Sustainable heating and cooling system options
Today, there are a lot of options available for heating and cooling systems, with sustainable solutions finally making their way into the fold. In this article, we give a brief overview of each, covering the basics, environmental impacts, and advantages/disadvantages.
Passive Solar Heating
Passive solar heating is a design approach rather than an actual heating system. Windows are placed on the south-facing side of the building, at a scale equivalent to 10-30 percent of the floor area. In the winter, all sun enters and heats the room, and in the summer, shading blocks summer sun which reduces cooling needs. Passive solar design is not enough alone, but it can reduce the load on heating systems. Computer modeling is available to calculate the ideal ideal passive solar design based on latitude and climate. Window glazing must be carefully selected. It's important to remember that passive solar principles can be used to varying degrees: this is not an not all-or-nothing system.
Solar Hydronic
Components: Solar thermal collector (flat plate or evacuated tube), circulation pump and controls, storage vessel, heat exchanger for delivery system.
Summary: A collector gathers the sun's energy, then is transferred to hydronic fluid, pumped to a storage vessel, and deposited heat via a heat exchanger. Solar energy is stored in a well-insulated tank so it can be used at will. Solar hydronic systems need to be carefully designed to calculate how much heat they can deliver based on climate. These systems can also be used to heat hot water.
Advantages: Very low environmental impacts, great for indoor air quality.
Disadvantages: Systems are very complex, expensive, labor intensive, not accepted by most codes without licensed professional.
Solar Hot Air Heating
Components: Solar thermal collector, ductwork, controls and fans.
Summary: Collectors (glazed or unglazed) collect solar radiation, heating the air in a plenum, which is then ducted into the building. These systems can use fresh air or recirculated indoor air. Most systems are homemade and are moderately difficult to install, but easy to use and maintain.
Advantages: Simple, inexpensive, great indoor air quality.
Disadvantages: Not accepted by most codes, may be accepted as an alternative with a licensed professional and backup system.
Ground Source Heat Pumps (GSHP)
Components: ground loops (two types, below) and circulation pump, heat pump unit with compressor, air or hydronic delivery system, controls as required.
Horizontal Ground Loop: collection loop dug into a depth below frost line or water below ice depth.
Vertical Ground Loop: collection loop placed in vertically drilled wells
Summary: Ground loops collect heat (in winter) or disperse heat (in summer). Btus are gained from additional loop length. Refrigerant collects heat from ground loop fluid, a compressor uses the refrigerant to create or extract heat for the building. These systems need to be designed with a precise heat loss calculation.
Advantages: Some can heat hot water, low GHG emissions if used with renewable energy, easily sourced, accepted in many codes
Disadvantages: Ecosystem disruption for ground loop digging, Uses toxic refrigerants, Expensive, labor intensive, need a licensed installer
Air Source Heat Pumps (ASHP)
Components: Heat pump, heat exchanger/plenum, air or hydronic delivery system, controls.
Summary: A heat pump unit outside uses large fans to move air over a heat exchanger coil, which transfers heat to the refrigerant. Heat is delivered through forced air, and can work in both directions.
Advantages: Low GHG emissions if used with renewable energy, easy maintenance, easy to source.
Disadvantages: Fans are noisy, backup heat source may be required in placed with very cold winters, toxic refrigerants, can be expensive, licensed installer required, accepted in many codes.
Boilers and Condensing Boilers
Components: combustion chamber, exhaust chimney, heat exchanger, direct vent, circulation pump, temperature and pressure relieve valve, expansion tank, air bleeder as required, controls as required
Summary: A burner ignites to heat water, a circulating pump moves the water through the system. Environmental impacts depend on the fuel source used to create the flame. There are three types:
Direct Heat: boiler water goes directly to heating system
Indirect Heat: boiler water goes to storage tanks
Condensing Boiler: exhaust gasses carrying waste heat use to warm water. used in both direct and indirect systems, is more efficient and used in newer systems
Advantages: Sourcing is easy, accepted in all codes (exception may be in alternative fuels like pellet and biofuel)
Disadvantages: Can be expensive, installations require licensed professional, combustion can have negative impacts on indoor air quality
Tankless or On-Demand Heaters
Components: Combustion chamber, exhaust chimney, direct vent intake and exhaust, circulation pump, relief valve/expansion take/air bleeder as required, controls.
Summary: Similar to direct heat boilers, but the heat exchanger can heat a small amount of water quickly. Environmental impacts depend on fuel source, but are generally high.
Advantages: Moderate costs, easy to source, accepted in all codes.
Disadvantages: Install requires licensed professional, combustion can have negative impacts on indoor air quality.
Tank or Batch Heaters
Components: Metal tank, tank insulation, heat exchangers (if required), direct vent intake and exhaust, relief valve/pumps/expansion tank as required, controls.
Summary: A tank holds water which is heated (in the tank) through combustion or electricity. Some systems can do heating and hot water, some use solar hydronic input. Be careful in selecting a tank with good insulation and even adding more after installing, as heat loss through the tank is the greatest disadvantage of this system. Environmental impacts depend on the fuel source.
Advantages: easy to install, use and maintain, easy to source, accepted in all codes (although uncommon to use as main heat supply)
Disadvantages: Combustion can have negative impacts on indoor air quality
Forced-Air Furnaces
Components: Combustion chamber, heat exchanger and plenum, direct vent intake and exhaust, circulation fan, controls.
Summary: A burner heats a heat exchanger plate and transfers it to air blown through the plenum. Cold air return ducts direct air to be reheated, a filter removes dust and allergens. Environmental impacts depend on fuel source but are generally high as most units use fossil fuels.
Advantages: Moderate labor, easy to course, accepted in all codes.
Disadvantages: Install requires licensed professional, Combustion can have negative impacts on indoor air quality.
Image: Perfect Home HVAC Design.
Wood and Pellet Stoves
Components: Burn chamber and ash pan, air inlet and controls, exhaust outlet and controls fuel delivery system, blower fan/water-heating coil and tank (depends on unit).
Summary: Fire burns in a burn chamber heating the metal or masonry around chamber. Air inlet lets air in, exhaust lets smoke out via a chimney. Central placement will allow for most ideal heating conditions.
Advantages: Low to moderate labor inputs, accepted in most codes.
Disadvantages: Maintenance and use, usually not accepted as primary heat source, dust and smoke from loading chamber.
Masonry Heaters
Components: Burn chamber and ash pan, masonry chimney, masonry shroud, air inlet and controls, cooking oven space, water heating coil or tank
Summary: A freestanding unit placed centrally in home contains a burn chamber for fire to be lit. Exhaust gasses travel through a winding masonry chimney providing additional heating opportunities. Fires need to be timed well to keep unit at optimal temperature range. Environmental impacts are low- these heaters make very efficient use of wood fuel.
Advantages: Low environmental impacts, very durable.
Disadvantages: Difficult insulation, must be fired on schedule, not accepted in any codes, dust and smoke from loading chamber.
References
Chris Magwood. Making Better Buildings: A Comparative Guide to Sustainable Construction for Homeowners and Contractors. New Society Publishers.