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How does heating work?

Author: CC

Mar. 07, 2024

116 0 0


Central Heat


The majority of North American households depend on a central furnace to provide heat. A furnace works by blowing heated air through ducts that deliver the warm air to rooms throughout the house via air registers or grills. This type of heating system is called a ducted warm-air or forced warm-air distribution system. It can be powered by electricity, natural gas, or fuel oil.

Inside a gas- or oil-fired furnace, the fuel is mixed with air and burned. The flames heat a metal heat exchanger where the heat is transferred to air. Air is pushed through the heat exchanger by the “air handler’s” furnace fan and then forced through the ductwork downstream of the heat exchanger. At the furnace, combustion products are vented out of the building through a flue pipe. Older “atmospheric” furnaces vented directly to the atmosphere, and wasted about 30% of the fuel energy just to keep the exhaust hot enough to safely rise through the chimney. Current minimum-efficiency furnaces reduce this waste substantially by using an “inducer” fan to pull the exhaust gases through the heat exchanger and induce draft in the chimney. “Condensing” furnaces are designed to reclaim much of this escaping heat by cooling exhaust gases well below 140°F, where water vapor in the exhaust condenses into water. This is the primary feature of a high-efficiency furnace (or boiler). These typically vent through a sidewall with a plastic pipe.

New furnace standards are currently under development by the U.S. Department of Energy, and are due to be finalized in the spring of 2016. The current furnace standards have not been updated since 1987.

Heating system controls regulate when the various components of the heating system turn on and off. The most important control from your standpoint is the thermostat, which turns the system — or at least the distribution system — on and off to keep you comfortable. A typical forced air system will have a single thermostat. But, there are other internal controls in a heating system, such as “high limit” switches that are part of an invisible but critical set of safety controls.

The best gas furnaces and boilers today have efficiencies over 90%

The efficiency of a fossil-fuel furnace or boiler is a measure of the amount of useful heat produced per unit of input energy (fuel). Combustion efficiency is the simplest measure; it is just the system’s efficiency while it is running. Combustion efficiency is like the miles per gallon your car gets cruising along at 55 miles per hour on the highway.

In the U.S., furnace efficiency is regulated by minimum AFUE (Annual Fuel Utilization Efficiency). AFUE estimates seasonal efficiency, averaging peak and part-load situations. AFUE accounts for start-up, cool-down, and other operating losses that occur in real operating conditions, and includes an estimate of electricity used by the air handler, inducer fan, and controls. AFUE is like your car mileage between fill-ups, including both highway driving and stop-and-go traffic. The higher the AFUE, the more efficient the furnace or boiler.


Boilers are special-purpose water heaters. While furnaces carry heat in warm air, boiler systems distribute the heat in hot water, which gives up heat as it passes through radiators or other devices in rooms throughout the house. The cooler water then returns to the boiler to be reheated. Hot water systems are often called hydronic systems. Residential boilers generally use natural gas or heating oil for fuel.

In steam boilers, which are much less common in homes today, the water is boiled and steam carries heat through the house, condensing to water in the radiators as it cools. Oil and natural gas are commonly used.

Instead of a fan and duct system, a boiler uses a pump to circulate hot water through pipes to radiators. Some hot water systems circulate water through plastic tubing in the floor, a system called radiant floor heating (see “State of the Art Heating”). Important boiler controls include thermostats, aquastats, and valves that regulate circulation and water temperature. Although the cost is not trivial, it is generally much easier to install “zone” thermostats and controls for individual rooms with a hydronic system than with forced air. Some controls are standard features in new boilers, while others can be added on to save energy (see the “Modifications by Heating System Technicians” section on the heating maintenance page).

As with furnaces, condensing gas-fired boilers are relatively common, and significantly more efficient than non-condensing boilers (unless very sophisticated controls are employed). Oil-fired condensing boilers are uncommon in the U.S. for several reasons related to lower latent heat potential, and potential for greater fouling with conventional fuel oil.

Heat Pumps

Heat pumps are just two-way air conditioners (see detailed description in the cooling systems section). During the summer, an air conditioner works by moving heat from the relatively cool indoors to the relatively warm outside. In winter, the heat pump reverses this trick, scavenging heat from the cold outdoors with the help of an electrical system, and discharging that heat inside the house. Almost all heat pumps use forced warm-air delivery systems to move heated air throughout the house.

