Greening Up the HVAC System

How do you green up an HVAC system? Four ways: the type of HVAC system you use, the duct location/duct sealants, the air filtration system, and the overall system efficiency.

Types of Ventilation Systems

There are three types of ventilation systems: (1) supply-only systems, (2) exhaust-only systems, and (3) balanced systems.
Supply-only systems just supply outside air to the home; they do not do anything to condition it with the energy contained in the exhaust air. When the HVAC system turns on, cool (in winter) or hot (in summer) outdoor air simply pours into the ducts from the outside and it needs to be conditioned (heated or cooled) before it gets to your rooms. If it is 0 degrees F outside and you have your thermostat set at 70 degrees F, you pay to heat that air up a full 70 degrees.

Exhaust-only systems simply pump air out of a room. These fans are commonly found in bathrooms and kitchens. Exhaust only systems require close watching because even small fans can depressurize a room and draw fumes back into the house (backdrafting) from flue pipes on heating and cooling appliances.

Balanced systems bring in fresh air while they exhaust the air from inside the home that is being replenished. Through a heat exchange system, 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 your greenest choice, because it saves you the energy you would otherwise expend to heat up or cool down the outdoor air for use in your home. Ask your HVAC contractor for advice. Installing a balanced system is not a task just anyone can do.

Ducts in Conditioned Spaces

If you are using ducts to heat and cool your home, your two main goals should be to (1) carefully seal the seams of the ducts, and (2) run the ducts through conditioned (insulated) space.

As for sealing ducts, common sense will tell you that if you are blowing air under pressure into a duct system that leaks like a sieve, you are going to lose conditioned air into nonliving spaces, which will dramatically drive down the efficiency of your HVAC system. After achieving a mechanical connection between ducts, seal them 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.

As for where to run ducts, common sense will tell you that if you blow heated air into a duct system that runs through ice cold — or non-conditioned — space, you will see a dramatic drop in the HVAC system’s efficiency. The same is true for an AC system that blows cold air into ducts that run through hot attic space. Ideally, you want to run ducts through conditioned space.

Whole-House Air Filtration

The technical group ASHRAE has put out a ventilation standard that says homes should have a fresh air flow of 7.5 cu. ft. per minute per resident, plus 1 cu. ft. per minute for every additional 100 sq. ft. of living space. As the air is delivered to the home through the HVAC duct system, it can be filtered to improve indoor air quality. Achieving superior indoor air quality can rarely be done with a standard nuisance-dust filter, but you can use filters that trap particulates as well as bacteria, viruses and even VOCs. Note that these filters often require your HVAC fan to blow harder, which will draw more power and even shorten the life of your blower motor. But for many people, that is a small price to pay for really clean air.

When choosing a filter, look for the “minimum efficiency reporting value” (MERV) scale, which ranges from 1 to 16. The higher the MERV, the tighter the filter, and the more particles the filter can take out of the air. The best nuisance-dust filter has a MERV of 4, whereas HEPA filters can achieve 16. A MERV 4 filter removes 20 percent of the particles in the air, but a MERV 16 filter removes more than 99 percent of particles in the air, down to 0.3 micron in size. That gets mold spores and pollen but viruses are less than 0.1 micron thick.


The greenest thing you can do to a home is to get high-efficiency HVAC systems in place. Here’s why: To heat and cool a home, the average U.S. home contributes 60 tons of CO2 each year. That’s the equivalent pollution from 10 cars per year.
This is where your clients should really stretch their budgets. The return on investment is increasingly easy to justify with high fuel costs, and frankly, it’s the right thing to do.