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What is HVAC? Print E-mail

First things first. What does HVAC mean?

HVAC is in common use in the heating and cooling industry. It stands for "heating, ventilation and air conditioning," three functions often combined into one system in today's modern homes and buildings. Warmed or cooled or dehumidified air flows through a series of tubes - called ducts - to be distributed to all the rooms of your house. A central HVAC system is the most quiet and convenient way to cool an entire home.

Unless you live in an amazingly temperate climate, the HVAC system in your home uses more energy and drains more energy dollars than any other system in your home. Typically, 44% of your utility bill goes for heating and cooling.

Like many other appliances, HVAC systems have improved in energy efficiency in the last decade. As a result, you can save money and increase your comfort by properly maintaining and upgrading your HVAC equipment.

Another development of the 1990s is the whole house approach to heating and cooling. Coupled with an energy efficient furnace, heat pump or air-conditioner, the whole house approach can have a great impact on your energy bills. By combining proper equipment maintenance and upgrades with appropriate insulation, weatherization and thermostat settings - properly regulated with a programmable thermostat, of course - you may be able to cut your energy bills in half.

If you are thinking about purchasing a new central furnace, check the ENERGY STAR® database. It displays information on most energy efficient appliances in a consumer-friendly, easy-to-use fashion.

 

Central Air Conditioning

Central air conditioning units are usually matched with a gas or oil furnace to provide heat through the same set of ducts.

There are also central HVAC units called heat pumps that combine both the heating and cooling functions. If you heat your home with electricity, a heat pump system is the most efficient unit to use in moderate climates. It can provide up to three times more heating than the equivalent amount of electrical energy it consumes. A heat pump can trim the amount of electricity you use for heating as much as 30-40%.

Even though air conditioners and heat pumps require the use of some different components, they both operate on the same basic principles.

 

How They Work

Heat pumps and most central air conditioners are called "split systems" because there is an outdoor unit (called a condenser) and an indoor unit (an evaporator coil). The job of the heat pump or air conditioner is to transport heat from one of these units to the other. In the summer, for example, the system extracts heat from indoor air and transfers it outside, leaving cooled indoor air to be recirculated through your ducts by a fan.

A substance called a refrigerant carries the heat from one area to another. Basically, here's how it works:

The compressor in your outdoor unit will change the gaseous refrigerant into a high temperature, high-pressure gas. As that gas flows through the outdoor coil, it loses heat. That makes the refrigerant condense into a high temperature, high pressure liquid that flows through copper tubing into the evaporator coil located in your fan coil unit or attached to your furnace.

At that point, the liquid refrigerant is allowed to expand, turning the liquid refrigerant into a low temperature, low pressure gas. The gas then absorbs heat from the air circulating in your home's ductwork, leaving it full of cooler air to be distributed throughout the house. Meanwhile, the low temperature, low pressure refrigerant gas returns to the compressor to begin the cycle all over again.

While your air conditioner or heat pump cools the air, it also dehumidifies it. That's because warm air passing over the indoor evaporator coil cannot hold as much moisture as it carried at a higher temperature, before it was cooled. The extra moisture condenses on the outside of the coils and is carried away through a drain. The process is similar to what happens on a hot, humid day, when condensed moisture beads up on the outside of a glass of cold lemonade.

The same process works in reverse in a heat pump during the winter. The heat pump takes heat out of the outside air - or out of the ground, if you have a geothermal heat pump - and it moves that heat inside, where it is transferred from the evaporator coil to the air circulating through your home.

That's not a typographical error, by the way- the heat pump moves heat from outside to warm your home, even on a cold day. That's because "cold" is a relative term. Air as cold as 30 degrees still contains a great deal of heat - the temperature at which air no longer carries any heat is well below -200 degrees Fahrenheit. A heat pump's heat exchanger can squeeze heat out of cold air, then transfer that heat into your home with the help of a fan which circulates the warm air through your ducts.

Heat pumps are often installed with back-up electric resistance heat or a furnace to handle heating requirements when more heat is needed than the heat pump can efficiently extract from the air.

Types of Systems

We've just described what is called a "split system" - the condensing unit is placed outside the house, and the evaporator coil is inside.

There is another configuration called a "packaged" air conditioner that combines the condensing unit and the evaporator coil into one outdoor unit (usually used for modular or mobile homes). Which type you should choose depends on your home's location and construction.

