Gas furnace efficiency ratings

Gas furnace efficiency ratings

This is a two-part article discussing the categories and efficiency ratings of the two most common natural gas furnaces. This article focuses on 80 percent furnaces. In the next article we'll take a look at a high-efficiency furnace.

Written by Jeremy James

Jeremy James is a veteran of the U.S. Navy who has grown his family's HVAC business into one of the most successful home service businesses in Knoxville, TN. More than 20 years of experience in the HVAC and Engineering industries and passion for what we do has translated into our customer service, professionalism, and quality. LinkedIn

Category Ratings and Efficiencies of Natural Gas Furnaces

Most furnace manufacturers offer two types of furnaces:

  • Mid-efficiency Category I fan assist

  • High-efficiency Category IV condensing-type furnace

The minimum energy efficiency for mid-efficiency furnaces is 80 percent annual fuel utilization efficiency (AFUE). 80 percent furnaces have a flue gas temperature at least 140F above the dew point of the flue gas.

A high-efficiency furnace has flue gas temperatures below the dew point which permits condensation within the furnace to pick up extra latent heat.

There are also standard low-efficiency furnaces. However, they are no longer being manufactured. Many earlier models of standard gas furnaces are still in operation and require service. These have natural-draft venting, and many use standing pilots.

Standard Low-Efficiency Gas Furnaces

Low-efficiency gas furnaces were manufactured before 1992 and a few remain in the field. With so many still in use, technicians should be familiar with some of the differences from the newer designs.

Venting

Below is a cross-sectional diagram for one of these units. One distinctive of the older design is the flow of combustion gases from the burner, past the heat exchanger (HX), and into the vent. This requires a different type of venting than newer, more efficient units that use fan assist.

Low efficiency gas furnace cross sectional diagram

Ignitors

Newer gas furnace burners like the one shown below use electronic igniters. Some of the older units use a standing, continuous-flame gas pilot. This pilot consumes a small amount of gas continuously.

Runner and thermocouple

There is a runner attached to the main burner. The runner carries the gas flame from the pilot to all of the burners. A thermocouple is used to sense the flame as a safety feature, turning off the gas if the flame goes out.

Natural Gas Furnace Category Ratings

There are four categories of modern gas furnaces:

Category I Gas Furnace

An appliance with a non-positive vent static pressure and vent gas temperature greater than 140F above the flue-gas dew point, avoiding excessive condensate production in the vent.

Category II Gas Furnace

An appliance with a nonpositive vent static pressure and vent gas temperature less than 140F above the flue-gas dew point, can cause excessive condensate production in the vent.

Category III Gas Furnace

An appliance with a positive vent static pressure and vent gas temperature greater than 140F above the flue gas dew point, avoiding excessive condensate production in the vent.

Category IV Gas Furnace

An appliance with a positive vent static pressure and vent gas temperature less than 140F above the flue-gas dew point, can cause excessive condensate production in the vent.

Category I Furnaces

Category I furnaces are divided into natural draft and fan-assisted. Most furnaces sold today are Category I, fan-assisted.

The fan does not produce a positive vent pressure because it is just large enough to overcome the resistance of the HX. A benefit of the induced draft fan-assisted furnace is that the HX operates under a negative pressure.

The negative pressure in the HX ensures added safety for the homeowner. If a hole develops in the HX, the air is pulled into the HX instead of harmful exhaust gases escaping into your home.

Category II and Category III Furnaces

Category II and Category III furnaces are rarely used. They require special vent materials. It is a special high-temperature plastic vent material that can be very costly. This venting material is no longer manufactured due to high failure rates and the resulting legal action.

Category IV Furnaces

Category IV furnaces (high-efficiency furnaces) are very common. They are the 90 percent AFUE condensing furnaces. PVC can be used as a vent material because the flue gas is relatively cool.

What are AFUE ratings?

AFUE, or Annual Fuel Utilization Efficiency, measures how efficiently a furnace converts fuel to heat energy. It determines how much fuel your furnace is turning into heat. In a mid-efficiency furnace, 80% of the fuel is turned into heat.

Mid-Efficiency Furnaces (80 Percent AFUE)

Furnace manufacturers achieve an energy efficiency of 80 percent by:

  • Eliminating the pilot light

  • Improving HX efficiency, and

  • Adding an induced-draft blower.

The internal components of a mid-efficiency furnace include the following:

  • The inducer blower assembly

  • The pressure switch

  • The gas control valve

  • The burner assembly

  • The blower door safety switch

  • The control box

  • The air filter retainer

  • The air filter

  • The wraparound casing

  • The heat exchanger

  • The blower and blower motor

When 80 percent furnaces were being introduced, many furnaces around has efficiencies in the 50-60 percent range. 80 percent furnaces were a midpoint in efficiency and cost between the traditional standing-pilot, natural-draft furnace, and the high-efficiency gas furnace. 80 percent furnaces are most manufacturers’ bottom line.

80 Percent AFUE Furnace Product Data

The following information for your new heating system will be supplied by the manufacturer. The technician needs to check the product data of the new gas furnace at the time of startup and during service calls following installation. This information appears on the nameplate:

  • Minimum gas-line pressure

  • Maximum gas-line pressure

  • Manifold gas pressure

  • Temperature rise

  • Electrical characteristics

  • Input capacity

  • Output capacity

These units, primarily designed for heating, can be combined with several accessories, such as a cooling coil, humidifier, or electronic air cleaner.

The 80 percent AFUE furnaces have output ranges of 35,000 BTU/hr to 124,000 BTU/hr.

Summary

Understanding your heating and air conditioning ratings and design will help you make better decisions for the comfort of your home. You will be able to weigh the upfront cost against monthly energy bills. The purchase price of the right furnace makes sense when compared to the savings on utility bills.

More about furnace energy efficiency ratings in the next article. Also, check out how you can get a new furnace free with the purchase of a complete HVAC system.

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