Inspecting Furnaces

by Ben Gromicko and Nick Gromicko, CMI®
Inspect, Identify and Describe a Furnace
According to the InterNACHI Residential Standards of Practice, a home inspection is a non-invasive, visual examination of a residential dwelling that is designed to identify observed material defects within specific components of that dwelling. Part of the home inspection includes the inspection, identification and description of the heating system.   
The inspector is required to inspect the heating systems using normal operating controls, and describe the energy source and heating method. The inspector’s report shall describe and identify, in written format, the inspected heating system and shall identify material defects observed.
In order to perform an inspection according to the Standards of Practice, an inspector must apply the knowledge of what s/he understands about the different types of residential heating systems. To fully inspect and identify a particular heating system, describe its heating method, and identify any material defects observed, an inspector should be able to explain and discuss with his/her client:
  • 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. 
The inspector must be able to thoroughly examine a heating system, understand how a particular heating system operates, and analyze and draw conclusions as to its apparent condition. An inspector should also be able to justify his/her observations, opinions and recommendations that were written in the inspection report.

Furnace Fundamentals
Let's focus on the fundamentals of a particular heating system called a furnace. There are many ways to inspect, identify and describe the different types of furnaces that may be found at a property using non-invasive, visual-only inspection techniques. It is up to the inspector’s judgment as to how detailed the inspection and report will be. For example, the inspector is not required to determine the capacity or BTU of the inspected heating system, but many inspectors record that detailed information in their reports. 
The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) defines a furnace as a “complete heating unit for transferring heat from fuel being burned to the air supplied to a heating system.” Another definition of a furnace is “a self-enclosed, fuel-burning unit for heating air by transfer of combustion through metal directly to the air.” Taking these two definitions into consideration, there are two basic characteristics of a furnace:
  1. There is a fuel used to produce combustion; and
  2. 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;
  • distribution;
  • airflow;
  • gravity or forced;
  • efficiency; and 
  • ignition.  

Fuel Type
One way to identify and describe a furnace is based on the type of fuel used to produce heat.  Based on fuel type, one can classify a furnace as:
  1. gas-fired;
  2. oil-fired;
  3. coal;
  4. wood;
  5. multi-fuel; or
  6. electric.
Fossil fuels are used to produce combustion in the first five types.  The last one uses electricity.  Whether or not electricity can be considered a fuel is not important here, since an electric furnace functions in the same manner as the other fossil-burning furnaces.  The electric furnace heats air and distributes it.  According to the Standards, an inspector is required to describe the energy source in their report.
The inspector is also required to describe the heating method. One way to do that is to identify the method of how the air is distributed throughout the house. Furnaces can be identified and described (or classified) by the way the air is distributed. There are two broad categories:
  1. gravity warm-air furnaces; and
  2. 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.

Forced warm-air furnaces can be identified and described by how the air flows through the heating unit in relation to the warm-air outlet and the return-air inlet locations on the furnace. There are three types of forced warm-air furnaces related to airflow:
  1. upflow (highboy or lowboy);
  2. downflow; and
  3. horizontal. 
Furnace manufacturers commonly use the terms "upflow," "downflow" and "horizontal" in their literature that describes their products, including their marketing materials, and in their installation and operation manuals.
Upflow Highboy
On a typical upflow highboy furnace, the warm-air outlet is located at the top of the furnace, so  warm air discharges out of the top. The return-air inlet is located at the bottom or sides of the furnace. A cooling unit is often added to the top of an upflow furnace. A typical upflow highboy furnace stands no higher than 6 feet and can occupy a floor space of 6 square feet (2 feet x 3 feet).
Upflow Lowboy
An upflow lowboy furnace is designed for low clearances. Both the warm-air outlet and return-air inlet are located at the top of the furnace. The lowboy is often installed in a basement where most of the ductwork is above the heating unit. This compact heating unit typically stands no higher than 4 feet. It is usually longer from front to back than either the upflow highboy or downflow furnaces.

A downflow furnace is also referred to as a counterflow furnace or a downdraft furnace. Warm air discharges out of the bottom of a downflow furnace, and the return-air inlet is located at the top. The downflow furnace is installed usually when most of the duct or pipe distribution system is below the furnace. The ducts might be embedded in a concrete floor slab or suspended in a crawlspace below the heating unit. The downflow furnace is similar in dimension to the upflow, but the warm-air outlet is located at the bottom instead of the top.

