Inspecting a High-Efficiency Gas Furnace - Part 1
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Inspecting a High-Efficiency Gas Furnace - Part 1

An overview of inspecting a high-efficiency gas furnace.

Modern homes are typically installed with a high-efficiency (HE) heating system and Canada has made them mandatory as of January 1, 2010 for natural gas forced-air heating systems. There are a few important differences between high-efficiency systems and conventional or mid-efficiency furnaces. September and October are the best months to inspect your HE furnace to make sure its ready for that first cold snap.

High-efficiency versus Standard-efficiency

A common misconception is that the combustion process is more efficient in high-efficiency furnaces than it is in standard-efficiency furnaces. In reality, HE furnaces simply collect more of the heat from the combustion products before they leave the house. Prior to these units, manufacturers had to maintain a minimum temperature for exhaust gases to ensure that they exited the chimney before condensing. The by-products of combustion of natural gas are mostly water and carbon dioxide. The condensation is corrosive and damaging to furnaces and chimneys as the water and carbon form carbonic acid.

HE furnaces have long heat exchangers or multiple heat exchangers to keep the hot gases in contact with the house air longer to capture more heat before it escapes to the outside. They are designed to handle the condensation safely with epoxy-coated or stainless steel components that may come in contact with the gas and condensation.

Condensation and Heat Gain

The exhaust gases contain a great deal of heat due to something called the latent heat of vaporization. This is the energy required to convert a liquid to a gas or the energy gained by reversing the process. If you increase the temperature of a pound of water or steam by one degree Fahrenheit, you would gain one British Thermal Unit (BTU). When we change one pound of gas at 212° to one pound of water at 212°, you would get 970 BTUs.

As stated before, there is a drawback to recovering this heat and that is corrosive condensation. High-efficiency furnaces have special heat exchangers constructed of stainless steel or plastic-coated steel. With some models, only the downstream heat exchangers come in contact with condensation and are constructed to be corrosion resistant. At the last pass of the heat exchanger there is typically some type of condensate collection system to remove the water from the unit. It can simply be plastic tubing connected to a fitting at the bottom of the flue that is run to a drain or pump.

The benefit of this design is that now the flue gases are cool enough to use PVC piping for the chimney instead metal or masonry.


Here are some of the ways to identify high-efficiency furnaces:

Unit with an Annual Fuel Utilization Efficiency (AFUE) rating on the nameplate greater than 90% are high-efficiency furnaces. If the AFUE is not stated, divide the output by the input to calculate the efficiency. Example: The output is 92,500 BTUs per hour and the input is 100,000 BTUs per hour, it’s a 92.5%.

High-efficiency furnaces will have a condensate collection system for the furnace. Usually there is also a central air conditioning system so there will be a condensate drain system for the air conditioning. In some areas, it has to go through a neutralizer to take away the acidity. The neutralizer is a salt bath the condensate passes through on its way to the drain. You should find out whether a neutralizer is required in your area. The condensate is not as acidic as vinegar, wine or soft drinks, but some municipalities require that it be neutralized anyway. A condensing furnace operating continuously for 30 minutes can generate up to a quart of condensate.

The vent piping on a HE furnace is PVC or ABS. Occasionally contractors use PVC or ABS piping mistakenly on mid-efficiency systems. A HE furnace will have the exhaust and possibly an intake adjacent to each other on the side of the home.

A high-efficiency furnace has a sealed combustion chamber. Newer high-efficiency furnaces draw in combustion air from outside, but the unit does not have to have outdoor combustion air to be high-efficiency.

A High-efficiency furnace (Note the condensate drain coming off of the inverted Y of the exhaust piping as well as from the humidifier on the right-side of the picture. The air conditioning condensate drain is just below the bare copper tubing and foam insulated tubing on the left.)


High-efficiency furnaces cycle on and off differently from conventional furnaces. It’s important to understand the sequence of operation so you can determine whether the system is responding properly during inspection.

1. Thermostat calls for heat.

2. There is a pre-purge cycle that takes about 25 seconds to remove any unburned gas or combustion products.

3. The system checks that there is enough combustion air or draft and that the exhaust is not blocked.

4. The igniter warms up for 15 or 20 seconds. Second generation high-efficiency systems do not use a pilot or spark ignition.

5. The gas valve opens.

6. The flame sensor verifies ignition within 2 to 8 seconds.

7. The supply fan (blower) comes on 30 to 60 seconds after ignition. There is fan switch that activates the blower when the furnace gets warm enough, it is set by programming.

8. The firing rate may change as the furnace operates if it is a multistage burner.

9. The thermostat is satisfied.

10. The burner shuts off.

11. The induced-draft fan stays on for 15 to 20 seconds, to do a post-purge of the combustion chamber and exhaust system.

12. The house air fan shuts off 60 to 240 seconds later.


High-efficiency furnaces are far more complex than conventional furnaces and require more service that traditional furnaces. Due to their complexity, they may also experience more frequent malfunctions that require the unit to be reset. Second generation high-efficiency furnaces are more reliable than earlier designs.

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