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Nothing in life is to be feared. It is only to be understood. -Marie Curie
by Emily Nicolosi, PhD. December 2020.
A good first step in starting to think about integrating a more environmentally sustainable heating and cooling system into your home is to understand how exactly they work! In this article, I'll cover how heat is produced and delivered with an eye to practical matters and sustainability.
A good first step in starting to think about integrating a more environmentally sustainable heating and cooling system into your home is to understand how exactly they work! In this article, I'll cover how heat is produced and delivered with an eye to practical matters and sustainability.
Means of Heat Production
Means of Heat Production
There are only 4 actual means of creating heat!
1. Solar Heat
1. Solar Heat
Summary: Directly harvesting solar energy, heat from the sun is collected at (in/on/near) the building. There are three types of solar heat:
Passive Solar: Collection of heat via glazing on the Southern exposure (sunny side) of building. The presence of sun raises the heat of the building.
Active Solar Air: Special units collect and concentrate the sun's heat to a flow of air that moves into a heat exchanger or directly to the building.
Active Solar Water: As above, but heat is collected into liquid
Efficiency: Rates appear low (10-70%), but are not comparable to combustion devices as "wasted" solar energy has no negative consequences.
Advantages/Disadvantages: Emission free, no fuel extraction, no transportation, minimal air pollution (unless using non-solar energy to run pumps)
Solar Thermal Heating Collectors. Image: Gallery Kitchen Design.
2. Combustion Devices
2. Combustion Devices
Summary: The most common means of heat production. All of these devices burn a fuel and extract heat from flame. All have 1) a supply of oxygen (for flame) 2) an exhaust (allows gasses to escape) 3) a heat exchanger (moves heat from flame to delivery system). There are two categories of combustion devices:
Gas/Liquid Fuel Combustion: Most common. Fossil fuels: natural gas, propane, oil; biofuels: biodiesel, vegetable oil
Solid Fuel Combustion: Burn solid fuels (e.g. wood, compressed pellets)
Efficiency: Range (50% wood burning - 98% new gas).
Advantages/Disadvantages: Exhaust gasses depend on fuel type, but all emit CO2 and other byproducts with negative environmental externalities.
Tarm BioMass Central Heating Fuel Burning Furnace. Image: Higgins Energy Alternatives.
3. Heat Pumps
3. Heat Pumps
Summary: The most common example in this category is the air conditioner. Basically, they transfer latent heat from a source by using a refrigerant and deliver it to a destination or heat sink. This process is difficult to understand, you might try this video for an explanation. Heat pumps can work in either direction, as both air conditioning and heating just by reversing the direction of the refrigerant.
Ground Source Heat Pumps (GSHP): Takes advantage of base ground temperature and can be used for heating and cooling.
Air Source Heat Pumps (ASHP): Ambient air temperature outside of building used as the source of heat, refrigerants and/or pumps extract heat from air, reversible air conditioners (AC) source of heat is warm air cooling only.
Efficiency: Much more efficient than combustion devices.
Advantages/Disadvantages: No exhaust gasses, environment impacts depend on source of electricity used to power system.
Ground Source Heat Pump. Image: Energy Company Numbers.
Air Source Heat Pumps. Image: Which?
4. Electric Resistance Heat
4. Electric Resistance Heat
Summary: Known as resistive, Joule, or ohmic heating, heat is produced when electrical current passes through a resistive conductor. The heat output is modified by adjusting the current, and the heat energy is transferred to the building via convection or infrared radiation.
Efficiency: 100% all potential energy becomes heat, but the source of power effects the overall system efficiency.
Advantages/Disadvantages: Environmental impacts depend on type of power generation.
A coiled heating element. Image: Wikipedia
Means of Heat Delivery
Means of Heat Delivery
Heat always moves from warm to cold, through three means:
Conduction: Heat transferred through direct contact
Convection: Heat moves warm object to air -> air to colder object. convection currents created by warm air (which is less dense) rising
Radiation: Heat transfer through electromagnetic waves
Heating systems always use a mix of the three, and are better defined by the medium of heat delivery.
Types of heat transfer. Image: Simscale
- Air Delivery
Summary: Using air to transfer heat.
Passive Air Delivery: Passive: natural convection only, no fans or ducts direct air.
Active Air Delivery (Forced Air): Use energy to force air in a desired direction, usually through ductwork.
Advantages/Disadvantages: Easy to change the temperature quickly energy required to move air is low, immediate comfort. Air loses heat quickly to denser objects, objects take a long time to heat up, because heat rises the warmest area is the ceiling and the coolest. The floor, ductwork and fans must be placed carefully in forced air systems. These systems move a lot of allergens (need filtration).
Passive v active cooling. Image: SimScale
2. Hydronic Delivery
2. Hydronic Delivery
Summary: Using a fluid to transfer heat. Water (the hydro in hydronic) is good at absorbing and releasing heat at building temperatures and can store a lot of heat. The term "radiant" is often used to describe these systems. Heat delivery is via a radiator with large surface area in floor, walls, and ceiling. Transfer fluid absorbs produced heat, which moved through pipes by a pump, released at points of delivery and finally recirculated to the heat source.
Advantages/Disadvantages: This system can take longer to deliver perceptible heat, with less frequent cycling than forced air systems. Systems can be simple to complex depending on the number of thermostats and zones.
Hydronic delivery. Image: Primex
Designing a Heating System
Designing a Heating System
Always start with the needs of the building. This is done through a heat loss calculation (here's an example online calculator). You may need this calculation for your building code. The calculation requires the dimensions of house components, R value, and temperatures for your area. Variables like insulation, windows, and the like can then be adjusted.
Example heat loss spreadsheet from nepwork.com
References
Chris Magwood. Making Better Buildings: A Comparative Guide to Sustainable Construction for Homeowners and Contractors. New Society Publishers.
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