April, 2010
Inspection News and Views from the American Society of Home Inspectors

Inspecting High-Efficiency Gas Heating Systems


This ain't your daddy's furnace!

This three-part series of articles is excerpted from the ASHI@HOME education program.

High-efficiency heating systems are becoming common in America, and, as of January 1, 2010, they are the only gas forced-air heating systems permitted in Canada. There are significant differences between high-efficiency systems and conventional or mid-efficiency furnaces. Professional home inspectors should be able to identify and inspect high-efficiency furnaces appropriately.

In this three-part series, we’ll define high-efficiency furnaces, then look at how to identify them in the field and their sequence of operation (quite different from conventional furnaces). Last, we’ll focus on things to watch for during inspections.

High-efficiency furnaces defined

The combustion process is not necessarily more efficient in high-efficiency furnaces than it is in conventional or mid-efficiency furnaces. High-efficiency furnaces simply collect more of the heat from the combustion products before they leave the house. For many decades, we worked hard to keep the exhaust gases warm enough to get out of the house and chimney without condensing. The products of combustion of natural gas are, after all, mostly water and carbon dioxide. The condensation is corrosive and damaging to furnaces and chimneys.

The downside is that it’s a waste of heat to keep the exhaust gases warm enough to avoid condensation. Wouldn’t it be great if we could remove more heat from the exhaust gases before they escape? If we could handle the condensation, we could capture a lot more heat. High-efficiency furnaces do exactly that. They have long heat exchangers or multiple heat exchangers to keep the products of combustion in contact with the house air longer, thereby capturing more heat before
exhaust gases escape outdoors. And, they handle the condensation safely.


Condensation is a good thing

The latent heat of vaporization is a magical thing. We get lots of heat from the exhaust products when they change from gas to liquid. Here’s how significant it is: When we change the temperature of a pound of water or steam by one Fahrenheit degree, we get one BTU. However, when we change one pound of gas at 212° to one pound of water at 212°, we get 970 BTUs!

We have to address the corrosion issue, and that’s why high-efficiency furnaces have special heat exchangers. They are typically stainless steel or plastic-coated. In some furnaces, only the downstream heat exchangers that are subjected to condensation are the special type. A condensate collection system gathers the water and safely carries it away.
When we cool the exhaust gases to the point of condensation, we can use inexpensive venting systems. We no longer need a chimney.

Photo: Plastic-coated clamshell heat exchanger

Identifying high-efficiency furnaces

High-efficiency furnaces have a number of distinguishing features. Unfortunately, we don’t usually get to see a cutaway view like the illustration below.


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

1. If the AFUE (Annual Fuel Utilization Efficiency) rating on the data plate is more than 90%, it is high-efficiency. Sometimes you have to divide the output by the input to calculate the efficiency. If the output is 92,000 BTUs per hour and the input is 100,000 BTUs per hour, it’s a 92% (92,000 divided by 100,000) efficient system. That’s high efficiency.

2. High-efficiency furnaces are condensing systems. If there’s a condensate collection system for the furnace, it’s high- efficiency. If there is a central air conditioning system, there also will be a condensate drain system for the air conditioning. Don’t confuse the two. In some areas, the furnace condensate has to pass through a neutralizing salt. This helps distinguish a high-efficiency furnace condensate system from an air conditioning condensate system.

3. If the venting piping is PVC or ABS, it should be a high-efficiency system, although we sometimes find PVC and ABS piping used inappropriately on mid-efficiency systems.

4. If the combustion chamber is sealed, it’s a high-efficiency system. Some high-efficiency furnaces draw combustion air from outdoors. Many high-efficiency furnaces are designed to take combustion air from either inside the house or from outdoors. It does not have to have outdoor combustion air to be high-efficiency.

Photo: sealed combustion chamber

5. If you see a combustion air intake adjacent to the exhaust vent on the outside of the home, it is a high-efficiency furnace.There are other components that may indicate the furnace is high-efficiency, but often they are not visible.For example, furnaces with multiple heat exchangers are high-efficiency.

Note: You may hear it said that if the venting system is through a side wall, it’s automatically high-efficiency. That’s not necessarily so. Some mid-efficiency systems vent through the side wall of the home.


For the most part, high-efficiency furnaces have the same components as conventional and mid-efficiency furnaces. The condensate collection system is the only component unique to high-efficiency furnaces.

The components include:

1. Cabinet
2. Combustion air
3. Burner
4. Ignition system
5. Heat exchanger
6. Condensate collection system
7. Induced-draft fan
8. Draft-proving system
9. Control board
10. Venting system
11. House air fan

Sequence of operation

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 the inspection.
  1. Thermostat calls for heat.

  2. There is a pre-purge cycle of roughly 25 seconds to get rid of any unburned gas or combustion products.

  3.  The system checks that there is good draft. We want to make sure that the exhaust is not blocked.

  4. Hot surface igniter (HSI) warms up for 15 or 20 seconds — most modern high-efficiency systems do not use a pilot or spark ignition.

  5. The gas valve opens.

  6. The flame-proofing system verifies ignition within 2 to 8 seconds.

  7. The house air fan (blower) comes on 30 to 60 seconds after ignition. There is no fan switch that activates the blower when the furnace gets warm enough. It is strictly a timing sequence.

  8. If it is a multistage burner, the firing rate may change as the furnace operates.

  9. The thermostat is satisfied.

  10. The burner shuts off.

  11. The induced-draft fan stays on for roughly an additional 15 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.

General comments

High-efficiency furnaces are far more complex than conventional furnaces.

Typically, they require more service and are more prone to malfunction than older furnaces. Newer high-efficiency furnaces, however, seem to be more reliable than earlier generations. Many home inspectors recommend a maintenance/service contract for high-efficiency furnaces. In any case, when this series is complete, you can inspect them with confidence!

Next Month: Part II, What can go wrong?

Note: This article is an excerpt from the ASHI@HOME training program.