- the heating system;
- its heating method;
- its type or identification;
- how the heating system operates;
- how to maintain it; and
- the common problems that may be found.
- There is a fuel used to produce combustion; and
- Heat is transferred to the interior air. Note that air – not water or steam – is used as the medium to convey the heat. This characteristic distinguishes warm-air heating systems from other types of heating systems.
Let’s look at identifying and describing some warm-air heating systems called furnaces.
Most modern furnaces are commonly referred to as central heating systems. The furnace is often centralized within the structure. The furnace is used as the main, central warm-air heating system. The heat of the furnace is forced (or rises) through a system of ducts or pipes to other places or rooms in the structure. The furnace does not necessarily need to be centrally located within the structure if the furnace is a forced warm-air system.
Furnaces that have no distribution ducts or pipes are used in some heating applications. They are limited in the size of the area that they can heat. They are installed within the room or area to be heated and have no way to distribute the heat to other places.
Identification and Description of Furnaces
There are several ways to identify and describe a furnace using non-invasive, visual-only inspection techniques, as required by the InterNACHI Standards of Practice. Furnaces can be identified and described by:
- fuel type;
- gravity or forced;
- efficiency; and
- multi-fuel; or
- gravity warm-air furnaces; and
- forced warm-air furnaces.
The gravity warm-air furnaces rely primarily on gravity for circulating the heated air. Warm air is lighter than cool air and will rise and move through ducts or pipes. After releasing its heat, the air becomes cooler and heavier. The air drops down the structure through return registers to the furnace where it is heated again, and the cycle continues. The very earliest types of furnaces were gravity-type furnaces. Sometimes they had a blower fan installed to move the heated air. They have mostly been replaced by modern, forced warm-air furnaces.
- upflow (highboy or lowboy);
- downflow; and
- a gravity warm-air furnace without a fan;
- a gravity warm-air furnace with an integral fan; or
- a gravity warm-air furnace with a booster fan.
- the direction of the air flowing through the heating unit;
- the heating efficiency of the unit; and
- the type of ignition system installed on the unit.
Airflow in Gas Furnaces
- intermittent-pilot or direct-spark; and
- hot-surface ignition.
The older gas furnaces have a standing-pilot light that is always burning. Modern furnaces with higher efficiency ratings are slowly replacing these older, conventional gas furnaces.
- one with an intermittent-pilot or direct-spark; and
- one with a hot-surface ignition system.
The production of excessive condensate is the main distinguishing characteristic.