Two-Stage Gas FurnacesTwo-Stage Gas Furnaces are ever-increasing in popularity among HVAC manufactures of gas furnaces. The product lines are ever-increasing from single-stage gas furnaces to a two-stage gas furnace. The reasoning is that the two-stage gas furnaces are more efficient than the single-stage furnaces.

Two-Stage Gas Furnaces

This is according to tests performed using the Department of Energy’s test guidelines. Test guidelines in accordance with testing AFUE ratings for gas furnaces’. Is this true and is the test of gas furnaces correct? Correct in making the claim that the two-stage is more efficient than the single-stage gas furnace?

Before we make any claims about the validity of the test we must first observe where the test originates. Where the origin of the DOE guidelines for testing heating appliances come from. These test guidelines come from ASHRAE or the American Society of Heating, Refrigerating, and Air Conditioning Engineers.

The current standards used to determine AFUE or Annual Fuel Utilization Efficiency was written and adopted by DOE in 1993 from multiple revisions dating back to 1982 where the origin of AFUE was made. The tests are applicable to gas furnaces/boilers, oil furnaces/boilers for determining AFUE.

Two-Stage Gas Furnace | AFUE Test Revision

ASHRAE is currently making a new revision in these tests. They use various manufacturers for determining AFUE in gas and oil furnaces and boilers. The reason for the revision is that the current test was made for single-stage gas furnaces.

Manufacturers are using the current single-stage test to test the two-stage gas furnaces. What will happen when the new tests are done using new proven methods for testing two-stage equipment? Two-stage gas furnaces/boilers and oil furnaces/boilers?

Methinks many people will be disappointed in the actual AFUE ratings of their new gas/oil boilers/furnaces. Although the true AFUE value will be lower it will not be a significant value lower. Not significantly lower than the AFUE rating listed on the furnace.

While the single-stage test had holes in it for two-stage furnace/boiler testing the test remains applicable. However, only for single-stage furnaces/boilers. The new tests will include:

Two-Stage Gas Furnaces | Test of Operation

  • On/off time for 2-stage or modulating furnace – this will include the time the burner ignites to the burner extinguishes.
  • Annual burner operating hours compared to annual energy consumption. The test will include ratios from low-fire to high-fire in this test. Additionally, the test will include the energy consumed by the blower and the induced draft motor. All these are factors in determining the true AFUE of a two-stage furnace.  Additionally, two-stage boilers, whether it be gas or oil, are included.
  • Calculation of Off-Period losses for oil boilers with a long post-purge time. The post purge cycle is generally used to scavenge heat in a furnace/boiler cool-down period. Flue losses will also be calculated in this test. The longer the post-purge cycle the more heat loss derived in the post purge. In layman’s terms heat lost up the stack.

The test will determine the true AFUE of two-stage gas/oil furnaces/boilers. HVAC manufacturers will be required to test the two-stage gas/oil furnaces/boilers using these new tests. The new tests based on the new guidelines. Additionally, they will be required to label the HVAC equipment accordingly.

Two-Stage Gas Furnaces | Components of a 2 Stage Gas Furnace

A 2-stage gas furnace needs the following for operation:

  • Two-stage gas valve
  • Two-stage induced draft motor
  • And a variable speed motor. Some manufacturers utilize a PSC multi-speed motor. Others use a true ECM variable-speed blower motor. The PSC motor is less efficient at lower speeds than the true ECM variable speed motor.
  • Dual-stage controls to control the dual-stage gas valve, induced draft motor, and the variable speed motor. This includes a dual-stage thermostat for the furnace’s dual-stage operation. Additionally, a control board to slow the induced draft motor and the variable speed motor. These components slow when the heating appliance is at low fire.

Use our handy calculator to see how many BTU's your natural gas appliances are producing:

Two-Stage Gas Furnaces | Conclusion

Most HVAC manufacturers list the dual-stage heating appliance with a range from 70% for low-fire to 100% for high-fire. The high-fire rate is for the coldest of days when demand is high. Therefore on the typical days when the temperature is not so cold outside, the furnace will consume less fuel. It will run at a lower rate.

Off the top, this seems to make one believe this will save you energy. That is because it is running at 70% rather than 100%. I am wondering when the new test is implemented how many furnaces will equal the same (or near the same) AFUE as what these furnaces are currently labeled as?

High Performance HVAC will keep you updated on the implementation of the new tests. The new tests when the DOE adopts these new test standards Two-Stage Gas Furnaces.

Currently, the new tests have been implemented for all two-stage gas furnaces.

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Two-Stage Gas Furnaces