A ground-source heat pump heats and cools in any climate by exchanging heat with the ground, which has a more constant temperature.

There are two relatively common types of heat pumps. Air-source heat pumps use the outside air as the heat source in winter and heat sink in summer. Ground-source (also called geothermal, GeoExchange, or GX) heat pumps get their heat from underground, where temperatures are more constant year-round. Air-source heat pumps are far more common than ground-source heat pumps because they are cheaper and easier to install. Ground-source heat pumps, however, are much more efficient, and are frequently chosen by consumers who plan to remain in the same house for a long time, or have a strong desire to live more sustainably. How to determine whether a heat pump makes sense in your climate is discussed further under “Fuel Options.”

Whereas an air-source heat pump is installed much like a central air conditioner, ground-source heat pumps require that a “loop” be buried in the ground, usually in long, shallow (3–6' deep) trenches or in one or more vertical boreholes. The particular method used will depend on the experience of the installer, the size of your lot, the subsoil, and the landscape. Alternatively, some systems draw in groundwater and pass it through the heat exchanger instead of using a refrigerant. The groundwater is then returned to the aquifer.

Because electricity in a heat pump is used to move heat rather than to generate it, the heat pump can deliver more energy than it consumes. The ratio of delivered heating energy to consumed energy is called the coefficient of performance, or COP, with typical values ranging from 1.5 to 3.5. This is a “steady-state” measure and not directly comparable to the heating season performance factor (HSPF), a seasonal measure mandated for rating the heating efficiency of air-source heat pumps. Converting between the measures is not straightforward, but ground-source units are generally more efficient than air-source heat pumps.

Direct Heat

Gas-Fired Space Heaters

In some areas, gas-fired direct heating equipment is popular. This includes wall-mounted, free-standing, and floor furnaces, all characterized by their lack of ductwork and relatively small heat output. Because they lack ducts, they are most useful for warming a single room. If heating several rooms is required, either the doors between rooms must be left open or another heating method is necessary. Better models use “sealed combustion air” systems, with pipes installed through the wall to both provide combustion air and carry off the combustion products. These units can provide acceptable performance, particularly for cabins and other buildings where large temperature differences between bedrooms and main rooms are acceptable. The models can be fired with natural gas or propane, and some burn kerosene.

Unvented Gas-Fired Heaters: A Bad Idea

Gas or kerosene space heaters that do not have an exhaust vent have been sold for decades, but we strongly discourage their use for health and safety reasons. Known as “vent-free” gas heating appliances by manufacturers, they include wall-mounted and free-standing heaters as well as open-flame gas fireplaces with ceramic logs that are not actually connected to a chimney. Manufacturers claim that because the products’ combustion efficiency is very high, they are safe for building occupants. However, this claim is only valid if you keep a nearby window open for adequate fresh air— which defeats the purpose of supplemental heat. Dangers include exposure to combustion by-products, as discussed in Ventilation, and oxygen depletion (these heaters must be equipped with oxygen depletion sensors). Because of these hazards, at least five states (California, Minnesota, Massachusetts, Montana, and Alaska) prohibit their use in homes, and many cities in the United States and Canada have banned them as well.

Electric Space Heaters

Portable (plug-in) electric heaters are inexpensive to buy, but costly to use. These resistive heaters include “oil-filled” and “quartz-infrared” heaters. They convert electric current from the wall socket directly into heat, like a toaster or clothes iron. As explained further under “Selecting a New System,” it takes a lot of electricity to deliver the same amount of useful heat that natural gas or oil can provide onsite. A 1,500- watt plug-in heater will use almost the entire capacity of a 15-amp branch circuit; thus, adding much additional load will trip the circuit breaker or blow the fuse. The cost to operate a 1,500-watt unit for an hour is simple to compute: it is 1.5 times your electricity cost in cents per kilowatt-hour. At national average rates—12¢ kWh for electricity— that heater would cost 18¢ per hour to run—and quickly cost more than its purchase price. On the other hand, for intermittent use, it is the “least-bad” solution when alternatives would require major investments to improve ductwork for a specific area, for example. Just remember, electric resistance heat is usually the most expensive form of heat, and it is, therefore, seldom recommended.