A horizontal furnace is designed primarily for installations with low, restricted space, such as a crawlspace or attic. A typical horizontal furnace is about 2 feet wide by 2 feet tall, and 5 feet long.
Gravity Warm-Air Furnace
A gravity warm-air furnace uses the fact that warm air is lighter than cool air, and warm air rises.  In a gravity warm-air furnace, warm air might rise 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. The air is circulated through the house in this manner.
The very earliest types of furnaces were gravity warm-air furnaces. They were popular from first half of the 19th century to the early 1970s. Sometimes they had a blower fan installed to move the heated air. But the primary way the air moved through the house relied on how gravity affected the different weights of warm and cool air. Gravity warm-air furnaces were sometimes described as "octopus" furnaces because of its appearance with all of the pipes coming out of the centrally located heating unit. Most of these gravity furnaces are obsolete and at the end of their life expectancy.

A gravity warm-air furnace can be described in one of the following three ways:
  1. a gravity warm-air furnace without a fan;
  2. a gravity warm-air furnace with an integral fan; or
  3. a gravity warm-air furnace with a booster fan.
A gravity warm-air furnace without a fan relies entirely on gravity and the different weights of air to circulate the air through the house. The airflow rate is slow. The air circulation and distribution of heated air is not efficient. It is all but impossible to effectively control the heat supplied to individual rooms of the house. Sometimes an integral fan is installed in the distribution ducts or pipes to reduce the internal resistance to airflow and increase air movement.  
A booster fan is installed to do the same, but does not interfere with air circulation when it is not in use. A booster fan might be a belt-driven fan unit, resting on the floor and attached to the outside of the heating unit.
Floor and space heaters operate using the same principles of gravity and air weights, as do the gravity warm-air furnaces. They differ by the way a floor or space heater is designed to provide heated air to a particular room or space, and do not distribute air throughout the house.
Warm Air Rises
When a certain amount of air is heated up, it expands and takes up more space. In other words, hot air is less dense than cold air. Any substance that is less dense than the fluid (gas or liquid) of its surroundings will float. Hot air floats on cold air because it is less dense, just as a piece of wood floats because it is less dense than water. Warm air is often described as weighing less than cool air.

