Indoor Climate Control

THE DELIVERY SYSTEMS. Every approach has its advantages and disadvantages, whether you're talking about deciding among a furnace, boiler, heat pump, or space heater, and pipes, tubes, or ducts. Let's look at the options.

Forced hot air. Forced hot air is the most common and the fastest mode of heat deliv­ery. The source of the heat may be an electric-, oil-, or gas-fired furnace or a heat pump. The heated air is then routed to the house through sheet metal, fiberglass, or plastic ducts, driven by a fan, emerging into the living spaces through registers.

The advantages of forced hot air are the speed with which heat is delivered to the house (these systems are significantly faster than, say, hot water systems) and the utility of the ductwork for other climate-control systems. Air-conditioning, filtra­tion, and ventilation, as well as humidification and dehumidification, can all be done using the same system of ducts and registers. The disadvantages are the risk of heat loss through leaky ducts and added difficulty (and expense) in separating different parts of the house into distinct zones. Hot air systems also can be noisy, as the fans that drive the air are usually audible in the living spaces.

Hot water. Also referred to as hydronic heat, hot water heating systems consist of a boiler that heats the water and a pump that circulates the water through a system of pipes (hidden in the skeleton of the house) and radiators (in the living spaces). Typ­ically the water is circulated at temperatures in the range of 130 to 180 degrees Fahrenheit.

Hot water systems are slower but quieter than hot air. They're easier to zone,but cost more to install. The radiators also pose a challenge to interior design since their sheer bulk interferes with furniture placement. Hot water systems cannot be adapted for air-conditioning and other climate-control uses.

Radiant floor. Although variations on this same theme have been around for millen­nia, this latest incarnation has only returned to wide use in recent years. Radiant floor heating is the least obtrusive method of heating. As with hot water systems, a boiler provides hot water, heated to temperatures in the range of roughly 85 to 140 degrees Fahrenheit. The hot water is distributed to the house via a system of manifolds and controls that bring the heat to a complex network of plastic or rubber tub­ing that is hidden in the floor.

Three basic approaches are used in radiant floor systems. When a house is built on a concrete slab that sits directly on the soil, the radiant pipes are embedded in the concrete. The second approach utilizes a thinner slab of concrete: once the tubing is fastened directly to the decking of a traditionally framed floor, a thinner slab of concrete is poured. The third uses aluminum heat- transfer plates that radiate the heat from the tubing. Tubing-and-plate systems can be installed on top of or below existing wood-framed flooring systems. The plate sys­tems lend themselves to retrofitting; since they can be installed from below, the existing flooring needn't be disturbed. However, keep in mind that radiant heat isn't well suited to homes with wall-to-wall carpets and thick pads or multiple layers of plywood. These have high thermal resistance and effectively insulate the room to be heated.

The growing popularity of this technology is explained largely by customer satisfaction: homeowners with radiant heat report that it heats evenly, with fewer hot or cool spots and less stratification. Radiant heat costs more to install: it requires careful design and skillful installation. But it is easy to zone.

Electric baseboard. Mounted on exterior walls at floor level, electric baseboard heaters consist of sheet metal housings that protect wires inside that, like those in a toaster, warm and glow when current is run through them. The heating elements are lined with metal fins that heat the air around them; the housing then allows air to circulate in the bottom and out the top. Electric baseboard radiators are inexpensive to install.

Baseboard heaters are wired like any other electrical appliance. A feed line is run through the walls or floors from the electrical panel to the baseboard unit. Some baseboard heaters come with their own thermostats, but in a room where several radiators are required, a thermostat is mounted on an interior wall as a control sys­tem. This also means that electrically heated homes are easily zoned; for the added expense of a few thermostats, every room becomes its own zone, where the heat can be lowered when it's not in use.

Electric baseboard heat is inexpensive to install, but it's very costly to run. That's one reason it's often found in spec houses—the builder wants to save money up front and doesn't have to worry about bloated electric bills later. On the other hand, electric heat is quiet, clean, and quite unobtrusive (the base­board units are modest in size and interfere little with furniture placement). I would not recommend using such systems for an entire house, especially in a cold climate. But for a small addition where the cost of enlarging an existing hot water or hot air system might be prohibitive, electric baseboard may be an appropriate choice.

Space heaters. There are other alternatives for heating individual spaces. Space heaters are direct heaters. Unlike systems where the heat is generated in one place and distributed another, these heaters are self-contained, directly heating the spaces where they are located. A fireplace is a space heater, albeit a very inefficient one. Others include woodstoves, gas and kerosene wall heaters, and freestanding heaters. The latter can be oil- or kerosene-fired or electric. Each of these has advan­tages—most are inexpensive to buy and fairly ecohomical to run. But burning wood produces environmental pollution (particulate matter and unburned gases) and free­standing kerosene heaters in particular have a very mixed safety record.

Air-conditioning. In any refrigerant system—whether it's inside your refrigerator or an air conditioner—the key element is the cooling medium or refrigerant. The refrig­erant is a gas at normal atmosphenc pressures but, when compressed as by the com­pressor of a cooling system, it becomes a liquid.

With central air-conditioning systems, the refrigerant is passed through the coiled tubing in an evaporator located in the house. There a flow of household air is passed over the coil. As the pressure is released, the refrigerant returns to its natural gaseous state, absorbing heat from the air as it does so. The cooled air is then dis­tributed to the living areas of a house via a network of ducts and registers. The refrig­erant is then pumped outside to a condenser where the heat is discharged, the refrigerant recompressed, and the cycle repeats. A window air-conditioning unit functions in the same way, but its components are self-contained.

Heat pump and geothermal systems. These systems are near relations of central air- conditioning systems. They rely upon an electrically powered compressor that com­presses a refrigerant from a gas to a liquid. In the process, heat is given off and, during the cool months of the year, that heat is distributed via ducts to warm the house. In warm weather, the process is reversed, and the system absorbs warm air indoors, releasing it outside.

One limitation of a heat-pump system is that it loses efficiency rapidly when the thermometer drops below 40 degrees. As a rsult, in colder climate a geothermal heat pump systemor ground-source heat pump may be used.

The earth's temperature 8 or 9 feet below the surface remains quite uniform all year round. That means that during the heating season, its temperature is warmer than the atmosphere's; during the hotter months, the earth's temperature is cooler than the air's. A ground-source heat pump puts that differential to work, again using a refrigerant and compressor system.

Because much of the energy is drawn from the environment, such systems are economical to run—typically, the electricity required to run them is roughly a third that of a traditional electric system. They're also clean. However, they're expen­sive to install, require annual maintenance, and typically their components have a shorter life expectancy than do traditional furnaces or boilers.