“Electric baseboard heat” is yet another kind of resistive heating, similar to a plug-in space heater except that it is hard-wired. It has two principal virtues: the installation cost is low, and it is easy to install individual room thermostats so you can turn down the heat in rooms that aren’t being used. Operating costs, as for all resistive systems, are generally very high, unless the house is “super-insulated.”

Wood-Burning and Pellet Stoves

Wood heating can make a great deal of sense in rural areas if you enjoy stacking wood and stoking the stove or furnace. Wood prices are generally lower than gas, oil, or electricity. If you cut your own wood, the savings can be large. Pollutants from wood burning have been a problem in some parts of the country, causing the U.S. Environmental Protection Agency (EPA) to implement regulations that govern pollution emissions from wood stoves. As a result, new models are quite clean-burning. Pellet stoves offer a number of advantages over wood stoves. They are less polluting than wood stoves and offer users greater convenience, temperature control, and indoor air quality.


Gas (and most wood) fireplaces are basically part of a room’s décor, providing a warm glow (and a way to dispose of secret documents), but typically not an effective heat source. With customary installations that rely on air drawn from the room into the fireplace for combustion and dilution, the fireplace will generally lose more heat than it provides, because so much warm air is drawn through the unit and must be replaced by cold outside air. On the other hand, if the fireplace is provided with a tight-sealing glass door, a source of outside air, and a good chimney damper, it can provide useful heat.

State of the Art Heating

Radiant floor heat generally refers to systems that circulate warm water in tubes under the floor. This warms the floor, which in turn warms people using the room. It is highly controllable, considered efficient by its advocates, and is expensive to install. It also requires a very experienced system designer and installer, and limits carpet choices and other floor finishes: you don’t want to “blanket” your heat source.

Contact the Radiant Panel Association

Ductless, Mini-Split, Multi-Split. Residential ductwork is relatively rare outside North America. “Ductless” heat pumps, which distribute energy through refrigerant lines instead of water or air, are widely used. Large field trials in the Pacific Northwest suggest that they can have good cold weather performance, and be very cost-effective where replacing electric resistance heating. Like ground-source systems, relative immaturity of the market helps assure that whole-house multi-split systems carry premium prices.

Combined heat and power (CHP) or cogeneration for houses is being seriously studied in some countries. The basic premise is to use a small generator to meet some of the electric demand of the house, and recover the waste heat (typically more than 70% of the heating value of the fuel) to heat the house (hydronic or water-to-air systems) and make domestic hot water. These systems are not yet widely available. They are likely to have the best economics in houses with high heating bills because the house cannot be feasibly insulated, such as solid stone or brick homes.


How Does My Home Heating Work?

No matter where you live in North America, there's a good chance that you're going to need some kind of home heating to keep your home comfortable. Now, while some folks in southern-most Texas may beg to differ, it's no surprise that practically every home from Houston to Dallas has a furnace. Of course, not every region in the U.S. favors the same type of heating system. For example, natural gas or propane heat is favored for most of the Midwest while many Northern cities still retain boilers and radiators.

The Two Basic Types of Heat Systems

Because we know our customers save more money by understanding how they use energy, we've put together this basic guide to the different types of home heating systems so that homeowners can learn about the kind of heating system they have and how it works.

Basically, there are two kinds of heating systems, forced air and radiant.

Forced air systems use a blower or fan to pull air into the system where it is heated and circulated throughout the home. They can be noisy with metallic squeaks from loose connections and require routine air filter changes to keep them running well. But because they heat the air, they tend to warm up homes quickly. A forced air system is composed of air return ductwork, a blower, a heating or cooling unit with heat exchangers housed inside the air handler cabinet, a plenum where the air exits the air handler, and supply ductwork. The supply ductwork carries the air to all the rooms of the home while the return ductwork carries all the air from the rooms back to the blower and air handler.

Radiant systems rely on the use of heat to move air through convection. That is, heated air rises and is replaced by cooler air, which is heated and rises and so on. Because these systems work passively, they tend to heat rooms slowly. Plus, they don't filter dust or allergens from the air, and in some cases, are not as energy efficient. However, most are inexpensive to buy, install, and maintain.

Forced Air Systems

Electric resistance heating is 100% energy efficient because all the electricity is converted to heat. And since the heating elements in an electric furnace are in direct contact with the air, the air heats up very quickly. This makes them very efficient but expensive to operate during extended cold weather.

