HVAC

This category of equipment is by far the most prevalent. Regardless of the type of system and fuel used, all combustion devices burn a fuel and extract heat from the flame. All these devices rely on a supply of oxygen to react with the fuel and create the flame; an exhaust to allow spent combustion gasses to exit the unit and the building; and a heat exchanger that passes the heat from the flame to the delivery system that supplies heat to the home. There are two broad categories of combustion devices:

Gas/Liquid fuel combustion — These are the dominant players in the market, and include all the various forms of fossil fuel such as natural gas, propane and oil, as well as biofuels like biodiesel and vegetable oil.

Solid fuel combustion — This group includes wood-burning devices as well as devices that use other forms of biomass such as compressed pellets.

Combustion devices have efficiencies that range from 50 percent for some wood-burning models to 98 percent for some new gas-burning ones. This means that almost all the heat potential of the fuel is captured and used to supply the building.

Exhaust gasses from combustion devices differ depending on the fuel, the combustion efficiency and the conditions, but all release CO2 and a host of other by-products with environmental effects (see Ranking Fuel Sources sidebar).

This category of equipment is already widely used in the form of air conditioners, and has started to capture a larger portion of the heating market. Heat pumps use the refrigerant cycle to transfer latent heat from a source and deliver it to a destination or heat sink. How this works exactly can be difficult to understand, but it is worth figuring out the principle at play in order to decipher manufacturer claims. Heat pumps can seem like they magically make heat from no heat if the refrigerant cycle is unclear.

Mechanical energy (usually from an electric motor) is used to cycle a volatile refrigerant that is chemically designed to boil and condense in the expected operating temperature range of the heat pump. The refrigerant is in its liquid state when it absorbs latent heat from a source (air, ground or water for most residential purposes). This heat does not need to be in a temperature range that feels warm or hot to the touch, as the refrigerant’s chemical properties will ensure its boiling point is at or near to the source temperature. Once the refrigerant has passed through the heat collection exchanger, the electric compressor pressurizes the warmed refrigerant, which is at or close to its boiling point. The pressurization causes the refrigerant to become a hot vapor. This vapor passes through a heat exchanger where the heat generated from pressurization is dispersed. The refrigerant condenses as the heat is removed. Condensed refrigerant now passes through an expansion valve where the pressure is released; the refrigerant then returns to its liquid state, goes back to the exchanger and repeats the cycle until a thermostat indicates the proper room temperature has been reached.

It is the process of boiling and condensing a refrigerant under different pressures that creates the heat exchange. Useful heat exists in this boiling/condensing cycle, regardless of the actual boiling point of the refrigerant. As long as the heat collection side is above absolute zero, there will be heat to extract. Consider the home refrigerator: the freezer does not generate cold; rather, heat is extracted from the freezer and released via the heat coils on the back of the fridge.

The refrigerant cycle can happen in either direction, and some types of heat pumps are designed to work as both heating and air-conditioning units by reversing the direction of flow of the refrigerant.

There are three broad categories of heat pumps for residential use:

Ground source heat pumps (GSHP) — The source of heat is the stable temperature of the ground (below the frost line in cold climates). Base ground temperature is very reliable and steady, and the ground provides a large surface area and capacity for heat exchange. Most GSHPs are reversible and can provide heat and cooling.

Air source heat pumps (ASHP) — The source of heat is the ambient air temperature outside the building. As this temperature can be quite variable, different refrigerants and/or pumps are used to continuously extract useful heat from changing air temperature. Most ASHPs are reversible and can provide heat and cooling.

Air conditioners (AC) — The source of heat is the uncomfortably warm air that is affecting comfort. These units provide cooling only.

Efficiencies for heat pumps far exceed those of combustion devices. The amount of energy needed to compress the refrigerant is significantly lower than the amount of heat energy that is extracted from the process, and this is how manufacturers claim efficiencies ranging from 200 to 500 percent. But the heat is not “free” as some claim — for every one unit of electrical energy applied to the system, two to four units of heat are returned. The systems don’t work without the electric motors, but they are much more efficient than combustion devices.

While there is no combustion and therefore no exhaust gasses from heat pumps, environmental impacts will vary depending on the source of the electricity used to power the system. The refrigerants used can also be powerful greenhouse gasses, though stricter regulations are resulting in less damaging formulations.

Electrical current can be passed through a resistive conductor to produce heat. This is known as resistive, Joule or ohmic heating. The heat produced is proportional to the square of the current multiplied by the electrical resistance of the wire or element. The amount of current supplied can be adjusted to vary the heat output. Heat energy may be supplied through convection and/or infrared radiation, depending on the kind of heating element used.

Efficiency of electrical resistance heating is considered to be 100 percent, as all the potential energy in the current is converted to heat. However, many sources of electrical power are less than 100 percent efficient, so overall system performance must take into account the type of generation used to supply the electricity.

The type of generation will also determine the environmental impacts, which can range from high for coal-fired power plants (with delivered power efficiencies as low as 35 percent) to negligible for renewable energy streams like solar, wind and microhydro.

Wood- and pellet-fired boilers are the subject of much debate in regard to environmental impacts. Proponents tout the abundance and renewability of biomass fuel and the carbon neutrality of burning biomass, which is part of the regular carbon cycle of the planet regardless of whether it is burnt or decomposes naturally. Opponents point out that biomass may not be a feasible fuel source for a high percentage of the population as need could outstrip demand and the renewability cycle. Burning biomass contributes particulate and other polluting gasses to the air, even if it is carbon neutral.

