Traditional Boiler and Radiator Systems

Forced Air Heating/Cooling Systems

By far the most common HVAC system in modern North American homes is the forced-air system that uses a furnace with a blower fan that delivers warmed air to the various rooms of the home through a network of ducts. Forced air systems are very quick at adjusting the temperature of a room, and because air conditioning systems can share the same blower and ductwork, this is an efficient overall HVAC System.

Fuel sources: The furnaces that power forced-air systems can be fueled by natural gas, liquid propane (LP), fuel oil, or electricity.

Distribution: Air that is warmed in by the furnace's burner or heating element air is distributed through a network of ducts to heating registers in individual rooms. Another system of ducts returns air back to the furnace through cold-air returns.

Advantages:

Forced air systems can be filtered to remove dust and allergens. However, they also can increase the amount of allergens that are airborne.
Humidifier (or dehumidifier) equipment can be integrated into the forced air system.
Forced air furnaces are relatively inexpensive.
These furnaces can achieve the highest AFUE (Annual Fuel Utilization Efficiency) ratings of any heating system (but that does not necessarily mean this is the most efficient way to heat a home).
Forced air systems can combine cooling with heating capability.

Disadvantages:

Requires ductwork and takes up space in walls.
Furnace fans can be noisy.
Moving air can distribute allergens.
Moving air can become dry unless it is humidified.
Because forced air systems heat the air and not the objects in a room, it is not considered the most comfortable form of heating.

Gravity Air Furnace Systems

A precursor to forced air systems, gravity air furnaces also distribute air through a system of metal ducts, but rather than forcing the air via a blower, gravity air systems operate by the simple physics of warm air rising and cool air sinking. A gravity air furnace in a basement heats air, which then rises into the various rooms through ducts. Cool air returns to the furnace via a system of cold-air return ducts. The so-called "octopus" furnaces found in many older homes are gravity air furnaces.

Gravity air systems are no longer installed, but in many older homes they continue to perform effectively.

Fuel source: Forced air furnaces can be fueled by natural gas, liquid propane (LP), fuel oil, or electricity.

Distribution: Conditioned air is circulated through a network of metal ducts.

Advantages:

Gravity systems have no moving parts and can last for many decades.
The system equipment is very dependable and requires little maintenance.

Disadvantages:

Air cannot be filtered effectively.
Energy efficiency is lower than with newer furnaces.
Temperature adjustments are slow because the systems operate by simple convection currents.

In-Floor Radiant Heating Systems

Modern in-floor heating is a type of radiant heating system. Radiant heating is different from forced air heat in that it heats objects and materials, such as furniture and flooring, rather than just the air. Most whole-home radiant systems distribute heat via hot water heated in a boiler or hot water heater.

In-floor heating involves plastic water tubing installed inside concrete slab floors or attached to the top or bottom of wood floors. It is quiet and generally energy efficient. It tends to heat more slowly and takes longer to adjust than forced air heat, but its heat is more consistent.

There are also in-floor systems that use electrical wiring installed under flooring materials, typically ceramic or stone tile. These are less energy efficient than hot water systems and are typically used only in small rooms such as bathrooms.

Fuel sources: Hot water tubing systems are usually heated by a central boiler, which can be fueled by natural gas, liquid propane (LP), or electricity. Hot water also can be provided by solar hot water systems, which are commonly used to supplement fuel-based systems.

Distribution: In-floor systems are usually distributed by hot water flowing through plastic tubing.

Advantages:

Radiant systems provide comfortable, even heat.
When heated by boilers, radiant systems can be very energy efficient.

Disadvantages:

Radiant systems are relatively slow to heat up and adjust to temperature changes.
Installation of in-floor systems can be expensive.
It is difficult access to hidden piping if maintenance problems emerge.
Boiler-based systems cannot be combined with air conditioning.

Traditional Boiler and Radiator Systems

Older homes and apartment buildings in North America often are heated with traditional boiler and radiator systems. These include a central boiler that circulates steam or hot water through pipes to radiator units positioned strategically around the house. The classic radiator—a cast-iron upright unit usually positioned near windows—is often called a steam radiator, although this term is sometimes inaccurate.

In reality, there are two types of systems used with these older radiators. True steam boilers actually do circulate gaseous steam through pipes to individual radiators, which then condenses back to water and flows back to the boiler for reheating. Modern radiator systems circulate hot water to radiators via electric pumps. The hot water releases its heat at the radiator, and the cooled water returns to the boiler for more heating. Hot water radiator systems are very common in Europe.

Fuel sources: Boiler/radiator systems can be fueled by natural gas, liquid propane, fuel oil, or electricity. Original boilers may even have been fueled by coal.

Distribution: Heat is produced by steam or hot water circulating through metal pipes to radiators shaped to facilitate the transfer of thermal energy.

Advantages:

Radiant heat is quite comfortable and does not dry out the air as forced-air heat does.
Radiators can be updated to low-profile baseboard or wall-panel radiators.
When old boilers are replaced, modern boilers can offer very good energy efficiency.

Disadvantages: