Two-Stage Gas Furnaces

Two-Stage Gas Furnaces2-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 two stage gas furnace. The reasoning is that the two-stage gas furnaces are more efficient than the single-stage furnaces. This is according to tests performed using the Department of Energy’s test guidelines in accordance with testing AFUE ratings for gas furnace. Is this true and is the test of gas furnaces correct in making the claim that the two-stage is more efficient than the single stage gas furnace? Before we make any claims to about the validity of the test we must first observe 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 tests used my 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 and manufacturers are using this 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 – 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 than the AFUE rating that came with the furnace. While the single-stage test had holes in it for two-stage furnace/boiler testing the test remains applicable and solid for single-stage furnaces/boilers. The new tests will include:

AFUE Test Revision Parameters

  • On/off time for 2-stage or modulating furnace – this will include the time the burner ignite 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 would be remiss if it did not 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 or boiler whether it be gas or oil.
  • Calculation of Off-Period losses for oil boilers with a long post-purge time – the post purge cycle which 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 or in layman’s terms heat lost up the stack.

The test will determine the true AFUE of gas/oil furnaces/boilers and HVAC manufacturers will be required to test the gas/oil furnaces/boilers using this new test guideline and label the HVAC equipment accordingly.

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

  • Two-stage gas valve
  • Two-stage induced draft motor
  • And a variable speed motor (there are a few manufacturers who utilize a PSC multi-speed motor instead of a true ECM variable speed 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 dual-stage operation and a control board to slow the induced draft motor and the variable speed motor when the heating appliance is at low-fire.

Most HVAC manufacturers list the dual-stage heating appliance with a range from 70% for low-fire to 100% for high-fire saving the high-fire rate for the coldest of days. Therefore on the typical days when the temperature is not so cold outside the furnace will consume less fuel and energy running at the lower rate. Off the top, this seems to make one believe this will save you energy because it is running at 70% 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 when the DOE adopts these new test standards Two-Stage Gas Furnaces.

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