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All About Building an HVAC

Keep reading to learn about the best HVAC technologies for indoor comfort and energy efficiency, according to This Old House plumbing and heating expert Richard Trethewey.

Building an HVAC system at home Grant Kessler

Not so long ago, staying warm meant shoveling coal or chopping wood. Those two fuels—which heated you twice, as the saying went—accounted for three-quarters of all residential heating in 1940. Today, with gas- and oil-fired central heat, attaining indoor comfort is no more strenuous than lifting a finger to adjust the thermostat.

But getting to that point is probably the most technically challenging part of any residential construction project. For starters, you want maximum efficiency, a fact made all the more acute by rising fuel prices. Heating, ventilating, and air-conditioning (HVAC) consume almost half of a home's energy bill, so every percentage of improvement makes a significant difference. You also want your HVAC pro to consider all the factors that influence comfort—including humidity, air velocity, air quality, and radiant energy—and to choose equipment based on its ease of maintenance and expected longevity.

"Proper sizing and installation are critical," says This Old House plumbing and heating expert Richard Trethewey. His advice: Hire reputable installers who know how to perform heat-loss calculations, and insist on components that have earned the Energy Star high-efficiency rating.

Keep reading for a look at the key components in the state-of-the-art heating and cooling system that Richard recommends if you're building from scratch.

All About Building an HVAC Ian Worpole

Heat With Water and Cool With Air

In cold-weather regions where temperatures are below 70 degrees for more than half the year, Richard Trethewey recommends a single, fuel-stingy boiler to supply both heat and hot water for maximum comfort and efficiency.

"Water conveys heat much better than air does," he says. Lower floors use hot water for radiant heat; it delivers heat to a forced-air system above. To keep the house cool, Richard specifies a traditional air-conditioning system, also shown here, which employs the same ducts used for upper-floor heating.

Building an HVAC system at home Ian Worpole

Fresh Air Without Energy Loss

When a house is built so tightly that it can't "breathe," it can suffer from high indoor moisture levels (which encourage mold growth), elevated levels of carbon dioxide and other toxins (from off-gassing finishes, fabrics, and glues), and plain old bad smells (from cooking and smoking).

A hybrid air exchanger (below) solves these problems by sucking in fresh outside air as it expels stale indoor air. In winter, it uses the expelled air to warm and humidify the incoming cold air. In summer, it cools and dehumidifies sticky outdoor air. And as it operates, it filters out airborne contaminants and odors.

"Sooner or later, every house will need one of these units," says Richard. He recommends connecting it to a separate duct system serving bathrooms, bedrooms, and the kitchen.

Outdoor Reset Control

Building an HVAC system at home Ian Worpole

These simple devices save energy by regulating the boiler or furnace temperature in response to changes in outdoor temperature. On a zero-degree day, for example, the water in a boiler might need to be 180 degrees to heat the house. But on a 35-degree day, 125-degree water might be enough.

The outdoor reset control makes the adjustments, saving a heating unit from excessive on-and-off cycles and smoothing out the uncomfortable fluctuations in interior temperature that result when a thermostat is in total control. Just make sure that your heating unit works with this kind of device. "A reset control can easily improve system efficiency by at least 10 to 15 percent," Richard says.

Condensing Gas Boiler

One by-product of burning natural gas is hot-water vapor, which until recently went up the stack with the rest of the exhaust gases. The loss of this heat prevented boilers from becoming more than 85 percent efficient. Condensing boilers employ a heat exchanger that wrings the heat out of the vapor before it can escape. "It's a quantum leap," says Richard. "Some of these units are achieving efficiencies of 96 percent."

Heat Your Home with In-Floor Radiant Heating

Building an HVAC system at home Ian Worpole

Radiant floors have changed the way Americans think about comfort. The principle is essentially the same as an old-fashioned radiator, except that instead of heating a large hunk of metal, the mass being heated is the floor itself, via hot-water pipes or electric wires under the surface. Not only is the heat more gentle and efficient, says Richard, it's more comfortable because humans prefer their feet to be warmer than their heads. "Forced-air heat does the opposite: it gives you cold feet and a hot head."

In-floor heating goes back at least to ancient Roman baths, where fires heated the air under stone floors. Modern in-floor heating uses loops of PEX pipe, either buried in poured concrete or fitted into grooved panels on top of a wood subfloor, as shown above. The same principle can be used outdoors to melt snow on a driveway or walkway.

Common HVAC Mistakes

No annual maintenance

All HVAC systems need annual cleaning and tuning, which is typically done in the fall by your heating professional or fuel provider. "You wouldn't let your car go more than a year without a checkup, would you?" says Richard. Homeowners can easily remove a register and vacuum out vents in the floor (particularly kitchen vents) at least once a year. Seriously filthy ductwork may need professional cleaning.

Not enough space around the heating and cooling units

Richard recommends leaving 3 feet of clear space to allow airflow, avoid fire risks, and conduct annual maintenance.

Building an HVAC system at home Grant Kessler

Oversized heating systems

"No contractor wants to hear a customer say, 'I don't have enough heat,'" Richard says. "So they install a boiler or furnace big enough for the coldest day of the year, plus a safety factor of 25 to 50 percent. But the coldest day of the year happens for just a handful of hours. So now you've paid for this appliance that's way too big for 99.9 percent of the time. That means more frequent on-off cycling, which wears out the unit and invites incomplete combustion."

Richard's solution: Make sure the contractor performs a heat-loss calculation—a measure based on a house's insulation levels, airtightness, and the local climate—and orders equipment that meets peak demand without any additional safety factor. The same goes for cooling equipment.

Not installing high-efficiency filters

Richard favors the thick, pleated media filters with antimicrobial coatings. "All the air you breathe comes through that filter," he says. "It makes no sense to use the cheap ones you can see through." Replace them annually when the system is being serviced.


Viessmann North America Headquarters
Waterloo, Ontario

Air exchangers:
Nutech Brands Inc.
London, Ontario


Indoor-outdoor reset:
Tekmar Control Systems Ltd.
Vernon, BC Canada

Radiant floor panels
Leesburg, VA

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