Other Types of Forced Air Systems

  • In natural gas/propane systems, you want the heat to from a series of flames to heat the air but you also want to send the combustion exhaust out of the house. Gas burns on several long ribbon burners (12" to 18" long). The heat exchanger resembles a tall, hollow hair comb that surrounds each ribbon burner on three sides. The hot gases move up into the heat exchanger and eventually exhaust out into the vent pipe. On highly efficient condensing gas furnaces, so much heat is wrung out of the exhaust that it won't rise well enough to leave. This is why the system uses a fan to blow the exhaust outside.
  • Heat pumps include both air condensers that are installed outside and geothermal condensers that are buried underground or in nearby water. Both use R-410A refrigerant that is also used in air conditioning, but the process runs in reverse so that instead of warm air being expelled outside as waste heat, it's blown inside to supply heat. Heat pumps use compressor/condensing systems similar to regular air conditioning systems except that they are reversible. Geothermal systems do basically the same thing, but instead of relying on the ambient air temperature to make heat, they use the temperature of subsurface ground which stays around 50°F, making it very reliable and energy efficient.
  • Ductless heat pump/AC systems are forced air systems that don't use ductwork. Sometimes called "split systems", the interior air handling and heat exchanging unit is directly connected to through the exterior wall to the outside unit. Because of their small size and the fact that they don't use ductwork, split systems like these are best suited for heating and cooling small areas.

Cost of Forced Air Systems

The installation costs of these systems vary significantly depending on the circumstances. Electric resistance, natural gas and propane systems are often the most affordable option in new construction and in homes with existing duct networks. But if you need to install a duct system in an existing home, the cost of the ductwork may be several times the cost of the heater itself.

The installation and equipment costs of air heat pumps are often twice as high or higher than the costs of installing electric, gas or propane furnaces. And geothermal heat pumps are typically the most expensive to install, with installation costs dependent on the complexity of the installation of underground piping. Some geothermal systems can cost more than $20,000 including equipment and installation.

Ductless systems may be the cheapest option if you're only heating one room, but because you need a separate system for each heated space, the installation and equipment cost increases with the number of systems.

As for operating costs, geothermal heat pumps are the most affordable, followed by air heat pumps and ductless systems. Among electric resistance, natural gas and propane systems, natural gas is typically the most affordable, but the operating costs are tied to the fluctuating costs of their energy sources: electricity, natural gas and propane.

Maintenance of Forced Air Systems

Airflow is the lifeblood of any forced air system, and the chokepoint for that airflow is the filter. Every forced air system has a filter that must be replaced or cleaned on a certain schedule, and neglecting to perform this maintenance on time can result in higher operating costs and increased system wear.

Any system with an outdoor condenser installed at ground level (this excludes geothermal heat pumps and ductless systems) requires some additional DIY maintenance. The condenser should be kept free of weeds and debris and should occasionally be gently hosed off to clear away dirt.

All other maintenance should be performed by a licensed HVAC professional once per year, ideally before the start of heating season. This maintenance should be performed every year, no matter how well the system is performing. Annual maintenance extends the life of the system, optimizes energy efficiency and ensures the system's safety.

Radiant Systems

In the case of all heat radiating systems, they work silently and don't buffet a room's occupants with blasts of hot air. However, they tend to work more slowly to warm up a room when compared to forced air systems. This is largely due to the fact that they rely on convection to heat the air and make it circulate through the room. That said, some types of radiant systems work faster than others.

Radiant heat can be more efficient than forced air systems with duct loss problems, and some people with allergies prefer it because the lack of air circulation doesn't stir up allergens. However, because these systems circulate water as either steam or liquid, radiator systems can be prone to problems such as blockages and leaks.

Types of Radiant Systems

  • Passive solar is the greenest and least expensive to operate because the sun's heat is stored in the thermal mass of the home. The sun's heat radiates and warms the space. However, your home needs to be very well air sealed and insulated plus have adequate southern exposure to allow enough sunshine to enter the windows and warm the home. Unfortunately, the further north you go, the more expensive it can be to build a passive solar home and you may well need to have backup heating during cold snaps.
  • Boiler-based systems include radiant floor heat that uses hot water, old-fashioned radiators that use steam or hot water, and also some hydronic (liquid-based) baseboard systems (see below). In these systems, a central boiler heats the water (or other liquid) to either steam or hot water and pumps it through pipes throughout the home to radiators or coils of tubing embedded in walls or flooring.
  • Radiant floor heat makes use of the floor's thermal mass. When you take hot water tubing and arrange it in loops on the floor and then a surround it with poured concrete (wet installation) or a sandwich of tile and plywood (dry installation), the floor will stay warm longer and radiate the heat longer, which keeps the room warm longer and more evenly. The bigger the floor space, the more of it can be heated and the more heat it will store.

