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Types of Heating Systems

Keys to obtaining design efficiency of a system in the field include:

·        Sizing the system for the specific heating and cooling load of the home being built;

·        Proper selection and proper installation of controls;

·        Correctly charging the unit with the proper amount of refrigerant;

·        Sizing and designing the layout of the ductwork or piping for maximizing energy efficiency; and

·        Insulating and sealing all ductwork.

Two types of heating systems are most common in a new home:  forced-air or radiant, with forced-air being used in the majority of the homes. The heat source is either a furnace, which burns a gas, or an electric heat pump. Furnaces are generally installed with central air conditioners. Heat pumps provide both heating and cooling. Some heating systems have an integrated water heating system.

Forced-Air System Components

Most new homes have forced-air heating and cooling systems. These systems use a central furnace plus an air conditioner, or a heat pump. Figure 7-1 shows all the components of a forced-air system. In a typical system, several of these components are combined into one unit. Forced-air systems utilize a series of ducts to distribute the conditioned heated or cooled air throughout the home. A blower, located in a unit called an air handler, forces the conditioned air through the ducts. In many residential systems, the blower is integral with the furnace enclosure.

Figure 7 – 1   Components of Horizontal Flow Forced-Air Systems

Radiant heating systems typically combine a central boiler, water heater or heat pump water heater with piping, to transport steam or hot water into the living area. Heating is delivered to the rooms in the home via radiators or radiant floor systems, such as radiant slabs or underfloor piping.

Advantages of radiant heating systems include:

·        Quieter operation than heating systems that use forced-air blowers.

·        Increased personal comfort at lower air temperatures. The higher radiant temperatures of the radiators or floors allow people to feel warmer at lower air temperatures. Some homeowners, with radiant heating systems, report being comfortable at room air temperatures of 60°F.

·        Better zoning of heat delivered to each room.

·        Increased comfort from the heat. Many homeowners, with radiant heating systems, find that the heating is more comfortable.

Disadvantages of radiant heating systems include:

·        Higher installation costs. Radiant systems typically cost 40% to 60% more to install than comparable forced-air heating systems.

·        No provision for cooling the home. The cost of a radiant heating system, combined with central cooling, would be difficult to justify economically. Some designers of two-story homes have specified radiant heating systems on the bottom floor and forced-air heating and cooling on the second floor.

·        No filtering of the air. Since the air is not cycled between the system and the house, there is no filtering of the air.

·        Difficulty in locating parts. A choice of dealers may be limited.

 

Heat Pump Equipment

Heat pumps are designed to move heat from one fluid to another. The fluid inside the home is air and the fluid outside is either air (air-source), or water (geothermal). In the summer, heat from the inside air is moved to the outside fluid. In the winter, heat is taken from the outside fluid and moved to the inside air.

Air-source heat pumps

The most common type of heat pump is the air-source heat pump. Most heat pumps operate at least twice as efficiently as conventional electric resistance heating systems in Climate Zone 4. They have typical lifetimes of 15 years, compared to 20 years for most furnaces.

Heat pumps use the vapor compression cycle to move heat (see Figure 7-2). A reversing valve allows the heat pump to work automatically in either heating or cooling mode. The heating process is:

1.      The compressor (in the outside unit) pressurizes the refrigerant, which is piped inside.

2.      The hot gas enters the inside condensing coil. Room air passes over the coil and is heated. The refrigerant cools and condenses.

3.      The refrigerant, now a pressurized liquid, flows outside to a throttling valve where it   expands to become a cool, low pressure liquid.

4.      The outdoor evaporator coil, which serves as the condenser in the cooling process, uses outside air to boil the cold, liquid refrigerant into a gas. This step completes the cycle.

5.      If the outdoor air is so cold that the heat pump cannot adequately heat the home, electric resistance strip heaters usually provide supplemental heating.

Periodically in winter, the heat pump must switch to a "defrost cycle," which melts any ice that has formed on the outdoor coil. Packaged systems and room units use the above components in a single box.

 

Figure 7 – 2   Air Conditioner Vapor Compression Cycle

The heating efficiency of a heat pump is measured by its Heating Season Performance Factor (HSPF), which is the ratio of heat provided in Btu per hour to watts of energy used. This factor considers the losses when the equipment starts up and stops, as well as the energy lost during the defrost cycle.

New heat pumps manufactured after 2005 are required to have an HSPF of at least 7.7.  Typical values for the HSPF are 7.7 for minimum efficiency, 8.0 for medium efficiency, and 8.2 for high efficiency. Variable speed heat pumps have HSPF ratings as high as 9.0, and geothermal heat pumps have HSPFs over 10.0. The HSPF averages the performance of heating equipment for a typical winter in the United States, so the actual efficiency will vary in different climates.

