The concept of a heating system is very broad, and the term can be used to describe many different types of installations. Some heating systems use fuel combustion as a heat source, while others are powered by electricity. Another difference is the way in which heat reaches indoor spaces; while some systems heat indoor air directly, others use an intermediate fluid like water or steam.

The most common types of heating equipment are boilers, furnaces, space heaters and heat pumps. Like in any engineering decision, each option has advantages and limitations.

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Boilers

Boilers heat a fluid that circulates through piping, and heat is then transferred from the fluid to indoor air with heat exchangers. The fluid is normally hot water or steam - hot water delivers heat through fan coils, and steam delivers it with radiators.

In modern applications, hot water is considered the most cost-effective heating fluid for boilers. Steam heating is viable when a building uses steam for other purposes beyond heating, such as manufacturing processes. When a boiler will be used only for space heating, mechanical engineers will normally recommend hot water distribution.

Boilers can also be classified based on the fuels they use. Fuels differ in terms of their cost and emissions, and the following are some examples:

• Natural gas is a common and cost-effective fuel for boilers. Since it is delivered as a utility service, there is no need to schedule fuel deliveries by truck.
• Heating oil is another common option in the US Northeast. It is also a cost-effective fuel, but has higher emissions than natural gas. Since heating oil must be delivered by truck, building owners must schedule deliveries to ensure they don’t run out.
• Biomass is a viable fuel for boilers when the owner has access to large amounts of organic waste that can be processed into biofuels. Biomass heating can be very expensive when biogas or biodiesel must be purchased from an external supplier.
• Propane is also used as a fuel for boilers. It must be delivered by truck like heating oil, but it operates with a higher efficiency and lower emissions.

There are electric resistance boilers as well, but their operating cost can be very high, especially in places with expensive electricity. If you are considering an electric heating option to avoid fuel consumption, a geothermal heat pump can save over 70% compared with a resistance boiler, and an air-source heat pump can save over 40%.

Furnaces

Furnaces operate with the same energy sources as boilers, and the two equipment types are often confused as a result. The main difference is that furnaces heat indoor air directly, while boilers deliver indirect heating with hot water or steam.

Furnaces are simpler and less expensive to install than boilers, but consider that heat is delivered with forced airflow only. Water is a more effective medium than warm air for vertical distances, and hydronic piping is more compact than air ducts. Furnaces can also be noisy, since they require powerful fans to establish the airflow required.

In general, furnaces are more affordable than boilers, and can be used in buildings with an adequate layout for the corresponding air ducts. When the building involves vertical distances and zoned heating, boilers are the recommended option.

Space Heaters

While boilers and furnaces are used in central heating systems, space heaters are a common option when building areas need independent heating. Space heaters operate like furnaces with a reduced scale, since they heat indoor air directly with fuel combustion or electric resistance.

• Electric space heaters are convenient and mobile because they use the building’s power supply. Space heaters have a low upfront cost, but a high operating cost.
• Gas-fired space heaters are less expensive to operate, but they require a gas supply. They must be properly vented, since enclosed combustion produces harmful substances for humans - carbon monoxide in particular can be lethal.

Heat Pumps

Heat pumps are modern devices that can achieve low operating costs like combustion heaters, while having clean operation like resistance heaters. A heat pump uses a refrigeration cycle, just like an air conditioner, but the heat movement direction is reversed:

• In an heat pump, the refrigerant is expanded and evaporated to gather heat from outside the building, and then compressed and condensed to release the heat inside.
• For comparison, an air conditioner gathers indoor heat with refrigerant expansion and evaporation, to release it outside through compression and condensation.

Heat pumps can be described as air-source or ground-source units, depending on the source from which they extract heat. While air-source heat pumps (ASHP) are the most affordable, ground-source heat pumps (GSHP) are the most efficient. Nevertheless, both heat pump types are much more efficient than resistance heaters.

• Air-source heat pumps are more affordable, but their effectiveness is reduced when outdoor temperatures reach the freezing range. The best models go as low as -20°C.
• Ground-source heat pumps do not have this limitation, since ground temperature varies less than air temperature throughout the year. However, they are more expensive.

Many heat pump models are designed for reversible operation, which means they can be used as air conditioners during summer. This way, you are consolidating two mechanical systems in a single installation. However, it is important to ensure that the heat pump is reversible; some models only have a heating mode and cannot be used as air conditioners.

Some heat pump models are designed to heat the air directly, just like furnaces, while others use hot water, just like boilers. Heat pumps are also available in compact versions, with a similar appearance to mini-split air conditioners.

Conclusion

There are many types of heating systems, and the recommended equipment changes depending on the budget available and building needs. Furnaces and boilers are the conventional options, while heat pumps are emerging as a viable heating solution.

In general, combustion heating achieves a lower operating cost than electric resistance heating, but heat pumps can often match the heating cost of fuels. When comparing furnaces and boilers, furnaces are normally the more affordable option, but boilers can adapt to more applications.