Climate Control

If you've been thinking of upgrading your heating and air-conditioning system, this is a good time to do it. Manufacturers have made real improvements in comfort, efficiency and durability. In most respects, residential equipment is just better than it was 10 years ago-even five years ago. For example, gas-furnace heat exchangers now have warranties of up to 30 years and central air conditioners have seen leaps in efficiency.

Today's forced-air furnaces and air conditioners are also more versatile, especially when mated with the latest zoning, ventilation, air-cleaning and humidity-control equipment. There's a lot to consider, and the best way to appreciate the new-and-improved is to acquaint ourselves with the old, at least enough to recognize the parts.

Gas Furnaces

Before the Environmental Protection Agency mandated improved energy efficiencies in the '80s--with a 78 percent minimum--gas-fired furnaces had changed little in 40 years. In these older units, you'll typically find three or four burners that fire into tall, narrow, cast-iron or steel heat-exchanger cells. The cells slow the movement of the exhaust gases to the vent, and transfer heat to household air that's blown over the warm cell walls.

These are reliable, easy-to-service furnaces, but they capture only 60 to 65 percent of the heat they generate. Add the fact that older furnaces were routinely oversized, wasting gas and electricity, and you could easily lose 50 to 55 percent of your heating investment each winter.

To improve efficiency, engineers slowed the rate at which the exhaust gases moved through the cells so more heat could be extracted. But at some point the gases become too cool to rise through the flue on their own, and you run the serious risk of leaking carbon monoxide into the living area.

Moreover, gas combustion produces water vapor and several other byproducts, including sulfuric, hydrochloric and carbonic acids. These pass easily through the system when hot, but if you subtract too much heat, the vapors can condense and corrode heat exchangers, cabinets and flue pipes.

To increase efficiency, manufacturers began by replacing standing pilots with electronic igniters, adding fans to kick the exhaust gases up the flue, and fine-tuning heat exchangers to capture more heat. With these few changes, the 78 to 80 percent furnace was born.

Eventually, most manufacturers added medium- (82 to 84 percent) and high-efficiency (90 to 94 percent) furnaces to their lines. Above about 82 percent, however, condensation comes into play, so engineers added a second heat exchanger made of corrosion-resistant materials--either aluminized steel, plastic-coated steel or stainless steel. Condensation is limited to this chamber, where it's safely drained away.

Called condensing furnaces, these units are so efficient, their exhaust gases can be expelled horizontally through 2-in. PVC pipe. In other words, a few feet of convoluted heat-exchanger surface area can extract 1800°F from a 2000°F flame.

About four years after their development, the 82 to 84 percent, midrange models began having problems--mainly from unmanaged condensate. The manufacturers' solution was to discontinue these furnaces, leaving us with two furnace categories: 80 percent noncondensing and 90 percent condensing.

How do you decide which to buy? While builder's-grade noncondensing furnaces run around $700 or $800, condensing furnaces sell in the $2000 to $2500 range--and their complexity can mean more maintenance. However, they make sense in colder climates and are a good choice for older, all-masonry homes, which can't be well insulated, and new ultratight, mechanically ventilated homes. Your dealer or utility company can calculate fuel savings against equipment costs in your area.

The most significant recent furnace developments are multispeed and variable-speed furnaces. Multispeed models feature burners and blower fans that operate at two or more levels--builder's-grade furnaces may have two, while better models may have four or five.

Blower speed and burner output are linked and electronically controlled by the thermostat--the amount and temperature of the air circulated is based on need. As there are more mild days than really cold days, the savings can be substantial--as much as 30 percent.

Variable-speed motors, available in deluxe models, offer even more in terms of efficiency and performance. Multispeed motors use about the same electricity, 400 to 500 watts, at low or high speeds, but variable-speed motors use dramatically less energy when throttled down--as little as 65 watts.

The economy of a variable-speed unit makes continually run blowers more feasible, an option that can make your house feel warmer with your thermostat set lower.

The latest electronic zoning equipment also increases comfort by allowing you to set different temperatures in various rooms. Some additional ductwork is required, so it's best for new construction. And ideally, each zone should have its own return-air duct.

To do the job, each room or group of rooms is controlled by dampers in the ductwork and a heat sensor in the room. You set the desired temperature for each zone at the thermostat, and when each zone reaches its target temperature, the damper closes. Prices vary with the size of the house and the number of zones, but you can easily add $1000 to an installation.

Set-back thermostats also can save energy by cutting back heat when you're away or asleep. They're widely available, easy to customize, and range in price between $35 and $200.

Central A/C

The best news in central air conditioning is another leap in efficiency, and again, variable-speed technology takes much of the credit. Air-conditioner efficiency is measured by a system called the Seasonal Energy Efficiency Rating (SEER). In the past few years, manufacturers have increased efficiency from SEER 14 to SEER 18--SEER 10 is the federally mandated minimum, soon to be SEER 12.

Prices vary, but if the equipment costs for a SEER 10 system run $900, each step up in SEER (from 12 to 14, 14 to 16, and so on) will run about $200. Ask your dealer or utility company to run the payback numbers.

Central air conditioners are split systems, with the noisy compressor and heat-shedding condenser installed outdoors, and the evaporator coil indoors, usually in the plenum of the furnace. Moisture condensing on the indoor coil drains away, making the air less humid.

