Indoor Climate Control

By Bob Vila | Updated Nov 11, 2013 9:45 PM

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The work of adapting your heating, ventilation, and air-conditioning (HVAC) systems begins at this stage. When the walls are open, running new pipes, ducts, tubes, or wires is relatively easy. Ironically, deciding what to do can actually seem like the hardest part of the process.

If the system in place requires no modification, then the decision is easy. That also means there’ll be fewer bills to pay and the overall cost of the renovation will be less. If you already decided no changes are necessary, you may want to skip to the next section of this chapter.

On the other hand, the systems in place may be insufficient to service the ren­ovated spaces. Or you may decide that while the workers are on site that you want to update, perhaps by installing air-conditioning, adding radiant heating, or otherwise changing the systems in place.

When it comes to HVAC work, you will need to decide upon what the source of energy is to be (gas, oil, and electric are the traditional ones) and then the nature of the system (hot air, hot water, and so on). So we’ll look at the choices: first, we’ll talk about the energy, then we’ll talk about the technology required to put it to use.

Two hundred years ago, heating systems were decidedly low-tech. If you wanted warmth, you lit a fire in the fireplace. If you were hot, you opened the doors and windows.

In the twentieth century, wood and coal, long the most common energy choices, have been largely superceded by oil, municipal gas, and electricity. Increas­ingly solar and geothermal energy are being used, too, often in conjunction with more traditional sources. The best choice for you depends upon many factors, including the nature of your existing system, the climate where you live, the relative energy costs in your area, and of course, your budget.

The choice isn’t always obvious. In many parts of the country, natural gas is the least expensive fuel and the clear favorite. In some rural parts of the country, however, where there are no pipelines, liquefied petroleum (LP) is the substitute. It often isn’t as economical as natural gas.

For many years, oil was the great energy bargain. Then in the 1970s, prices rose rapidly. As the cost of oil skyrocketed, countless homeowners in northern areas installed woodstoves, seeking to take advantage of a resource that was widely available, cheap, and renewable. But the rules continue to change. Today we think of burning wood as labor intensive and, particularly with older stoves that lack the latest advances, it’s dis­tinctly unfriendly to the environment. A woodstove sends up the chimney a range of unburned gases and releases particulate matter as well. In the meantime, oil prices have come back down, and today oil is, once again, favorably priced.

All of which is to say the right fuel isn’t obvious. Your decision about which source you should use should be made in tandem to your decision about the heat distribution system.

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.

If you’re considering a new system for your home, talk first to your architect or designer. Conversations with HVAC contractors will probably fol­low, although you or your designer may also want to consult a specialist, a heating engineer, in the event your remodeling presents unusual demands.

Talk through in detail exactly what your needs are. If your budget is tight, you’ll need to identify essentials. If you can afford to think more broadly, consider the added comfort of, say, radiant floor heating. If you’re unhappy with your present sys­tem or would like to add humidification or a filtration system, get bids for those costs. In most cases, extending your existing system or adding a smaller area heater will be the least expensive.

Here are a few other considerations:

The air-conditioning option. As a rule of thumb, if local temperatures rarely rise above 85 degrees Fahrenheit, you probably don’t need central air-conditioning. On the other hand, central air is often regarded by realtors as a valuable selling point, so if there’s a chance you’ll be transfened to another region or are likely to put your home on the market for any reason in the near future, central air-conditioning may be a good investment. Top-of-the-market houses get top-of-the-market prices because they have all the bells and whistles. For people with asthma and other allergy problems, central air with its ability to filter and “condition” household air can also have health benefits.

Beware of oversize systems. Strange though it may sound, too much heating capacity will make a system less efficient. It will cause the system to cycle on and off frequently, producing excess wear and tear on the components. The system may never reach peak operating temperatures.

To be sure your system is suited to your home, ask your HVAC contractor, heating engineer, or whoever designed the system to walk you through the calcula­tion. The process consists of determining what the heating load is (based on an arith­metical formula that factors in the size of your home, its insulation, and the local climate). The system capacity should be no more than 25 percent greater than the calculated heating load.

■ Simple is usually less expensive. Staying with your existing system is almost cer­tainly the cheapest route. If your system has enough capacity that it can be extended to heat (or cool) new spaces, that approach will probably be less expensive than installing an all-new system.

Buy quality. Good shoppers don’t always buy bargains. Buying durable boilers or furnaces that come with long warranties often costs more initially but, over the years, presents fewer headaches. Good furnaces often are guaranteed for twenty years, boilers for thirty, heat pumps for less.

■ Think locally. Don’t buy equipment that no one in your area can service. If the only HVAC contractor who’ll bid your job is a long-distance call away, you could be ask­ing for trouble. These sophisticated modem systems require occasional checkups by service people familiar with their design, installation, and individual characteristics.

One industry study found that half of all service calls were the result of improper or insufficient maintenance.