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Two-Stage Gas Furnaces

Update for this page coming soon to refelct new changes in testing two-stage and modulating gas furnaces

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Two Stage Gas Furnace AFUE Testing

Two-Stage Gas Furnace AFUE Test Revision

Two-Stage Gas Furnace AFUE Test Revision Parameters

Two Stage Gas Furnace Operation

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Two Stage Gas Furnace AFUE Testing

Two-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 furnaces. The reasoning is that the two-stage gas furnaces are more efficient than the single-stage gas furnaces. This is according to tests performed using the Department of Energy’s test guidelines in accordance with testing AFUE ratings for gas furnaces. 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 gas furnaces and boilers 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, oil furnaces, gas boilers, and oil 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 suing 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 gas furnaces - gas boilers and oil furnaces – oil boilers? Methinks many people will be disappointed in the actual AFUE ratings of their new gas or oil boilers and 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 and boiler testing the test remains applicable and solid for single-stage furnaces and boilers. The new tests will include:

Two-Stage Gas Furnace AFUE Test Revision Parameters

• On/off time for two-stage or modulating furnaces – 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 or 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 and oil furnaces and boilers and HVAC manufacturers will be required to test the gas and oil furnaces and boilers using this new test guideline and label the HVAC equipment accordingly.

Two Stage Gas Furnace Operation

A two-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 gas furnace 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)
• Two-stage controls to control the two-stage gas valve, induced draft motor, and the variable speed motor. This includes a two-stage thermostat for the furnace two-stage operation and a control board to slow the induced draft motor and the variable speed motor when the furnace is at low-fire.

Most HVAC manufacturers list the two-stage gas furnace 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?

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High Performance HVAC will keep you updated on the implementation of the new tests when the DOE adopts these new test standards for two-stage gas and oil furnaces and boilers.

Related Furnace Heating and Gas Controls Pages

Modern Condensing Gas Furnaces | ECM Variable Speed Blower Motors | Gas Furnace Components | The Filter Page | Water Heaters | Site Map/Content | Gas Furnace AFUE Ratings| Furnace Page Top

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