The first gains in efficiency occurred in the early 1990s, and were achieved mostly by sifting the old technology for refinements. When engineers had squeezed everything they could from single-speed compressors, they came out with multispeed compressors, with even better results. At the same time, highly efficient scroll compressors arrived that promised longer life through reduced wear.

And now there are variable-speed compressors, which run only as hard as they need to, bumping performance up to SEER 16 and 18. Other improvements include better protection of the vulnerable outdoor coil, and plastic bases that don't rust. Warranties run 10 years for compressors and five years for coils.

Ventilation

Indoor air quality is a real issue these days. The problem is that houses are built much tighter than they were a generation ago. And after 30 years of energy-conscious home improvements, even older homes can become dangerously tight. With less fresh air seeping in, excessive moisture buildup, plus the combustion gases from a range, water heater or dryer, it's easy to see how some people get sick.

One popular low-budget approach is to cut a 4-in. vent into the side of the house and connect it to the return-air duct. In this way, each time the furnace blower kicks on, it draws in fresh air. The problem, of course, is that you have to heat or cool this air.

With this in mind, the HVAC industry developed the heat-recovery ventilator (HRV). It draws fresh air into the system, but uses outgoing air to temper the incoming air. It's about 75 percent efficient, so you still lose some heat, but with an inlet and outlet fan, indoor pressures remain balanced, and that prevents backdrafting and heat loss.

A similar unit, called an energy-recovery ventilator (ERV), is able to recover moisture from the exhaust stream as well. It helps maintain indoor moisture levels in cold, dry climates. HRVs and ERVs can be spliced between the return-air duct and an exterior wall, but they're best connected to dedicated ducts. Bath and kitchen vents are piped into these ducts, as well. HRVs cost about $1200, plus the installation and ductwork.

Air Filtration

When you consider that a forced-air furnace recycles all the air in a house many times a day, it's easy to see why this is the best place to clean the indoor air. And if you opt for a high-efficiency air cleaner, continuous blower operation makes the system more effective.

Air-filter efficiency is all about particle size, because it's the smallest particles that lodge deepest in our lungs. Filters are rated by the size of the particles they can trap. Particles 10 to 20 microns across are still large enough to be visible. By comparison, fungi are as small as 0.5 microns, bacteria 0.3 and smoke 0.1 microns.

There are really only two good choices when it comes to air filters: paper-media filters ($250) and electronic filters ($500, plus installation). Media filters do a reasonably good job on allergens. Pollen is as small as 6 microns, and paper-media filters trap 99 percent of particles that size. A $30 filter cartridge lasts about a year.

As good as media filters are, nothing compares with electronic filters for efficiency. They capture 70 percent of particles 0.3 microns in size. Like other filters, these are installed in a cabinet between the furnace and the return duct. The cabinet contains two metal pre-filters, which catch pet hair and the larger fluff. Behind these two cells are rows of high-voltage/low-amperage wires and rows of metal plates. The wires negatively charge the dust particles floating by, which causes the particles to stick to the positively charged plates. The pre-filters and cells need to be washed every 30 days to remain efficient. If they're not washed, effectiveness drops off dramatically.

Most air cleaners are 6 in. wide, so retrofits require modifying the return duct, which adds expense. Now there's an electronic filter that fits a standard 1-in. filter slot. The Airscreen 1000 (about $300, Cimatec Environmental Engineering, PMB 096, 60 Industrial Pkwy., Cheetowaga, NY 14227; www.cimatec.com) also creates a polarized charge, trapping particles in a plastic mesh. Its low-voltage transformer plugs into a standard household receptacle.

Humidity Control

Humidity really influences comfort because the higher it is, the warmer we're likely to feel. In winter, though, too much moisture in the air can rot windows, dampen insulation and grow mold. Too little moisture--20 percent or less--cracks skin and dries out sinuses and lungs, leaving us vulnerable to infections. In summer, high humidity can leave even an air-conditioned room feeling clammy.

In winter, furnace-mounted humidifiers and remote humidistats are best. Again, continuously running furnaces improve performance. A flow-through humidifier with a continuous stream of water (about $230) is less likely to grow mold or develop calcification problems, but this type does waste some water. These work slightly better when tapped into a hot-water pipe, but using cold water saves water-heating costs.

Some of the latest humidistats (about $50) can read outdoor temperatures and throttle back the moisture output when it senses a cold snap. A drop in outdoor temperature really increases the tendency for heat and moisture to move through the building envelope. And, with high indoor humidity and low outdoor temperatures, moisture condenses on window glass and seeps into window frames. Before these self-regulating humidistats came along, a homeowner's only defense was to watch the weather report and make the adjustments manually.

Humidity control in summer is a job for air conditioners. Again, variable-speed blowers offer an advantage. Sophisticated controls in the thermostat matched to the electronics in the blower motor can slow the motor down enough to provide additional humidity control without much additional cooling.

In fact, Carrier's Thermidistat--a combination set-back thermostat/humidistat (about $215, Carrier Corp., 7310 W. Morris St., Indianapolis, IN 46231; www.carrier.com)--can trigger a run cycle based solely on humidity requirements. You get a little cooling in the bargain, but the focus is humidity control.

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