The varied technology we have today for mechanically heating and cooling indoor spaces is truly amazing: furnaces that are nearly 100% efficient at extracting heat from fuel, and heat pumps that use the earth as a heat source or heat sink, for example. But there are a few constants that apply to all heating and cooling systems and it’s good to keep these in mind as you evaluate options.
The thermostat is the first constant. Unless you rely on a woodstove as your main heat source, the thermostat allows you to maintain consistent indoor temperatures. Once you’ve set your target temperature, your heating or cooling system will come on when the thermostat’s built-in thermometer indicates a departure of just a degree or two. Once the target temperature is attained, the thermostat tells the heating (or cooling) system to shut off.
Insulation is the second constant. All houses need insulation to slow down heat loss during the winter and to slow heat gain on hot summer days. Insulation can improve interior comfort by eliminating cold drafts in winter and hot rooms in summer. In addition to being more comfortable, a well-insulated house is more energy efficient and thus less expensive to heat and cool.
Efficiency is the final constant in any assessment of heating and cooling requirements. About half of a typical household’s total energy budget is spent on heating and cooling. It makes sense to optimize efficiency because it saves money, while also minimizing environmental impact from fossil fuel emissions. The efficiency of your heating and cooling systems depends on a number of factors, which are explained just ahead.
Heating & Cooling System Efficiency
Age & maintenance history. Heating and cooling systems function more efficiently with regular maintenance. DIY maintenance tasks include changing the air filter on a forced-air system (check the filter once a month and replace it if it’s dirty) and keeping an outdoor AC unit free of leaves and other debris that can impede air circulation. It’s also important to have heating and cooling equipment serviced at least once a year by a professional HVAC contractor.
Even with proper maintenance and professional servicing, the life of heating and cooling equipment is limited. Some furnaces and boilers can continue to function well for 15 or 20 years, but if their peak efficiency is low (65%, for example), it often makes sense to replace the unit and enjoy much higher efficiency (see “Efficiency ratings,” below).
Zoned systems. In larger homes, zoned systems enable you to conserve energy and save money by only heating or cooling areas that are in use at a particular time of day. For example, by putting second-floor bedrooms on a separate heating zone, this area of the house wouldn’t need to be heated during the day, when the upstairs is unoccupied. Each zone is controlled by a dedicated thermostat.
Programmable thermostats. Equipping your home with a programmable thermostat is an affordable and reliable way to save energy and money on heating and cooling. Basic models enable you to automatically adjust temperature settings to avoid heating or cooling living space that isn’t being used. More advanced models offer weekend/weekday settings.
Efficiency ratings. The heating and cooling equipment sold today comes with an efficiency rating assigned by the EPA. Furnaces and boilers have an AFUE rating—Annual Fuel Use Efficiency. The most efficient units are “condensing” or “direct-vent” models rated at 90% AFUE or higher. Heat pumps and central air conditioning equipment is given a SEER designation—Seasonal Energy Efficiency Rating. The most efficient units will be in the 20 range.
Heating Systems & How They Work
Room heaters. This large category includes both portable and built-in heating appliances that function independently of a central heating system. Portable electric heaters are the least expensive and easiest to operate. Built-in room heaters come in the form of baseboard or wall-mounted units. Room heaters that burn propane or natural gas are also available. These heaters should vent to the outdoors to prevent harmful combustion gasses from accumulating inside.
Forced-air heating. Designed to distribute heated air throughout a building, this system typically consists of a centrally located furnace and a network of air ducts. The furnace can use electricity, fuel oil, natural gas, propane, or (in rare cases) firewood as the heat source. When the indoor temperature drops below the temperature setting on your thermostat, the furnace turns on automatically. A blower forces heated air through supply ducts that terminate in heat registers located throughout the living space. At the same time, the blower draws a matching volume of cooler air through return ductwork, to be reheated at the furnace.
In hot weather, the same ductwork that delivers warm air to the living space can be used to distribute cool air from a central air conditioning system. This is one of the main reasons builders and homeowners choose forced-air heating. Forced-air heating offers another notable advantage: It’s faster than hydronic heating at raising room temperature.
Hydronic heating. This popular central heating system works by circulating heated water to wall-mounted radiators or long runs of plastic pipe placed under the finished floor. The latter system is referred to as radiant floor heating. Instead of heating air, hydronic systems heat objects like furniture, flooring, and wall surfaces. Hydronic systems use a boiler or a tankless water heater to heat water, which is then pumped to piping or radiators.
Hybrid or combi systems. These systems combine functions to save money on installation and operating costs. One common hybrid system uses a single boiler or tankless water heater to heat water for bathing and heating. Another example is a furnace that incorporates a heating coil for water, enabling a single unit to supply hot water along with heated air.
