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

Inspecting High-Efficiency Gas Heating Systems


What Can Go Wrong

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

In part I, we defined high-efficiency gas furnaces, discussed how to identify them, and reviewed their components and sequence of operation. In part II, we discussed what can go wrong, and in part III we’ll continue to cover what an inspector needs to be aware of when inspecting a high-efficiency gas heating system, beginning with the control board.

Control board

The control board is the electronic brain of the furnace. It is a complex and expensive black box that typically costs several hundred dollars to replace. Watch for evidence of water leakage onto the control board, which, unfortunately, often is located in the fan compartment immediately below the condensate collection system and induced-draft fan.

Photo: Control Board

Venting system

The ABS or PVC venting system can go up through the roof or out through the wall. Each manufacturer has specific rules for its venting system. Generally speaking, the vents point straight out through the wall or point down. Some manufacturers call for the exhaust to be 12 inches above the combustion air inlet. Vents should not be tucked under roof overhangs because the warm, moist exhaust can cause water damage. The vent should not be located at the inside corners of buildings because airflow may be obstructed.


The vent should not be obstructed or close to the ground, but there is no need to worry about combustible clearances because the temperatures are very low.

Horizontal venting systems should slope slightly back and down toward the furnace so the condensate will not freeze at the discharge end. The condensate should run back and down to the furnace, where it can be carried away by the condensate collection system.

The manufacturer determines the length of the vent system. Most fans have to be at least 15 feet long. Vents that are too short may allow water accumulation at the vent termination, which can cause a freeze-up.

On the other hand, if the vent is too long, the exhaust may be obstructed. We question many vents that are more than 40 feet in cumulative length. We also question systems that have more than three or four elbows. We think of elbows as adding about five equivalent feet to the vent length.

Vents should terminate several feet from gas regulators and mechanical air inlets. They should be at least a foot above grade level and at least a foot from windows, and should not terminate above walkways because ice may build up on the walkways, which can be dangerous. Local rules and manufacturers’ requirements may vary.

Vents that pass through unheated areas, including attics, should be insulated.

House air fan

There may be nothing unique to high-efficiency furnaces with respect to these fans. Nevertheless, if the fans are multiple speed, they may be expensive. High-efficiency ECM-driven fans use much less electricity than conventional fan motors, but can add roughly $1,000 to the cost of the furnace.
By the way, some induced-draft fans also are ECM types. ECM stands for Electronically Commutated Motor.

There is a balancing act with high-efficiency furnaces. We have to determine that the fan is moving the correct amount of air across the heat exchanger. If the fan is too fast, the temperature rise of the house air will be too low. This may cause comfort issues and condensation in the primary heat exchanger. If the fan is too slow, the temperature rise may be too high, which can overheat the heat exchanger. The cleanliness of the air filter plays a role in this as well.

Typically, high-efficiency furnaces have the permitted temperature rise stamped on the data plate — 45 to 75°F is common. A temperature rise outside of this range may shorten furnace life and void the warranty.

Excess temperature rise is a common problem when a conventional furnace has been replaced with a high-efficiency furnace. Conventional furnace ductwork, both supply and return, is often too small and restrictive for a high-efficiency furnace. It may be difficult to get adequate airflow across the furnace heat exchanger(s). This can result in excess temperature rise, which is hard on the furnace and may void the warranty.

When inspecting the house air fan, listen for excess noise, check the airflow at supply registers and return grills; check the temperature rise, and make sure filters are in place and clean.

Photo: Temperature rise is indicated on the data plate.

Note: Sometimes, it is hard to detect airflow at return grills. We use a facial tissue or piece of toilet paper to make sure air is moving.

Safety devices

Safety devices include limit switches, rollout switches, air-proving switches (differential pressure switches), flame sensors and a blower cover interlock.

A professional home inspection does not include testing safety devices. Nevertheless, we should understand their function and know where to find them.

The limit switch shuts off the furnace on high temperature. Most limit switches reset automatically. Typically, they are located near the burner or the top of the heat exchanger. Some furnaces have a secondary limit switch in the fan compartment of horizontal or upflow furnaces because that's where the heat is likely to collect.

The rollout switch or spillage switch detects flames coming out the front of the burner opening (vestibule). This will shut off the furnace. These may require a manual reset.

Photo: The top arrow indicates the flame rollout swtich and the bottom arrow indictes the limit switch.

The air-proving switch or, as it is sometimes called, the draft-proving switch, ensures there is airflow from the intake out through the vent termination. This was discussed previously. Differential pressure switches perform the same function.

The flame sensor replaces the thermocouple used on older conventional furnaces. It is an electronic device that responds quickly to flame. If it doesn't see flame at the burner, it will shut off the furnace within two to eight seconds of startup. Flame sensors use flame rectification, converting AC voltage to DC and generating a current.

The blower door interlock is a safety device that shuts off the furnace when the door is removed from the blower compartment.

Photo: Air-proving switch

Photo: Flame sensor

Photo: The red arrow indicates the blower door interlock.

Life expectancy of high-efficiency furnaces

The ASHI Standards of Practice and Code of Ethics asks us to identify components that are near the end of their life expectancy. Our research suggests that the normal life expectancy for a typical high-efficiency gas furnace is 15 to 20 years. We suggest that any high-efficiency gas furnace that is more than 13 years old may be considered near the end of its life expectancy.

How can you tell how old the furnace is? Often, the manufacturing date may be obvious on the data plate. If not, you have to decipher the serial number. There are books, such as the Carrier Blue Book (no longer widely available), Preston's Guide and the CDW Technical Reference Guide that can help do that.

Closing comments

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. More complex than conventional furnaces, typically high-efficiency furnaces require more service and are more prone to malfunction than older types of gas heating systems. Even though newer models of high-efficiency systems seem to be more reliable than earlier generations, many home inspectors recommend a maintenance/service contract to their clients.

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

Read Part 1 of this article here. Read Part 2 of this article here.