Residential Heating System Basics
The ability to employ mechanical systems to automatically modulate the temperature (and often humidity levels) of our homes is a radical change from the previous centuries of human habitation. Our heating and cooling systems are often complex and high performance devices that give us fingertip control over indoor climate that would have been unthinkable less than a century ago. Until quite recently, the devices we used to achieve stable temperatures were functional but quite inefficient, using large quantities of fuel to meet our thermostat settings. A lot of development has gone into increasing efficiency, and in many cases this has come with increased complexity and cost. The following residential heating systems basics will help you understand your options.
Though most heating devices are intricate systems, it is quite easy to understand the basic technology behind each of them. It is worthwhile as a homeowner to understand these systems, and not leave it to company reps or installers to provide selling points.
It is easiest to think of heating and cooling systems as falling into categories of means of heat production and means of heat delivery. From this understanding, it is possible to narrow down the pool of options to those that suit the needs of a project.
Means of Heat Production
Despite all the competing products in the heating and cooling market, there are just four kinds of heat production. Details for each system are provided individually later in the chapter.
1) Solar Heat
In effect, all sources of heat are based on solar energy, as the fuels used in every heating system are the result of captured and stored solar energy. However, this classification of heating systems is based on direct harvesting of solar energy in real time. Heat from the sun can be collected (and sometimes concentrated) in, on or near the building and distributed for use throughout the building.
There are three basic types of solar heat, which may be used in any combination.
Passive Solar – A building may be designed with sufficient glazing on the sunny side of the building to allow for a measurable increase of indoor temperature when the sun is shining.
Active Solar Air – Collector units are used to gather and concentrate the sun’s heat in a flow of air that is supplied to a heat exchanger or directly to the building.
Active Solar Water – Collector units are used to gather and concentrate the sun’s heat in a flow of liquid that is supplied to a heat exchanger or directly to the building.
This category of heating devices does not include photovoltaic cells, which use solar energy to generate electrical current, and not directly to produce heat. Heat created by solar electric current is considered in the category of electric resistance heating.
Solar energy systems may appear to have low efficiency rates, with figures ranging from 10-70%, depending on ambient temperatures and type of collector, among other factors. These figures represent the percentage of available potential energy from the sun: approximately 1000 Watts per meter squared (W/m2) for a surface perpendicular to the sun’s rays at sea level on a clear day. A reduced figure of 800 W/m2 is often used in generating comparative figures for solar devices. While it is beneficial to increase efficiency rates to produce more heat from less collector area, the efficiency rates aren’t directly comparable to those of combustion devices as no sunlight is actually “wasted” and no harmful byproducts are generated by the solar energy that is not absorbed by the collector.
Solar heating systems do not generate emissions, fuel extraction and transportation impacts or air pollution, with the exception of those systems that use non-solar energy to drive small pumps or fans.
2) Combustion Devices
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 bio-fuels like biodiesel and vegetable oil.
Solid Fuel Combustion – This group includes wood burning devices, as well as those that use other forms of biomass such as compressed pellets.
Combustion devices have efficiencies that range from 50% for some wood burning devices to 98% for some new gas burning devices. This means that less than all of the heat potential of the fuel is captured and used to supply the building.
Exhaust gasses from combustion devices differ depending on the fuel used and the combustion efficiency and conditions, but all devices release CO2 and a host of other byproducts with environmental effects.
3) Heat Pumps
This category of equipment is widely used in the form of air conditioners, and has started to capture a larger portion of the heating market. These systems use the refrigerant cycle to transfer latent heat from a source and deliver it to a destination or heat sink. It can be difficult to understand exactly how a heat pump works, but it is worth figuring out the principle at work 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 in the form of 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 vapour. This hot vapour 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 and the refrigerant returns to its liquid state and returns again 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 heat collection side is above absolute zero, there will be heat to extract. Consider the home refrigerator: cold is not being generated in the freezer, rather heat is being 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 input to provide compression of the refrigerant is significantly lower than the amount of heat energy that is extracted from the process and this how manufacturers claim efficiencies ranging from 200-500%. The heat is not “free” as some claim, but for every one unit of electrical energy applied to the system, 2-4 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 electricity used to power the system. The refrigerants used can also be powerful greenhouse gasses, though stricter regulations are resulting in less damaging formulations.
4) Electric Resistance Heat
Electrical current can be passed through a resistive conductor to produce heat. This type of heat production 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%, as all the potential energy in the current is converted to heat. However, many sources of electrical power are less than 100% 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%) to negligible for renewable energy streams like solar, wind and microhydro.
Means of Heat Delivery
Heat flow always moves from a warmer object to a colder one. Heat flow can occur in three ways:
Conduction – Heat energy is transferred from a warmer object to a colder one by direct contact.
