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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 They have mostly been replaced by modern, forced warm-air furnaces.
Airflow
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:
- upflow (highboy or lowboy);
- downflow; and
- 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.
Downflow
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.
Horizontal
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:
- 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.
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:
- 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
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.
BTU
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.
AFUE
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:
- standing-pilot;
- 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.
Standing-Pilot
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.
Mid-Efficiency
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
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:
- 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.
How it Operates
When a heat pump is operating in the heating mode or heat cycle, the outdoor air is relatively cool and the outdoor coil acts as an evaporator. Under certain conditions of temperature and relative humidity, frost might form on the surface of the outdoor coil. The layer of frost will interfere with the operation of the heat pump by making the pump work harder and, therefore, inefficiently. The frost must be removed. A heat pump has a cycle called a defrost cycle, which removes the frost from the outdoor coil.
A heat pump unit will defrost regularly when frost conditions occur. The defrost cycle should be long enough to melt the ice, and short enough to be energy-efficient.
In the defrost cycle, the heat pump is automatically operated in reverse, for a moment, in the cooling cycle. This action temporarily warms up the outdoor coil and melts the frost from the coil. In this defrost cycle, the outdoor fan is prevented from turning on when the heat pump switches over, and the temperature rise of the outdoor coil is accelerated and increased.
The heat pump will operate in the defrost cycle until the outdoor coil temperature reaches around 57° F. The time it takes to melt and remove accumulated frost from an outdoor coil will vary, depending on the amount of frost and the internal timing device of the system.
Interior Heating Element
During this defrost cycle with older heat pumps, the indoor unit might be operating with the fan blowing cool air. To prevent cool air from being produced and distributed inside the house, an electric heating element can be installed and engaged at the same time as the defrost cycle. In defrost mode, this heating element will automatically turn on, or the interior blower fan will turn off. The heating component is wired up to the second stage of a two-stage thermostat.
The Typical Cycle
The components that make up the defrost cycle system includes a thermostat, timer and a relay. There is a special thermostat or sensor of the defrost cycle system, often referred to as the frost thermostat. It is located on the bottom of the outdoor coil where it can detect the temperature of the coil.
When the outdoor coil temperature drops to around 32° F, the thermostat closes the circuit and makes the system respond. This causes an internal timer to start. Many heat pumps have a generic timer that energizes the defrost relays at certain intervals of time. Some generic timers will energize the defrost cycle every 30, 60 and 90 minutes.
The defrost relays turn on the compressor, switch the reversing valve of the heat pump, turn on the interior electric heating element, and stop the fan at the outdoor coil from spinning. The unit is now in the defrost cycle.
The unit remains in the defrost cycle (or cooling cycle) until the thermostat on the bottom of the outdoor coil senses that the outdoor coil temperature has reached about 57° F. At that temperature, the outdoor coil should be free of frost. The frost thermostat opens the circuit, stops the timer, then the defrost cycle stops, the internal heater turns off, the valve reverses, and the unit returns to the heating cycle. A typical defrost cycle might run from 30 seconds to a few minutes. The defrost cycles should repeat regularly at timed intervals. An inspector should not observe a rapid cycling of the defrost operation.
In summary, certain conditions can force a heat pump into a defrost cycle (or cooling cycle) where the fan in the outdoor coil is stopped, the indoor fan is stopped or electric heat is turned on, the frost melts and is removed from the outdoor coils. When the frost thermostat is satisfied or a certain pre-set time period elapses, the outdoor fan comes back on, and the heat pump goes back into the heating cycle.
One problem of many older heat pump systems is that the unit will operate in the defrost cycle regardless of whether ice is present. On these systems, if it’s cold outside, the defrost cycle might turn on when it is not needed.
If the defrost cycle is not functioning properly, the outdoor coil will appear like a big block of ice, making the unit non-functional. Damage could result if the heat pump operates without a functional, normal-operating defrost cycle.
Causes of Frost
There are many reasons why an inspector might find frost and ice stuck on an outdoor coil of a heat pump that is not properly defrosting. The cause of the frost and ice problem may include:
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a bad reversing valve;
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a damaged outdoor coil;
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a wiring problem;
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a bad thermostat;
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a leak in the refrigerant;
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a dirty outdoor coil covered with grass, dirt, debris and/or pet hair;
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a fan that won’t turn on;
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a fan installed backwards with the blades running in the wrong direction;
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a motor operating in the incorrect direction; and/or
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a replacement fan motor spinning at a very low rpm.
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