Integrating the Heating and Cooling of Your Home Integrating the Heating and Cooling of Your Home
Many conventional home heating and cooling appliances perform a single function, such as space heating or cooling (heat pumps supply both space heating and cooling), water heating, or ventilation. Appliances that perform two or more of these functions, however, are becoming more popular. There are many different types of integrated, or multi-function, appliances available that may offer one or more of the following features:
- Reduced equipment needs.
- Simplified installation.
- Lower purchase and installation cost.
- Compact size for the limited space of small homes, multiple-dwelling units, and apartments.
- A design that enables the individual components to work together to achieve overall higher efficiency, energy savings, and lower operating costs.
There are several different types of multi-function appliances available. The following is a description of some of those systems and their characteristics.
Combined Space and Domestic Hot Water Heating:
Tankless, combined hydronic (or steam) space heating and water heating systems have been available for years. These systems have a heat exchanger inside the boiler that heats domestic water. This eliminates the need for a conventional water heater. Although this type of system is inexpensive to buy and install, it is, however, more inefficient and expensive to operate than two separate appliances. This is because the boiler must operate continuously even during warm weather in order to heat the domestic water supply.
A boiler with a separate storage tank for domestic water is an increasingly popular alternative to the above system. A pump forces hot boiler water through a heat transfer coil in the tank. A single burner, inside the boiler, supplies heat for one or more space heating zones and a domestic water heating zone. Thermostats control both the operations of the boiler and the solenoid valves that control the flow of water. Once the storage tank water reaches its set temperature, the boiler ceases operation; thus efficiency is higher (30% to 50% or more during warm weather) than for the tankless coil combined units. Since they have a storage tank, these indirect-fired systems cost more than the combined hydronic space and hot water heater systems described above. Unlike direct-fired systems, the tanks usually last longer since they are not subjected to the extreme changes in temperature caused by open flame. Other, less common, indirect-fired systems incorporate such features as an immersed cold coil tank (cold domestic water passes through a heat exchanger inside a tank of boiler water), a "booster tank" (the above system without a heat exchanger the domestic water zone shuts off when the tank is full), and double tank configurations, where the boiler water runs between and/or around internal tanks.
Combined space heating/domestic hot water systems are not limited to hydronic (boiler) systems. Some high-efficiency condensing forced-air furnaces circulate a liquid to cool the furnace's heat exchanger. This heated liquid may either circulate through a liquid-to-air heat exchanger to warm room air, or it may circulate through a coil inside a separate indirect-fired water heater. This type of condensing furnace, when connected to an indirect-fired water heater, provides an improvement in water heating efficiency.
Many new integrated systems also feature sealed-combustion technology. These systems use outdoor air for combustion. Exhaust gases vent to the outside through small diameter plastic pipes. Thus, heated indoor air is not used for combustion purposes. "Power burners" or "power vents" are often used (especially in retrofit situations) to increase the efficiency of the air flow.
Heat Pump Water Heaters:
While more common in space heating or cooling applications, dedicated heat pumps called heat pump water heaters (HPWH) can provide hot water for homes as well. These are different from heat pump desuperheaters (discussed below) that provide hot water only as a side benefit of space conditioning. Since they move heat from the air to the water, HPWHs use only 35% to 50% as much energy as a typical electric resistance water heater. These units are usually about one ton (12,000 Btu/hr) in size. Models and adapter kits are available for retrofits or new installations. Because heat pump water heaters cost more to purchase and maintain than standard water heaters, their use is usually economical only in situations where:
- Water use is high (four or more people)
- Electricity rates are high.
- No natural gas is available.
- Warm or mild air temperatures exist where the unit is installed.
- The extra space cooling the system provides is needed and can be easily ducted to the area where it is used.
The geothermal heat pump industry has recently developed separate heat pump water heaters that operate off the same ground loop as the main space conditioning heat pump. These transfer heat from the ground loop water to the domestic hot water. Although their efficiency depends upon the temperature of the ground loop water, they can use as little as 20% as much energy as an electric resistance water heater.
Hydronic furnaces are air handlers that resemble forced-air furnaces. In these systems, a liquid-to-air heat exchanger replaces the conventional combustion chamber in oil and gas furnaces, or electric resistance heating elements in electric furnaces. A boiler or domestic hot water (DHW) heater furnishes the hot water that circulates through the heat exchanger. A fan forces air over the heat exchanger coils. Heat transfers from the hot water to air, which is then transferred by ducts into the house. The air handlers generally have direct expansion coils for summer air conditioning. Hydronic furnaces are gaining popularity because of the reduced heating load required by today's energy-efficient houses. In super-insulated houses, the DHW heater can effectively heat both domestic water and the living space. Using the hot water heater for space heating utilizes the heater more fully, which increases the efficiency of the system.