Convection – Heat energy is transferred from a warmer object to the air surrounding the object and then to cooler objects in contact with the warmed air. Warmed air become less dense and rises, creating convection currents that affect objects in the path of the current.
Radiation – Heat energy is transferred from a warmer object to a cooler object by electromagnetic waves, caused by energy released by excited atoms.
These neat categorizations do not adequately describe heat delivery systems in buildings, as any heating system will warm a space in all three ways to varying degrees. Consider the element on an electric range: anything that touches the hot element will be heated by conduction. At the same time, air near the element will be warmed, become buoyant and warm objects near the range by convection and the heated element will radiate heat to nearby cooler objects, warming the surface of the range and nearby utensils that are not touching the element nor in the path of heated air.
Certain heating systems will rely on one of these methods of heat flow more than others, but cannot be categorized by a particular kind of heat flow.
Instead, it is more useful to consider heating systems in regard to the medium of delivery, of which there are two. Almost any kind of heat production can be twinned with any kind of delivery system.
1) Air Delivery
Passive Air Delivery – These systems rely only on natural convection currents to move heat from a source to the desired locations in the building. No fans or ducts are used to direct warmed air.
Active Air Delivery – These systems use some form of mechanical energy to force air movement in a desired direction. Ductwork is often used to deliver a concentrated stream of warmed air to a particular location.
Air can be an effective medium for delivering heat in some circumstances. It is not very dense, and it is therefore easy to change the temperature of a large volume of air quite quickly. The energy required to move air from one location to another is low, as it flows and changes direction easily and large volumes can be moved quickly. Natural convection loops of rising warmed air and falling cool air can be exploited to good effect to contribute some or all of the required flow.
Occupants in buildings will feel heated air against their skin and have an immediate awareness of warmth and perceived comfort.
These advantages of air as a heating medium are also the disadvantages. The low density of air means that it loses its heat very quickly to more dense objects; raising the temperature of objects in a building via air flow can take a long time, and often objects and the surfaces of the building remain significantly cooler than the ambient air temperature. This can lead to discomfort as the warm occupants in the home will unwittingly be trying to heat the building’s surfaces radiantly, one of the reasons it’s possible to feel a “chill” even in a warm building.
The convection loops associated with air delivery ensure that the warmest area of the building is at the ceiling and the coolest is at the floor. Since occupants reside on the lower side of this balance, some heat is “wasted” by being concentrated outside the contact zone for occupants. Convection loops can also cause cool air to move against occupants in some areas of the building, causing the feeling of chilly drafts even in an air tight building.
In forced air systems with ductwork and fans, the layout must be done carefully if it is going to be efficient. Limiting the number of bends and restrictions increases flow, and proper positioning of outlets can reduce unwanted convection loops and create a relatively even distribution of heat.
Air heating systems will move a significant amount of dust and allergens as it flows. In forced air systems, inline filtration is highly recommended. For natural convection systems, an active filtration system is worth considering.
2) Hydronic Delivery
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 that has a lot of surface area. 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 through the action of 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 will take a longer time to deliver perceptible heat, 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 they can be complex with multiple pumps and valves responding to individual thermostats in each radiator zone.
Heating System Design: Heat Loss Calculation
Regardless of type of heat production and means of delivery, the design of an effective heating system starts with an accurate assessment of the expected heating needs for the building. This is achieve through the completion of a heat loss calculation. Many building codes now require such a calculation, and even if it is not a legal requirement it is recommended. Oversized or undersized heating systems are not efficient, and will cost a lot more than the calculation.
There are free, simple spreadsheets that can be used for heat loss calculations. These involve gathering dimensions for wall, floor, ceiling, window and door surface areas from the building plans and assigning each its expected heat loss rate (U-value or R-value). These figures are tallied and factored with the number of degree days and minimum anticipated temperature. The results include the hourly heat loss for the coldest expected day (expressed as BTU/hr) and total yearly heat loss (in millions of BTU).
More complex and comprehensive computer modeling programs will include more variables in the calculation, giving consideration to solar gain, thermal bridging in the building enclosure, anticipated air leakage rates and occupant behaviour among other factors. The more detailed the calculation, the more useful the resulting figures for sizing heating systems.
The hourly heat loss figure determines the maximum required output of the heating system, and the total yearly heat loss helps to anticipate energy requirements and costs. Figures from a good heat loss calculation also allow the design of the building to be tweaked for maximum efficiency at the design stage, as variables can be adjusted to determine ideal levels of insulation, window size and quality and air tightness.
With the parameters established by the results of the heat loss calculation, the particulars of the system can be designed to meet these needs in the most efficient and comfortable way.
This content is based on the book Making Better Buildings by Chris Magwood.