Heat exchangers: balancing the heat and air quality of your home

When trying to live more sustainably, you often face tradeoffs. The washer that uses the least water uses more electricity; your old car burns fossil fuels, but electric vehicles use toxic batteries; large refrigerators work more efficiently but encourage food waste. It is difficult to find the balance point between two choices. Fortunately, heat exchangers maintain the balance between energy efficiency and indoor air quality (IAQ).

Competing values

It may seem counterintuitive to use a machine to bring fresh air into your home when you can open a window. But when temperatures are low (or very high), it’s important to keep a house sealed to maintain comfortable temperatures. Older homes allowed air to seep through walls, attics, fireplaces, and windows, but the air that replaced it had to be heated. Even in modern homes, heating and air conditioning accounts for almost a fifth of America’s carbon footprint. But insulation and other strategies that make your home energy efficient also trap air and moisture inside the home.

Some insulation contributes directly to indoor air pollution, but in most cases, insulation simply prevents pollutants from escaping. The most effective way to improve indoor air quality is to reduce or eliminate individual sources pollution. But for indoor pollutants that you cannot control, it is necessary to bring fresh air into the house. American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Packages The standard for residential ventilation at a minimum of 0.35 air changes per hour and not less than 15 cubic feet per minute (cfm) per person. Even a drafty house can fall below the ventilation standard under certain conditions.

What are heat exchangers?

Different from furnace heat exchanger, air-to-air heat exchangers are simple devices that keep heat in your home while exhausting stale air, allowing you to maintain your energy efficiency and the quality of your indoor air. Attached to your existing HVAC system, an air-to-air heat exchanger puts two air streams of different temperatures into thermal contact so that heat is transferred from indoor air leaving to outdoor air entering through your home’s existing mechanical ventilation system. Heat exchangers can recover up to 85% of the heat in the outgoing air and they filter particles from the incoming air.

Typical characteristics of an air-to-air heat exchanger. Picture The source

Most air-to-air exchangers installed in northern climates are heat recovery ventilators (HRVs). These units recover heat from indoor air when it is exhausted from the building. Energy recovery ventilators (ERVs) are heat exchangers that also transfer moisture. Historically, they performed best in warmer climates with higher humidity. Recent technological advancements have made them useful for a wider range of climates.

Heat exchanger or heat pump?

Heat exchangers simply improve ventilation. Aerothermal heat pumps actually heating and cooling your home. They can replace your home’s heating and air conditioning systems; they can heat specific areas of the house or work in tandem with your existing heating system. Homeowners typically add them to existing HVAC systems, using the fuel furnace as a back-up solution during more extreme weather conditions.

Use of two heat exchangers (one outside the unit and one inside), an air-source heat pump transfers heat rather than burning fuel. Heat pumps can provide up to three times more BTUs than they consume, while the most efficient gas boiler cannot produce as much thermal energy as it consumes. Early versions of the technology had some issues. But recent advances have improved their reliability and widened the climatic range of their utility. While it’s hard to imagine drawing heat from the outside air in winter, new heat pumps can heat homes. more efficiently than oil, even in the northeastern United States

Do you need it?

You probably already know if your home needs insulation. Your heating bill will be high and there will be drafts in your home. An energy audit of your home will help you prioritize your improvements. Once you have sealed your home sufficiently, a heat exchanger will help maintain indoor air quality.

It is not always immediately obvious if your home has poor ventilation. Most indoor air pollutants do not produce odors. But in narrow houses, condensation from windows and other moisture problems, like mold, are usually the most visible signs of poor ventilation. If your house was built after year 2000, it’s probably airtight enough that you need a heat exchanger for proper ventilation.

Unless you have a new efficient home heating system, consider replacing your furnace with a heat pump instead of adding a heat exchanger for ventilation. And if you live in a hot climate, a heat pump is always a good idea, because any method that keeps you cool is better for the environment than using an air conditioner.

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