Indoor air quality is of particular concern to residents of airtight, energy-efficient houses. A heat recovery ventilator (HRV), also known as an air-to-air heat exchanger, is one way to provide mechanical ventilation without wasting energy. The HRV recovers heat from warm, stale air before exhausting it to the outside. The indoor air circulates through the core of the heat recovery ventilator along with cold, fresh, outdoor air. The two streams of air are separated by thin metal, specially-treated paper, or plastic sheets. As the warm exhaust air passes through the HRV, it gives up some of its heat to the incoming air. Efficient units operate at 80% to 90% efficiencies during the winter and 70% to 80% efficiencies during the summer. HRV units may have variable speed blowers connected to air ducts in bathrooms and kitchens, and may be equipped with automatic sensors that operate according to indoor humidity levels. They also should have electrically operated dampers to limit and control the amount of outside air entering the return side of the air handler. Two-speed blowers or adjustable speed drives are more efficient, because the fan operates at a lower speed when the system is in the ventilation mode. In cold climates, the condensation and frosting of water vapor when warm exhaust air meets subzero outdoor temperatures is a concern. Most HRVs have a defrost cycle to prevent this problem.
Integrated ventilation units may have additional heat exchangers for space heating, water heating, and space cooling. Waste heat recovered from stale indoor air can heat water in domestic hot water tanks. These integrated units may have either electric resistance heating elements that make the units function as electric furnaces; or liquid-to-air heat exchangers, where hot water circulates from a boiler or domestic hot water heater. The latter allows the units to function as hydronic furnaces. When properly designed and installed in houses with minimal duct and envelope air leakage (infiltration), integrated residential heating and ventilating systems supply fresh, heated air at a lower cost than separate heating and ventilating systems provide. Leaky houses, long duct runs, improperly sealed ducts, and inefficient heating and cooling systems prevent integrated systems from operating economically.
High Efficiency Triple-Function Heat Pumps:
A heat pump functions as a space heater in the winter and an air conditioner in the summer. A triple-function heat pump also functions as a water heater or ventilator.
Many heat pumps (as well as some air conditioners) can heat water with desuperheaters. These are devices that use some of the heat pump's hot refrigerant to heat water. One disadvantage of a desuperheater is that it only operates when the heat pump is heating or cooling the indoor air.
Certain air source heat pumps have dedicated water heaters, which allow them to heat water when no heating or cooling is required. These include new advanced heat pump models. For example, one unit uses a variable speed compressor and indoor fan. Variable speed increases the heat pump's efficiency, eliminates blasts of cool air when the unit starts up on cold winter days, and provides better humidity control. It reportedly achieves heating and cooling efficiencies that are 15% to 25% better than single-speed air-source heat pumps. This provides considerable savings in energy costs. These heat pumps also have other energy-saving features, including the capacity for multi-zone control and utility management of peak loads. These heat pumps cost about 30% more than conventional single-speed heat pumps.
One triple-function heat pump system stores ice and heated water in two large tanks one pressurized for domestic water, and the other non-pressurized for space heating and cooling. It is designed to operate during a utility's off-peak hours. This allows the owner to take advantage of reduced off-peak rates offered by some utilities. In some areas, electricity costs half as much during off-peak hours as during peak hours. This creates significant savings. (Some electric utilities also offer rebates to encourage their customers to purchase energy-efficient equipment such as this.) In the cooling mode, the compressor operates only at night, making ice in a large storage tank. During the day, chilled water is pumped from the tank to one or more indoor heat exchangers to cool the living space. During the heating season, the compressor, operating during off-peak hours, heats the water in the storage tank for later space heating use. Zone heating and cooling are other energy-saving features of triple-function heat pumps. These integrated units are considerably more expensive to purchase than conventional gas and electric equipment, but provide significant energy cost savings over the long term.
Another type of triple-function unit uses ground water or a closed-loop heat exchanger buried in the ground to heat or cool space and to heat water year-round. Ground source or water source heat pumps are inherently more efficient than air source heat pumps. They draw heat from the ground or groundwater, both of which maintain more stable temperatures than outdoor air. They are, however, more expensive to buy and install than air-source heat pumps.
Purchasing an Integrated Product:
Information on integrated products is not as widely available as it is for conventional equipment. Use the sources listed below to learn more. Many individual products are reviewed. Note that with fuel-fired systems, equipment is often simplified, eliminating a redundant second burner and venting system. This may or may not translate into a lower purchase cost. Cost and energy savings vary widely, depending upon the types of heating and cooling, ventilating, and water heating features integrated into the equipment. Thus, efficiency ratings may not be comparable to non-integrated systems. The Gas Appliance Manufacturers Association (GAMA) began to rate gas combined space/water heating systems in April 1994. Some of the systems represent state-of-the-art technology, and demand a premium price; however, energy savings are normally much higher than with conventional equipment. Most of the equipment described above integrate some components, but few have all of the components housed in a single package. Before making a purchase, investigate the options thoroughly. Select a unit that will meet your needs. The systems discussed above represent only a small sampling of integrated appliances. New products are continually being introduced. For further information, consult the following sources, and contact your local heating, ventilating, and air conditioning (HVAC) contractors.
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