Heat pumps. Depending on the size and design of a heat pump, it can supply heating, cooling, or both. Air-source heat pumps are the most common, and function best when the outside temperature is above freezing. In colder weather, a backup heat source is a good idea. Ground-source (aka geothermal) and water-source heat pumps can function effectively and efficiently at all times of the year, but they’re much more expensive to install.
Wood stoves. In areas where firewood is readily available, an EPA-certified wood stove can be a good source of supplemental heat. It can also be the primary heat source for a small house or a single room. EPA certification means that the stove can operate safely, while keeping harmful emissions at a minimum.
Solar heating. Home design priorities today often include green building features like energy efficiency and sustainability. Solar heating is a major element in green building. It typically includes the following features: insulated windows oriented for maximum exposure to direct sunlight, high levels of insulation and airtight construction to minimize heat loss during cold weather, thermal mass that can effectively store solar heat and radiate this heat after the sun goes down, plus shading and ventilation features that prevent overheating during warm weather.
Cooling Systems & How They Work
Central air conditioning. The same system of ductwork that circulates heated air throughout a house can be used to distribute cooled in a central air conditioning system. A network of supply ductwork delivers conditioned air to different rooms in the house, while return ductwork conveys air back to the system’s blower (aka air handler) to be reconditioned.
A central AC system operates on the same principles your refrigerator uses to keep food cool. The outdoor part of this split system includes a compressor, a condenser, and a fan to circulate air over the condenser coils. Working together, these components cause a “phase change” that turns refrigerant gas into a liquid. This liquid refrigerant is then pumped indoors, where it enters evaporator coils in the air handler’s plenum. As warm indoor air blows over the coils, the liquid refrigerant evaporates and removes heat from the air -the same cooling effect you experience when sweat evaporates from your skin. Because air loses its capacity to hold moisture as it cools, your AC system also dehumidifies your air. To complete the cycle, refrigerant gas moves back to the outdoor unit to be turned back into a liquid.
Window and portable AC units. Window air conditioners are affordable (prices start around $200), easy to install, and effective at cooling rooms of all sizes. Today’s models come with programmable controls designed to increase comfort and operating convenience. The cooling capacity of these units is expressed in Btu’s (British thermal units); the range begins at around 5,000 and goes up to 12,000. The rule of thumb for sizing a window air conditioner is 20 Btu for every square foot of room area. But it’s smart to add cooling capacity if there are high ceilings, low levels of insulation, or large windows that admit plenty of direct sunlight. Because a window air conditioner can detract from a home’s exterior appearance, many homeowners choose to install them on a less visible side of the house. Just remember that the unit will work best when it’s aiming cooled air toward the center of a room.
Portable air conditioners can also provide cooling in houses without central air conditioning. These units typically come with wheels, and are designed to sit on the floor next to a window. Flexible ductwork extends through a narrow manifold that mounts in the window opening to discharge warm air. Although these units can be a workable solution when building codes don’t allow window-mounted AC, they are large, heavy (barely portable), and generally not as efficient as window units.
Passive cooling. Strategies for cooling without mechanical assistance are long established, and often worth implementing. Shade is a great weapon against overheated interiors, and it can be provided by trees, porch roofs, awnings, and window-mounted shades. Opening windows to take advantage of natural breezes and cross ventilation is another strategy. Folks who grew up without AC might also recall the efficacy of opening windows when temperatures cooled in the evening, and then closing them in the morning, as heat began to build.
Common Problems
System design errors. It takes a skilled HVAC contractor to design heating and cooling systems. Many factors figure in the selection of equipment and the layout of ductwork or radiant heating zones: house size, insulation and air tightness conditions, window area, fuel preferences, budget, and more. Installing heating and cooling equipment with excess capacity (a common error) can actually cause comfort and efficiency problems due to excessive cycling.
Poor maintenance. It’s easy and inexpensive to replace the filter(s) in a forced-air system; this is basic maintenance that any homeowner can perform. Check the filter once a month and replace it if it’s dirty. Have an HVAC technician service your heating system in advance of the heating season, and do the same for your cooling system. These basic maintenance tasks will keep your equipment in top condition.
Old and/or outdated equipment. A 20-year-old furnace that has been well maintained to work at peak efficiency may still only operate at 75% AFUE (Annual Fuel Use Efficiency). In contrast, a new, high-efficiency, direct-vent furnace will deliver an efficiency above 90%.
Building envelope issues (air leaks, poor insulation). Old houses can place high demands on heating and cooling systems. This happens when there’s too little insulation and too many air leaks that allow “conditioned” indoor air to escape, while also admitting outdoor air that is often too hot or too cold for comfort. The good news is that these building envelope problems can often be corrected by air-sealing gaps, replacing outdated windows, and increasing home insulation.