 

Rating a Unit's Efficiency

The efficiency of central air conditioning systems is rated by a Seasonal Energy Efficiency Ratio (SEER). SEER ratings typically range from 13 to 23, with the highest numbers indicating the most efficient units that offer the most energy savings year after year. Fortunately, great strides have been made in the last 10 years to increase the efficiency of new air conditioners and heat pumps.

The typical SEER rating of air conditioners manufactured before 1992 is about 6.0. In 1992, the federal government established the minimum cooling efficiency standard for units installed in new homes at 10. The minimum SEER value changed again on January 23, 2006 to a SEER of 13. To be considered as high-efficiency units, air conditioners must have a SEER rating of at least 14. The federal government is in the process of increasing the minimum cooling efficiency standards again next year. The SEER rating is usually shown on a yellow and black EnergyGuide sticker attached to the outside unit of the air conditioner.

Central air conditioners that are in the top 25 percent of efficient models may carry the Energy Star label. To qualify, they must have a minimum SEER efficiency level of 14. Additionally, Energy Star® models must also have a minimum Energy Efficiency Ratio (EER) of at least 11.5 for split systems, and of at least 11.0 for single-package models. Air conditioners that bear the Energy Star label may be twice as efficient as some existing systems.

Heat pumps also have heating efficiency ratings, indicated as a Heating Seasonal Performance Factor (HSPF). In general, the higher the HSPF rating, the less electricity the unit will use to do its job.

In 1992, the federal government established the minimum heating efficiency standard for new heat pumps at 6.8 HSPF. (Most heat pumps manufactured before 1992 had HSPF ratings below 5.) The minimum HSPF standard changed again on January 23, 2006 to an HSPF of 7.7. Today, an HSPF of 8.2 or higher is considered "high-efficiency"; the maximum available is 9.35.

High-efficiency central air-conditioning heat pumps can also qualify as Energy Star models. In addition to meeting the minimum SEER and EER requirements, they must also meet minimum HSPF requirements of 8.2 for split systems and 8.0 for single-package models.

Higher efficiency units usually cost more to purchase initially, but save money in the long run on operating costs.

 

Sound Levels

Few people think about how loud an air conditioner or heat pump will be - at least until the unit is installed and running in their back yard. With some units, the noise created by the condensing unit outside can even interfere with indoor peace and quiet. That's why you should compare the sound levels produced by different models when you are shopping for a new unit.

The sound level of outdoor units is measured in bels (a term similar to decibels). The rating scale goes from 0 - the rating for a barely perceptible sound - to 13 - the threshold of pain. Most air conditioners and heat pumps operate in the range of 8 to 9 bels, although some are quiet enough to rate as 6.8 bels. (While that may not sound like a wide range, consider this: the noise output at 9 bels is 10 times louder than 8 bels. That means one 9-bel air conditioner is as loud as 10 units rated at 8 bels!)

 

GEOTHERMAL or GROUND SOURCE HEAT PUMPS

Heat pumps move heat from one place to another - from outside to inside a home, for example. That's why they're called "heat pumps."

Here's a simplified version of how a heat pump works:

All heat pumps have an outdoor unit (called a condenser) and an indoor unit (an evaporator coil).

A substance called a refrigerant carries the heat from one area to another. When compressed, it is a high temperature, high-pressure liquid. If it is allowed to expand, it turns into a low temperature, low pressure gas. The gas then absorbs heat.

In the winter the normal heat pump system extracts heat from outdoor air and transfers it inside where it is circulated through your home's ductwork by a fan.

Even cold air contains a great deal of heat; the temperature at which air no longer carries any heat is well below -200 degrees Fahrenheit. But the coldest temperature ever recorded in the lower 48 states was -70 degrees, recorded at Roger Pass, Montana on January 20, 1954. Obviously in such weather, a heat pump would have to work pretty hard to produce 68-degree temperatures inside your home.

That's why geothermal heat pumps are so efficient.

Geothermal heat pumps are similar to ordinary heat pumps, but instead of using heat found in outside air, they rely on the stable, even heat of the earth to provide heating, air conditioning and, in most cases, hot water.

From Montana's minus 70 degree temperature, to the highest temperature ever recorded in the U.S. - 134 degrees in Death Valley, California, in 1913 - many parts of the country experience seasonal temperature extremes. A few feet below the earth's surface, however, the ground remains at a relatively constant temperature. Although the temperatures vary according to latitude, at six feet underground, temperatures range from 45 degrees to 75 degrees Fahrenheit.

