Ever since people moved away from the tropics and began living in cold climates, home heating has been a central part of life. Early civilizations used individual fireplaces, often connected to a bed or other part of the house. The Roman hypocaust system, which ducted furnace-heated air under floors and between wall is a well-known example of early technological advancement. After the Romans, Britain did not see central heating in homes until the 1900s. Even before the Romans, people figured out how to heat homes creatively, without exposing occupants to smoke from fires. The kang, an invention pioneered in China and Korea is a bed-sized stone or concrete block that has been hollowed out. Built into the side of a room, a kang is connected to a furnace at one end and a chimney that vents to the roof at the other end. Because heat from the fire warms the bed, but smoke is completely separated from the room. Even long after the fire dies down at night, the large mass of the stone bed radiates heat into the room.
Today, kangs still exist in rural China, but for modern homes, the idea has a few drawbacks. One is that because a kang is built into the room, it cannot be moved around. Modern families tend to rearrange furniture from time to time. Also, a concrete bed is much too hard for most people to feel comfortable sleeping on. While a modern mattress could theoretically be placed on top of a kang, this somewhat defeats the purpose of having the heated surface.
Modern heating methods fall into several main categories. Places that have extreme cold and mild summers often have only heating, which is not combined with an air conditioning system. Cold winters and hot summers require a cooling system and a separate heating system. Hot summers and mild winters necessitate air conditioning which is often combined with a heating system.
To heat a very cold place one of the most common methods is by burning natural gas. The heat can be delivered to the rooms in several ways. Underfloor heating is one method. A mixture of water and antifreeze is pumped from boiler through a network of lines that runs under the floor. The boiler is able to heat the room quite efficiently, as the occupants of the room are nearly in direct contact with the heating element at any given time. The boiler can operate at a comparatively low temperature, because of the large area from which the heat diffuses (the entire floor).
Other underfloor systems use electric elements to heat the floor. These are attractive because they can be used in a much more limited space, and are slightly easier to manipulate when installing, making them generally cheaper.
Drawbacks to an underfloor system are that it is generally dedicated to heating only. While cool fluid can be run through a hydronic system, this is not as effective at cooling a room as other methods. Electric underfloor heaters strictly heat, and cannot be used for cooling at all. So, these systems, while quite effective, are best for places that primarily need heating, but not cooling in summer.
Forced air is another popular heating method. A system of air ducts runs throughout the house, connecting each room with a central heating unit. The heating unit might warm the air via a gas furnace, or by using a heat pump. Forced-air gas heaters are common, especially in older homes that were built before air conditioning was widely available. This type is less fuel efficient than a heat pump, but has its own advantages.
First of all, the cost of replacing an older, but still functioning gas heater with a fuel-efficient electric heat pump often exceeds the benefits gained by running the newer system. Also, gas heaters work better in extreme cold than heat pumps. While a gas heater actually produces heat by combusting fuel, a heat pump simply moves heat from the outside into the house. The coils on the outside of the house can get very cold (it’s essentially air-conditioning the outdoors), and can freeze. When this happens, the unit will reverse for a few minutes, thawing the coils, but this loses its effectiveness when the temperature drops much below freezing. By contrast, a gas heater will function normally even in extreme cold. Also, since a gas heater is a dedicated heat system, the forced air vents are usually located near the floor, so that the heat can more effectively heat the room.
A heat pump is an ideal situation for cold to mild winters and hot summers. It heats winter homes more efficiently in cold weather, and can be reversed in summer to run as an air conditioning unit. In places that get cool but not often subfreezing, such as Phoenix, AZ a heat pump is ideal. Phoenix air conditioning units run double duty, since the extreme summer heat is combined with chilly winter nights. While freezing in Phoenix is rare (an average of 10 days per year), temperatures definitely dip low enough to warrant a central heating system.