When people I meet find out I am a green-building consultant, they often ask me, “What’s the greenest thing I can do to my home?” They suggest things like woven, pesticide-free carpets, and low-VOC paints. Bamboo is a big one, too. I point out that the house I live in was built in 1894 and that all the VOCs cooked off around 1895. Since then, the house has been heated for 105 Vermont winters (we don’t use A/C up here).
Not to diminish the importance of low toxicity, let me ask you how important, in relative terms, are the green products to, say, the efficiency of the HVAC system or the integrity of the thermal envelope? If the original builder of my home had not paid attention to the HVAC system, the green benefits of a low-VOC floor finish would have been negated hundreds of times over by an inefficient heating system. So, as in my last column when I urged that designers and builders pay attention to thermal enveloping before opting for exotic insulation alternatives, I urge you now to look at HVAC basics … then focus on the next entirely necessary level of green.
Anyone can green-up an HVAC system in four ways: 1) type of HVAC system used; 2) duct location/duct sealants; 3) air filtration system; and 4) overall system efficiency.
As for types of heating, ventilation and air-conditioning systems, there are three: supply-only, exhaust-only and balanced HVAC systems.
Supply-only systems supply outside air to the home. They do not do anything to “condition” it for use (e.g., heat it up or cool it down with the energy contained in exhaust air). When this HVAC system turns on, cool (in winter) or hot (in summer) outdoor air simply pours into the ducts from the outside. If the air is 0 F outside and you have your thermostat set at 70 F, you pay to heat that air up to 70 F.
Exhaust-only systems simply pump air out of a room. These ventilator fans are commonly found in bathrooms and kitchens. Even small fans can depressurize a room and draw furnace fumes back into the house — called backdrafting — from flue pipes.
Balanced systems bring in fresh air while they exhaust air from inside the home. Through a heat exchanger, the balanced system uses the energy contained in the outgoing air to condition the incoming air. Yet these systems do not let the air actually mingle. A balanced system is the greenest choice because it saves the most energy. After you pick your type, then pick the most efficient unit you can get your hands on.
For forced-air duct systems, there are two main goals: 1) carefully seal the seams of the ducts so they do not leak air; and 2) run the ducts as straight as possible through conditioned (insulated) space.
After achieving a good mechanical connection, seal the ducts with an acrylic- or butyl-based specialty tape, or a duct-sealing mastic. Don’t use “duct tape,” which has a rubberized adhesive and dries out over time. Look for “UL 181 compliant” products. For duct-sealing mastic, use low-VOC products like those from RCD Corp., McGill Airseal and Hardcast.
Ideally, run ducts through conditioned spaces. Even if the temperature of the conditioned space is not the same as the living space, the aim is to lessen the degree differential between the air temperature inside the duct and the temperature of the air the duct is running through.