Ever been inside a cave in the summer? The air underground is a constant, cooler temperature than the air outside. During the winter, that same constant cave temperature is warmer than the air outside.

That's the principle behind geothermal heat pumps. In the winter, they move the heat from the earth into your house. In the summer, they pull the heat from your home and discharge it into the ground.

Studies show that approximately 70% of the energy used in a geothermal heat pump system is renewable energy from the ground. The earth's constant temperature is what makes geothermal heat pumps one of the most efficient, comfortable, and quiet heating and cooling technologies available today. While they may be more costly to install initially than regular heat pumps, they can produce markedly lower energy bills - 30-40% lower, according to estimates from the U.S. Environmental Protection Agency, who now includes geothermal heat pumps in the types of products rated in the EnergyStar® program. Because they are mechanically simple and outside parts of the system are below ground and protected from the weather, maintenance costs are often lower as well.

As an added benefit, systems can be equipped with a device called a "desuperheater" can heat household water, which circulates into the regular water heater tank. In the summer, heat that is taken from the house and would be expelled into the loop is used to heat the water for free. In the winter, the desuperheater can reduce water-heating costs by about half, while a conventional water heater meets the rest of the household's needs. In the spring and fall when temperatures are mild and the heat pump may not be operating at all, the regular water heater provides hot water.

 

How Do They Compare?

Surveys taken by utilities have found that homeowners using geothermal heat pumps rate them highly when compared to conventional systems. Figures indicate that more than 95% of all geothermal heat pump owners would recommend a similar system to their friends and family.

 

Cost

You will have to add the cost of drilling to the total amount of the project. The final cost will depend on whether your system will drill vertically deep underground or will put the loops in a horizontal fashion a shorter distance below ground. The cost of drilling also will vary depending on the terrain and other local factors.

 

Durability

Geothermal heat pumps are durable and require little maintenance. They have fewer mechanical components than other systems, and most of those components are underground, sheltered from the weather. The underground piping used in the system is often guaranteed to last 25 to 50 years and is virtually worry-free. The components inside the house are small and easily accessible for maintenance. Warm and cool air are distributed through ductwork, just as in a regular forced-air system.

Since geothermal systems have no outside condensing units like air conditioners, they are quieter to operate.

 

How Do They Work?

Remember, a geothermal heat pump doesn't create heat by burning fuel, like a furnace does. Instead, in winter it collects the Earth's natural heat through a series of pipes, called a loop, installed below the surface of the ground or submersed in a pond or lake. Fluid circulates through the loop and carries the heat to the house. There, an electrically driven compressor and a heat exchanger concentrate the Earth's energy and release it inside the home at a higher temperature. Ductwork distributes the heat to different rooms.

In summer, the process is reversed. The underground loop draws excess heat from the house and allows it to be absorbed by the Earth. The system cools your home in the same way that a refrigerator keeps your food cool - by drawing heat from the interior, not by blowing in cold air.

The geothermal loop that is buried underground is typically made of high-density polyethylene, a tough plastic that is extraordinarily durable but which allows heat to pass through efficiently. When installers connect sections of pipe, they heat fuse the joints, making the connections stronger than the pipe itself. The fluid in the loop is water or an environmentally safe antifreeze solution that circulates through the pipes in a closed system.

Another type of geothermal system uses a loop of copper piping placed underground. When refrigerant is pumped through the loop, heat is transferred directly through the copper to the earth.

 

Types of Loops

Geothermal heat pump systems are usually not do-it-yourself projects. To ensure good results, the piping should be installed by professionals who follow procedures established by the International Ground Source Heat Pump Association (IGSHPA). Designing the system also calls for professional expertise: the length of the loop depends upon a number of factors, including the type of loop configuration used; your home's heating and air conditioning load; local soil conditions and landscaping; and the severity of your climate. Larger homes requiring more heating or air conditioning generally need larger loops than smaller homes. Homes in climates where temperatures are extreme also generally require larger loops.