These heating systems work best when they are mounted at least 3/4" above the floor or carpet. This allows cooler air on the floor to flow through the heater fins and be heated. One drawback is that fur from shedding pets can get pulled into these kinds of heaters and block airflow.

  • Electric baseboard heaters (that use electric resistance heating elements) are mostly zonal heaters so each is controlled by a built-in thermostat. Available in lengths from 3 to 6 feet, each foot draws about 250 watts. Electric baseboard heaters tend to be the least expensive, easiest to install heating system. They only need to be wired up (with either 120 or 240 wiring) and fastened to the wall.
  • Hydronic (liquid-based) baseboard systems use water or oil instead of electric resistance and tend to be a bit more expensive. In electric resistance systems, once the current shuts off, the heating element goes cold in just a few minutes. But in hydronic systems, once the liquid is hot, it stays hot longer —making them a little more efficient to operate than electric resistance baseboards. Hydronic systems can be installed as individual units or as a single whole-house system that uses a single heater, much like a radiant or radiator system.

Cost of Radiant Systems

The installation costs of radiant systems tend to be even more difficult to estimate than those of forced air systems. With passive solar heating, for example, the heating elements are integral to the construction of the home and could add anywhere from a few thousand to several tens of thousands of dollars to the total cost of designing and building new construction.

With boiler-based systems, the cost of boiler installation ranges from a few thousand dollars (comparable to electric, gas or propane furnaces) for smaller boilers into five-figure sums for larger ones. If radiators, hydronic baseboards or floor heat piping must be installed, the cost is tied directly to the number of units or square feet of flooring. So the cost of both the boiler and the heat distribution equipment increases with the size of the home.

Similarly, the cost of electric radiant floor heat installation usually boils down to a price per square foot, so the total cost depends on the size of the home.

In the typical home, the operating costs of these radiant systems tend to be lower than those of electric, gas and propane furnaces but higher than those of heat pumps. As with installation, however, this can vary along with the size of the home. Electric radiant floor heating is expensive to operate, for example. In a very small home, this may be cheaper overall than installing a furnace and ductwork or a series of mini-split systems. But in a huge home, heating entirely with electric radiant flooring could be a costly mistake.

Maintenance of Radiant Systems

Compared to forced air systems, radiant systems are much easier and usually cheaper to maintain. An annual boiler inspection and tune-up is typically the only routine maintenance cost associated with boiler-based systems.

Passive solar homes are maintenance-free on the inside, but can require ordinary outdoor maintenance like clearing gutters, trimming trees and washing windows to ensure sufficient exposure to the sun's warming rays. Electric radiant flooring is also essentially maintenance-free; unless the system isn't performing as expected, you may only want to schedule an electrical inspection every several years, as you would with the other electrical infrastructure of your home.

Electric baseboard heaters do require routine cleaning of vents, especially if your home is dusty or you have pets.

Which Type of Heating System Is Best?

It really depends on how your home is built, what you can afford, and what you prefer. For example, if you are building an addition or are modernizing your home's HVAC, you may find that it might not be feasible to run new ductwork to different parts of your home. In which case, you might need to consider some type of baseboard system coupled with a ductless mini system for summertime cooling. And while it has been argued that forced air systems do stir up allergens, when they are equipped with HEPA air filtration, they do a much more efficient job of purging allergens from the air throughout the entire home. If it's a matter of energy efficiency but passive solar is not a practical choice, the most efficient is a geothermal heat pump followed by its cousin, the air heat pump. While these are very effective heating systems, during events like cold snaps they require heating backups — usually in the form of built-in auxiliary electric resistance heating elements.

If you're thinking about updating your home's heating system or even just need some maintenance, it's best to contact a skilled professional like One Hour Air Conditioning and Heating for expert service.

Keep your home and wallet comfortable with an energy plan from Direct Energy. We have the tools and tips you need to track your appliances' usage to save energy and money.

How does heating work?

How Does My Home Heating Work?




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