Gas Furnaces 
There are a variety of ways to describe different types residential gas furnaces. Gas furnaces can be classified by:
  1. the direction of the air flowing through the heating unit;
  2. the heating efficiency of the unit; and
  3. the type of ignition system installed on the unit.
Airflow in Gas Furnaces
One way to identify and describe a gas furnace is by the direction of the air flowing through the heating unit, or the location of the warm-air outlet and the return-air inlet on the furnace.  Gas furnaces can be described as upflow, downflow (counterflow), highboy, lowboy, and horizontal flow.  Air can flow up through the furnace (upflow), down through the furnace (downflow), or across the furnace (horizontal).  The arrangement of the furnace should not significantly affect its operation, or your inspection.
Gas furnaces can be classified by their different capacities. A furnace capacity can be described by BTU output. The BTU is determined by what is required by the heating unit for the structure, which is the amount of heat the unit needs to produce to replace heat loss and provide the occupants a good comfort level.
Furnaces can be identified and described by heating efficiency. The energy efficiency of a natural gas furnace is measured by its annual fuel utilization efficiency (AFUE). The higher the rating, the more efficient the furnace. The U.S. government has established a minimum rating for furnaces of 78%.  Mid-efficiency furnaces have AFUE ratings from 78 to 82%. High-efficiency furnaces have AFUE ratings from 88 to 97%. Old, standing-pilot gas furnaces have AFUE ratings from 60 to 65%. Gravity warm-air furnaces might have efficiencies lower than 60%.
BTU and Efficiency
BTU stands for British Thermal Unit. The BTU is a unit of energy. It is approximately the amount of energy needed to heat one pound of water 1 degree Fahrenheit. Once cubic foot of natural gas contains about 1,000 BTUs. A gas furnace that fires at a rate of 100,000 BTUs per hour will burn about 100 cubic feet of gas every hour. 
On a gas furnace, there should be a data plate. On that plate there might be written the input and output capacities. For example, the data plate may say, “Input 100,000 BTU per hour.” And it may also say, “Output 80,000 BTU per hour.” While this furnace is running, about 20% of the heat generated is lost out through the exhaust gases.  The ratio of the output to the input BTU is 80,000 ÷ 100,000 = 80% efficiency. This is the "steady state efficiency" of the furnace.  
Steady state efficiency measures how efficiently a furnace converts fuel to heat, once the furnace has warmed up and is running steadily.  However, furnaces cycle on and off as they maintain their desired temperature.  Furnaces typically do not operate as efficiently as they start up and cool down.  As a result, steady state efficiency is not as reliable an indicator of the overall efficiency of your furnace.
AFUE and Efficiency
The AFUE is the most widely used measure of a furnace's heating efficiency.  It measures the amount of heat delivered to your house compared to the amount of fuel that must be supplied to the furnace. Thus, a furnace that has an 80% AFUE rating converts 80% of the fuel that is supplied to heat. The other 20% is lost and wasted.
Note that the AFUE refers only to the unit's fuel efficiency, not its electricity usage. The U.S. Department of Energy (DOE) determined that all furnaces sold in the U.S. must have a minimum AFUE of 78%, beginning January 1, 1992. Mobile home furnaces are required to have a minimum AFUE of 75%.
The DOE's definition of AFUE is the measure of seasonal or annual efficiency of a furnace or boiler. It takes into account the cyclic on/off operation and associated energy losses of the heating unit as it responds to changes in the load, which, in turn, is affected by changes in weather and occupant controls.
Ignition Type 
Gas furnaces can be identified and described by the type of ignition system on the furnace.  The different types of ignition systems are:
  1. standing-pilot;
  2. intermittent-pilot or direct-spark; and
  3. 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.

Standing-pilot gas furnaces represent a significant number of residential gas furnaces that are still in use today. A standing-pilot gas furnace is equipped with a naturally aspirating gas burner, a draft hood, a solenoid-operated main gas valve, a continuously operating pilot light (standing- pilot), a thermocouple safety device, a 24-volt AC transformer, a heat exchanger, a blower and motor assembly, and one or more air filters. The standing-pilot is the main distinguishing characteristic of the low-efficiency conventional gas furnace.
A mid-efficiency gas furnace is equipped with naturally aspirating gas burner and a pilot light. The pilot light is unlike a standing-pilot. It does not run continuously. The pilot light is shut off when the furnace is not in operation (when the thermostat is not calling for heat). The heat exchanger is more efficient than one inside a conventional furnace. There is no draft hood. There may be a small fan installed in the flue pipe to create an induced draft, so these furnaces are sometimes referred to as induced-draft furnaces. A mid-efficiency gas furnace is also equipped with automatic controls, blower and motor assembly, venting, and air filtering. Some mid-efficiency furnaces will have a motorized damper installed in the exhaust flue pipe. A mid-efficiency furnace is about 20% more energy-efficient than a conventional gas furnace. A mid-efficiency furnace has an AFUE rating of 78 to 82%. The intermittent-pilot is the main distinguishing characteristic. 
High-efficiency gas furnaces have AFUE ratings of 90%and greater.  A solid-state control board controls the ignition.  There is no continuous pilot light.  There are two or sometimes three heat exchangers installed inside a high-efficiency gas furnace.  Condensate is produced when heat is extracted from the flue gases.  The temperature of the flue gases is low enough to use a PVC pipe as the vent exhaust pipe.  There is no need to vent the exhaust gases up a chimney stack.  There are two different types of high-efficiency furnaces:
  1. one with an intermittent-pilot or direct-spark; and
  2. one with a hot-surface ignition system. 
The production of excessive condensate is the main distinguishing characteristic.

The Best Techniques
There are many ways to identify and describe a furnace.  According to the InterNACHI Standards of Practice, the inspector is required to inspect the heating systems using normal operating controls, and describe the energy source and heating method. The inspector’s report shall describe and identify, in written format, the inspected heating system, and shall identify material defects observed.