Here are the typical loop configurations:

Horizontal Ground Closed Loops

This type is usually the most cost effective when trenches are easy to dig and the size of the yard is adequate. Workers use trenchers or backhoes to dig the trenches three to six feet below the ground in which they lay a series of parallel plastic pipes. They backfill the trench, taking care not to allow sharp rocks or debris to damage the pipes. Fluid runs through the pipe in a closed system. A typical horizontal loop will be 400 to 600 feet long for each ton of heating and cooling.

Vertical Ground Closed Loops

This type of loop is used where there is little yard space, when surface rocks make digging impractical, or when you want to disrupt the landscape as little as possible. Vertical holes 150 to 450 feet deep - much like wells - are bored in the ground, and a single loop of pipe with a U-bend at the bottom is inserted before the hole is backfilled. Each vertical pipe is then connected to a horizontal underground pipe that carries fluid in a closed system to and from the indoor exchange unit. Vertical loops are generally more expensive to install, but require less piping than horizontal loops because the Earth's temperature is more stable farther below the surface.

Pond Closed Loops

This type of loop design may be the most economical when a home is near a body of water such as a shallow pond or lake. Fluid circulates underwater through polyethylene piping in a closed system, just as it does through ground loops. The pipes may be coiled in a slinky shape to fit more of it into a given amount of space. Since it is a closed system, it results in no adverse impacts on the aquatic system.

Although they are less applicable to California, there are other loop systems described at the Geothermal Heat Pump Consortium's Web Site. These include an Open Loop System in which ground water is pumped into and out of a building, transferring its heat in the process; and Standing Column Well Systems, which can be up to 1,500 feet deep and can also furnish potable water.

In a few places, developers have installed large community loops, which are shared by all of the homes in a housing project.

 

 

RADIANT HEATING

Radiant heating systems involve supplying heat directly to the floor or to panels in the wall or ceiling of a house. The systems depend largely on radiant heat transfer: the delivery of heat directly from the hot surface to the people and objects in the room via the radiation of heat, which is also called infrared radiation. Radiant heating is the effect you feel when you can feel the warmth of a hot stovetop element from across the room. When radiant heating is located in the floor, it is often called radiant floor heating or simply floor heating.

Radiant heating has a number of advantages: it is more efficient than baseboard heating and usually more efficient than forced-air heating because no energy is lost through ducts. The lack of moving air can also be advantageous to people with severe allergies. Hydronic (liquid-based) systems use little electricity, a benefit for homes off the power grid or in areas with high electricity prices. The hydronic systems can also be heated with a wide variety of energy sources, including standard gas- or oil-fired boilers, wood-fired boilers, solar water heaters, or some combination of these heat sources.

Despite their name, radiant floor heating systems also depend heavily on convection, the natural circulation of heat within a room, caused by heat rising from the floor. Radiant floor heating systems are significantly different than the radiant panels used in walls and ceilings. For this reason, the following sections discuss radiant floor heat and radiant panels separately.

Air-Heated Radiant Floors

Because air cannot hold large amounts of heat, radiant air floors are not cost-effective in residential applications, and are seldom installed. Although they can be combined with solar air heating systems, those systems suffer from the obvious drawback of only being available in the daytime, when heating loads are generally lower. Because of the inefficiency of trying to heat a home with a conventional furnace by pumping air through the floors, the benefits of using solar heat during the day are outweighed by the disadvantages of using the conventional system at night. Although some early solar air heating systems used rocks as a heat-storage medium, this approach is not recommended.

Electric Radiant Floors

Electric radiant floors typically consist of electric cables built into the floor. Systems that feature mats of electrically conductive plastic are also available, and are mounted onto the subfloor below a floor covering such as tile.

Because of the relatively high cost of electricity, electric radiant floors are usually only cost-effective if they include a significant thermal mass, such as a thick concrete floor, and your electric utility company offers time-of-use rates. Time-of-use rates allow you to "charge" the concrete floor with heat during off-peak hours (approximately 9 p.m. to 6 a.m.). If the floor's thermal mass is large enough, the heat stored in it will keep the house comfortable for eight to ten hours, without any further electrical input (particularly when daytime temperatures are significantly warmer than nighttime temperatures). This saves a considerable number of energy dollars compared to heating at peak electric rates during the day.

Electric radiant floors may also make sense for additions onto homes for which it would be impractical to extend the heating system into the addition. However, homeowners should examine other options, like a non ducted gravity wall furnace.

