hard to justify.
The Seasonal Energy Efficiency Ratio (SEER) is a measure of the cooling efficiency of your air conditioner or heat pump. The higher the SEER number the better the system is at converting electricity into cooling your home. SEER ratings above 13 or more are generally considered high-efficiency units.
In summary, sizing and installation of your HVAC system should be completed after you have completed your other weatherization projects (windows, insulation, etc). With that I again also strongly recommend having an enery audit or at minimum a blower door test to determine the air tightness of the dwelling. This will present the contractor with the proper information for him to determine the proper sizing.
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