About Furnaces

A furnace is specifically a heating device that generates warm air for distribution. Key characteristics include:

• Single-purpose heating appliance
• Operates independently or as part of HVAC system
• Available in gas, electric, or oil models
• Typically paired with air conditioner in split systems

Use heating elements similar to electric heaters to warm air. These systems may require additional or larger electrical circuits to provide the amperage needed for energizing electric resistance heat elements. Benefits include:

• No combustion or exhaust requirements
• Quieter operation
• Easier installation

Use natural gas or propane combustion to generate heat. They require the availability of natural or propane gas to function. They also require gas piping, in some cases fresh air ventilation piping, and exhaust ventilation piping. They feature:

• Burners for fuel combustion
• Heat exchangers to transfer warmth
• Flue for exhaust gases
• Can utilize secondary heat exchangers for better efficiency

Use oil combustion to generate heat. They rely on oil tanks and oil pumps to transfer oil to the burners. These systems are commonly found in areas were gas availability is limited. They feature:

• Burners for oil combustion
• Heat exchangers to transfer warmth
• Flue for exhaust gases

AFUE stands for Annual Fuel Utilization Efficiency. For furnaces and boilers it’s kind of like miles-per-gallon or MPG for cars. It’s a standard measurement of how efficiently a furnace converts energy from the fuel it uses into warm air for your home. All furnace manufacturers are required by the Department of Energy to display each unit’s AFUE so consumers can compare efficiencies between one furnace and another. The rating for a furnace is found on the familiar yellow Energy Guide label, similar to the one seen on virtually every appliance in the United States. Keep in mind that while annual fuel utilization efficiency ratings are a good way to compare efficiencies, calculating actual operating costs for a new heating system or an older furnace is a bit more complicated. This is due to a number of variables including the differences in pricing for natural gas and heating oil. Currently, the minimum standard for new furnaces and boilers is 80% AFUE.

The furnace AFUE rating is calculated using the total annual heating output from the furnace versus the amount of fuel input over the same time period. The published rating of a furnace should be considered its average rating, not the efficiency it will achieve every single day. When making a purchase decision based on ratings, keep in mind that models with higher numbers are often deluxe heating units with extra features and benefits. Those units also typically come with a higher price tag as well, so you may want to research the potential savings on your utility bills when making your decision.

Because your actual operating costs take into account a number of variables such as the size of your home, thermostat settings, fuel type and more, your local Carrier dealer can be an excellent source for obtaining a reasonable estimate of these costs. Professional HVAC dealers have access to estimating tools that can help show your potential savings by stepping up from an 80% AFUE furnace model to a 90% or higher model.

For consumers making choices based on environmental impact, selecting the highest energy efficiency furnaces or boilers makes a lot of sense. Because they are more efficient furnaces, they can use less fuel compared to the same-sized 80% AFUE unit. And because they convert more of the fuel into heating energy that warms your home, fewer combustion gases are vented outside during the heating process.

So, now the question is, what is a good AFUE furnace rating?

When comparing annual fuel utilization for furnaces and boilers, keep in mind the current minimum allowed by the Department of Energy is a rating of 80% for gas furnaces. By today’s industry standards, a minimum efficiency model actually falls into the mid-efficiency category of 80 – 83% AFUE. These furnaces usually include fans that control the flow of combustion air and gases, and typically have electronic ignition instead of a continuously burning pilot light. Low efficiency units are older, existing systems that may reach 56 – 70% AFUE. These units often include a continuous pilot light and natural draft induction. High efficiency units are in the 90 – 98.5% AFUE and include features like two heat exchangers, a sealed combustion system, two or more heating stages, variable speed fans and “intelligent” controls. Learn more about the efficiency of gas and oil furnaces here.

The Heat Rise value is the rated value the manufacturer of the furnace has designed for. The Heat Rise of a furnace is important to understand. This rated value is usually labeled on the furnace or contained in the installation or technical manuals. The actual Heat Rise value is basically the supply air temperature of the furnace minus the return air temperature of the furnace. The actual Heat Rise value must fall within the rated Heat Rise value. If the actual Heat Rise is low, the furnace will be inefficient and home comfort will be affected. Low Heat Rise can cause condensation and poor heating. If the actual Heat Rise is high, the furnace will have a short lifespan, may short-cycle and may overheat.

If the actual Heat Rise is high, adjust it by increasing airflow slowly until the desired value is reached. If the actual Heat Rise is low, adjust it by decreasing airflow slowly until the desired value is reached.

The formula for Heat Rise is CFM = BTUs / (1.08 * Heat Rise)

Where 1.08 is a constant based on air properties.
The following is an example for the target Heat Rise of a furnace.
A 80,000 BTU furnace with a target Heat Rise of 50°F.
80,000 ÷ (1.08 × 50)
= 80,000 ÷ 54
= 1,480 CFM

With this CFM value, configure the blower to deliver 1,480 CFM of airflow and design your ductwork to deliver 1,480 CFM of airflow.

Furnaces use different heat sources, depending on their type (such as electric, gas or oil). When choosing the best type of furnace, one must take into account the availability of electrical capacity (Electric Furnace), gas capacity (Gas Furnaces) or oil capacity (Oil Furnaces).

Additionally, when selecting a gas or oil furnace, proper fresh air intake and exhaust air ventilation must be used. This ventilation may need to be added to the structure if it does not exist. The ventilation must be installed properly to prevent deadly fumes from being released into your structure. Additional precautions might need to be taken into consideration to prevent negative and positive pressure imbalances in the structure.