Hydronic Radiant Floors

Hydronic (liquid) systems are the most popular and cost-effective radiant heating systems for heating-dominated climates. Hydronic radiant floor systems pump heated water from a boiler through tubing laid in a pattern underneath the floor. In some systems, the temperature in each room is controlled by regulating the flow of hot water through each tubing loop. This is done by a system of zoning valves or pumps and thermostats. The cost of installing a hydronic radiant floor is approximately $4-$6 per square foot ($40-$60 per square meter), depending on the size of the home, the type of installation, the floor covering, remoteness of the site, and the cost of labor.

 

Types of Floor Installations

Whether cables or tubing, the methods of installing electric and hydronic radiant systems in floors is about the same.

So-called "wet" installations embed the cables or tubing within a solid floor and are the oldest form of modern radiant floor systems. The tubing or cable can be embedded in a thick concrete foundation slab (commonly used in "slab" ranch houses that don't have basements) or in a thin layer of concrete, gypsum, or other material installed on top of a subfloor. If concrete is used and the new floor is not on solid earth, additional floor support may be necessary because of the added weight. You should consult a professional engineer to determine the floor's carrying capacity.

Thick concrete slab systems have high heat capacity and are ideal for storing heat from solar energy systems, which have a fluctuating heat output. The downside of the thick slabs is their slow thermal response time, which makes strategies such as night or daytime setbacks difficult if not impossible. Most experts recommend maintaining a constant temperature in homes with these heating systems. Due to recent innovations in floor technology, so-called "dry" floors, in which the cables or tubing run in an air space beneath the floor, have been gaining in popularity, mainly because a dry floor is faster and less expensive to build. But because dry floors involve heating an air space, the radiant heating system needs to operate at a higher temperature. Some dry installations involve suspending the tubing or cables underneath the subfloor between the joists. This method usually requires drilling through the floor joists in order to install the tubing. Reflective insulation must also be installed under the tubes to direct the heat upward. Tubing or cables may also be installed from above the floor, between two layers of subfloor. In these instances, liquid tubing is often fitted into aluminum diffusers that spread the water's heat across the floor in order to heat the floor more evenly. The tubing and heat diffusers are secured between furring strips (sleepers), which carry the weight of the new subfloor and finished floor surface. At least one company has improved on this idea by making a plywood subfloor material manufactured with tubing grooves and aluminum heat diffuser plates built into them. The manufacturer claims that this product makes a radiant floor system (for new construction) considerably less expensive to install and faster to react to room temperature changes. Such products also allow for the use of half as much tubing or cabling since the heat transfer of the floor is greatly improved over more traditional dry or wet floors.

Floor Coverings

Ceramic tile is the most common and effective floor covering for radiant floor heating, as it conducts heat well from the floor and adds thermal storage because of its high heat capacity. Common floor coverings like vinyl and linoleum sheet goods, carpeting, or wood can also be used, but any covering that helps to insulate the floor from the room will decrease the efficiency of the system. If you want carpeting, use a thin carpet with dense padding and install as little carpeting as possible. If some rooms, but not all, will have a floor covering, then those rooms should have a separate tubing loop to make the system heat these spaces more efficiently. This is because the water flowing under the covered floor will need to be hotter to compensate for the floor covering. Wood flooring should be laminated wood flooring instead of solid wood. This reduces the possibility of the wood shrinking and cracking from the drying effects of the heat.

Radiant Panels

Wall- and ceiling-mounted radiant panels are usually made of aluminum and can be heated with either electricity or with tubing that carries hot water, although the latter creates concerns about leakage in wall- or ceiling-mounted systems. The majority of commercially available radiant panels for homes are electrically heated. Like any type of electric heat, radiant panels can be expensive to operate, but they can provide supplemental heating in some rooms or can provide heat to a home addition when extending the conventional heating system is impractical. Unlike other types of radiant heating systems, radiant panels have very low heat capacity and have the quickest response time of any heating technology. Because the panels can be individually controlled for each room, the quick response feature can potentially result in cost and energy savings compared to other systems when rooms are infrequently occupied: when entering a room, the occupant can increase the temperature setting and reach a comfortable level within minutes. But as with any system, the thermostat must be maintained at a minimum temperature that will prevent pipes from freezing. Radiant heating panels operate on a line-of-site basis: you'll be most comfortable if you're close to the panel. Some people find the ceiling-mounted systems uncomfortable, since the panels heat the top of their heads and shoulders more effectively than the rest of their body.