To modify the HSPF for a specific climate, a modeling study was conducted and an equation was developed that modifies the HSPF, based on the local design winter temperature. In colder climates, the HSPF declines and in warmer climates, it increases. In Climate Zone 4, the predicted HSPF is approximately 15% less than the reported HSPF.

Geothermal heat pumps

Unlike an air-source heat pump, which has an outside air heat exchanger, a geothermal heat pump relies on fluid-filled pipes, buried beneath the earth, as a source of heating in winter and cooling in summer, Figures 7-3, 7-4. In each season, the temperature of the earth is closer to the desired temperature of the home, so less energy is needed to maintain comfort. Eliminating the outside equipment means higher efficiency, less maintenance, greater equipment life, no noise, and no inconvenience of having to mow around that outdoor unit. This is offset by the higher installation cost.

 

 

 

 

 

Geothermal Heat Pumps

 

 

 

 

 

Figure 7 – 3   Deep Well Loop

 

 

 

 

 

Figure 7 – 4   Shallow Trench Loop

 

 

 

There are several types of closed loop designs for piping:

·        In deep well systems, a piping loop extends several hundred feet underground.

·        Shallow loops are placed in long trenches, typically about 6 feet deep and several hundred feet long. Coiling the piping into a "slinky" reduces the length requirements.

·        For homes located on large private lakes, loops can be installed at the bottom of the lake, which usually decreases the installation costs and may improve performance.

Proper installation of the geothermal loops is essential for high performance and the longevity of the system. Choose only qualified professionals, who have several years experience installing geothermal heat pumps similar to that designed for your home.

Geothermal heat pumps provide longer service than air-source units do. The inside equipment should last as long as any other traditional heating or cooling system. The buried piping usually has a 25-year warranty. Most experts believe that the piping will last even longer because it is made of a durable plastic with heat-sealed connections, and the circulating fluid has an anticorrosive additive.

Geothermal heat pumps cost $1,300 to $2,300 more per ton than conventional air-source heat pumps. The actual cost varies according to the difficulty of installing the ground loops as well as the size and features of the equipment. Because of their high installation cost, these units may not be economical for homes with low heating and cooling needs. However, their lower operating costs, reduced maintenance requirements, and greater comfort may make them attractive to many homeowners.

Geothermal Heat Pump Efficiency

The heating efficiency of a geothermal heat pump is measured by the Coefficient of Performance (COP), which measures the number of units of heating or cooling produced by a unit of electricity. The COP is a more direct measure of efficiency than the HSPF and is used for geothermal heat pumps because the water temperature is more constant. Manufacturers of geothermal units provide COPs for different supply water temperatures. If a unit were installed with a COP of 3.0, the system would be operating at about 300% efficiency.

Furnace Equipment

Furnaces burn fuels such as natural gas, propane, and fuel oil to produce heat and provide warm, comfortable indoor air during cold weather. Furnaces come in a variety of efficiencies. The comparative economics between heat pumps and furnaces depend on the type of fuel burned, its price, the home’s design, and the outdoor climate. Recent energy price increases have improved the economics of more efficient equipment. However, due to the long-term price uncertainty of different forms of energy, it is difficult to compare furnaces with various fuel types and heat pumps.

Furnace Operation

Furnaces require oxygen for combustion and extra air to vent exhaust gases. Most furnaces are non-direct vent units—they use the surrounding air for combustion. Others, known as direct vent or uncoupled furnaces, bring combustion air into the burner area via sealed inlets that extend to outside air.

Direct vent furnaces can be installed within the conditioned area of a home since they do not rely on inside air for safe operation. Non-direct vent furnaces must receive adequate outside air for combustion and exhaust venting. The primary concern with non-direct vent units is that a malfunctioning heater may allow flue gases, which could contain poisonous carbon monoxide, into the area around the furnace. If there are leaks in the return system, or air leaks between the furnace area and living space, carbon monoxide could enter habitable areas and cause severe health problems.

Most new furnaces have forced draft exhaust systems, meaning a blower propels exhaust gases out the flue to the outdoors. Atmospheric furnaces, which have no forced draft fan, are not as common due to federal efficiency requirements. However, some furnace manufacturers have been able to meet the efficiency requirements with atmospheric units. Atmospheric furnaces should be isolated from the conditioned space. Those units located in well ventilated crawl spaces and attics usually have plenty of combustion air and encounter no problem venting exhaust gases to the outside.