All of these points are valid, and as with most options for sustainable building, there is no one right or wrong approach. Context is everything, and both choices can be environmentally sound in the right circumstances.

Wood-fired 

Boilers fired by cordwood or stick wood have traditionally been very inefficient, with fuel to heat conversion rates of 35–60 percent. New technologies, including gasification boilers, can raise efficiency into the 75–85 percent range, although often at increased cost and complexity.

There are inherent inefficiencies to cordwood burning that limit the upper range of efficiency. Stick wood fires take a relatively long time to reach an efficient ignition temperature, during which time they are releasing unburnt gasses and essentially wasting fuel potential and releasing toxic smoke. With an efficient fire in the burn chamber a wood boiler will likely be producing many more Btus than required for residential heating, but to avoid the hassle (and inefficiency) of having to put out and relight a fire to match demand needs, overproduction of heat is normal. Older wood boiler technology choked oxygen supply to the fire when heat demands were met, allowing the fire to smolder wastefully and dirtily until the damper was opened to reignite the flame. Properly sized storage can mitigate this issue, but is difficult to do accurately and requires careful monitoring and input by the homeowner.

Cordwood is not a feasible fuel for vast urban populations, but it is a reasonable and often ecologically sound choice in areas where wood lots can provide adequate fuel for a certain population in a completely sustainable way. In forested areas of Canada, “an area of ~3.5 ha of woodlot or forest would provide an indefinite supply” of firewood for a single family dwelling. The work involved in the collecting, splitting, stacking, fire-building and maintaining such systems limits the feasibility of this fuel source.

Cordwood fuel offers a degree of self-sufficiency and resilience to a certain population that is not available from any other fuel source, but one that would be short-lived if a high percentage of the population attempted to make use of it.

Pellet-fired

Compressed biomass pellets are a relatively new fuel source, especially in North America. A wide range of biomass stock, much of it currently waste material from the lumber and food industries, can be pelletized. High-yield grass crops as well as urban and agricultural waste can also be turned into pellets. The process is relatively low-energy, with an average input of energy required to manufacture and transport pellets that represents less than 2 percent of the energy content of the pellets. That compares very favorably with widely accepted estimates that crude oil requires an energy investment of 20–33 percent of its energy content (and more still to process into usable fuel stock).

The many potential biomass feedstocks make pellets a suitable fuel source in almost all regions. Pelletization can be accomplished on a micro- or macro-scale with equal effectiveness, encouraging the development of small-scale, local production close to feedstocks and end users. Estimates of the amount of feasible biomass stock are only just starting to be made and vary widely, but even the most conservative estimates show that biomass could practically become a significant percentage of our fuel economy.

Beyond their widespread availability, pellets have many advantages over solid wood fuel. Their density and moisture content can be easily regulated, allowing Btu output to be calculated and consistent. The compact form allows for efficient transportation, storage and movement.

Their burn characteristics are far superior to cordwood. The small size allows for fast ignition times and very complete combustion. Combined with automatic ignition and feeding systems, it is easy to modulate the output of a pellet-fuelled device, ensuring that only the required amount of heat is generated.

Pellet-burning devices require more maintenance than fossil fuel equivalents, as a certain amount of ash remains and must be removed and disposed of. New devices are making this a simpler, less frequent requirement, but it will always be necessary.

Pellet fuel is likely to play an increasingly prominent role in the North American energy market, and the technology and devices will see a steep improvement curve in the coming years.

Using a fluid (typically water) to transfer heat from a source to a destination is a strategy with a long history. Water has a high capacity for absorbing and releasing heat in the range of temperatures used in buildings, and its high density means it can store a lot of heat.

Hydronic heating systems are often called “radiant” heating systems, especially when the heat is delivered to an entire floor area, but this is not an accurate description. Floor heat is conducted through feet, and a very even and useful form of convection accompanies the radiant transfer of heat.

Hydronic heat delivery can be achieved via a radiator with a large surface area. Such radiators can be the floor, walls and/or ceilings of the building, or purpose-built radiator systems. Water-to-air systems use a radiator inside forced-air ductwork to create a hybrid system.

In all of these systems, produced heat is absorbed into the transfer fluid and moved through pipes by a mechanical pump. The fluid is delivered through branch pipes to the point(s) of delivery where the heat is released, before being recirculated to the heat source again.

Hydronic heating systems take longer to deliver perceptible heat than air systems, as the water requires more heat input to reach a perceptible temperature change. Once delivered to the radiator, the quantity of heat in the mass of the water and radiator creates much slower release times. The mass of air (and in some systems, even people) in the building will be lower than the heated mass, and all will rapidly be warmed to the radiator’s temperature without “draining” the radiator of its stored heat, resulting in longer but less frequent cycling of the heating system compared to air systems.

The design of hydronic systems needs to account for the surface area and distribution of the radiator(s), the length of piping and head, and the temperatures required to provide comfort based on the radiator layout. Systems can be quite simple, with a single pump and just a few radiator loops, or complex, with multiple pumps and valves responding to individual thermostats in each radiator zone.