However, units located in closets or mechanical rooms inside the home, or in relatively tight crawl spaces and basements, may have problems. Furnace mechanical rooms must be well sealed from the other rooms of the home (see Figure 7-5). The walls, both interior and exterior, should be insulated. Two outside-air ducts sized for the specific furnace should be installed from outside into the room, one opening near the floor and another near the ceiling, or as otherwise specified in your locality’s gas code.

 

Measures of efficiency for furnaces

The efficiency of a gas furnace is measured by the Annual Fuel Utilization Efficiency (AFUE), a rating that takes into consideration losses from pilot lights, start-up, and stopping. The minimum AFUE for most furnaces is now 78%, with efficiencies ranging up to 97% for furnaces with condensing heat exchangers. The AFUE does not consider the unit’s electricity use for fans and blowers, which can easily exceed $50 annually. An AFUE rating of 78% means that for every $1.00 worth of fuel used by the unit, approximately $.78 worth of usable heat is produced. The remaining $.22 worth of energy is lost as waste heat and exhaust up the flue. Efficiency is highest if the furnace operates for longer periods. Oversized units run intermittently and have reduced operating efficiencies.

Furnaces with AFUEs of 78% to 87% include components such as electronic ignitions, efficient heat exchangers, better intake air controls, and induced draft blowers to exhaust combustion products. Models with efficiencies over 90%, commonly called condensing furnaces, include special secondary heat exchangers that actually cool flue gases until they partially condense, so that heat losses up the exhaust pipe are virtually eliminated.

A drain line must be connected to the flue to catch condensate. One advantage of the cooler exhaust gas is that the flue can be made of plastic pipe rather than metal and can be vented horizontally through a side wall.

There are a variety of condensing furnaces available. Some rely primarily on the secondary heat exchanger to increase efficiency, while others, such as the pulse furnace, have revamped the entire combustion process.

A pulse furnace achieves efficiencies over 90% using a spark plug to explode gases, sending a shock wave out an exhaust tailpipe. The wave creates suction to draw in more gas through one-way flapper valves, and the process repeats. Once such a furnace warms up, the spark plug is not needed because the heat of combustion will ignite the next batch of gas. The biggest problem is noise, so make sure the furnace is supplied with a good muffler, and do not install the exhaust pipe where any noise will be annoying.

Because of the wide variety of condensing furnaces on the market, compare prices, warranties, and service. Also, compare the economics carefully with those of moderate efficiency units. Condensing units may have longer paybacks than expected for energy efficient homes due to reduced heating loads. Table 7-1 compares the break-even investment for high efficiency gas furnaces in Code and in ENERGY STAR® homes.

 

 

Table 7-1   Economic Analysis of Gas Furnaces

Type of Treatment

AFUE 0.95

Energy Savings*($/yr) Compared to AFUE 0.80

Break-even Investment‡ ($)

Code Home

42

477

ENERGY STAR® Home

31

352

*For a system in Lexington, KY

‡See Chapter 2 for information on break-even investment.

 

 

Electric integrated systems

Several products use central heat pumps for water heating, space heating, and air conditioning. These integrated units are available in both air-source and geothermal models. To be a viable choice, integrated systems should:

·        Have a proven track record in the field;

·        Cost about the same, if not less, than comparable separate heating and hot water systems;

·        Provide at least a five-year warranty; and

·        Be properly sized for both the heating and hot water load.

Make sure the unit is not substantially more expensive than a separate energy efficient heat pump and electric water heater. Units within $1,500 may provide favorable economic returns.

Unvented Fuel-Fired Heaters

Unvented heaters that burn natural gas, propane, kerosene, or other fuels are not recommended. While these devices usually operate without problems, the consequences of a malfunction are life threatening—they can exhaust carbon monoxide directly into household air. Unvented heaters also can cause serious moisture problems inside the home.

Most devices come equipped with alarms designed to detect air quality problems. However, many experts question putting a family at any risk of carbon monoxide poisoning; they see no rationale for bringing these units into a home (Figure 7-6).

 

 

 

 

 

 

 

 

 

 

 

 

Figure 7-6   Unvented Heater

 

 

 

Examples of unvented units to avoid include:

·        Vent-free gas fireplaces. Use sealed combustion, direct vent units instead.

·        Room space heaters.

 

Choose forced draft, direct-vent models instead (Figure 7-7).

 

 

 

 

 

 

 

 

 

 

 

 

Figure 7-7   Direct Vent Heater

 

 

 


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