Service & Maintenance / Extra Edition

Troubleshooting Challenge: A Gas Furnace That Heats, But Not Enough

September 3, 2012

This troubleshooting problem brings you to a three-bedroom, ranch style home on a concrete slab, which, as most typically do in the particular area of the Southwest in which this home is located, employs a rooftop evaporative cooling system in the summer and an upflow forced-air gas furnace for heating in the winter. The furnace, according to the customer, isn’t heating enough because one of the bedrooms “just won’t get comfortable.” Other technicians have been there before you to check on this problem over the past three years, but the problem persists. When you arrive, you find that this air handling system employs an extended plenum with supply registers in the ceiling as shown in Figure 1.

You also note in your overall survey of this home that the return duct system is under the slab, and each room has its own return air grille. The last bedroom on the supply duct system is served by the branch shown at the bottom right of our illustration, and the customer explanation of the situation is that the system has never performed properly. You also note that some of the registers in the system have been adjusted almost totally closed in an effort to supply adequate airflow to the problem bedroom.

As you begin your evaluation of the equipment, you set the thermostat to call for heat and note the following:

• The induced draft motor starts.

• The glow-coil ignition system is initiated.

• The gas valve opens and a proper flame is established.

• The air handler starts after a short delay.

Once you have confirmed that the evaporative cooler damper system is not allowing any supply air to escape, that there is no problem with the supply duct system in the attic space, and that both the filter and the squirrel-cage blower wheel are clean, your initial check of the airflow from the supply registers confirms that the airflow to the problem bedroom is insufficient.

When you check the equipment tag, you note that the manufacturer has specified a 65°F temperature rise. When you perform a temperature-rise test, you find that the supply air temperature is 140°, and the return air temperature is 70°. As your next step, you check the wiring for the indoor blower motor, and find the wiring configuration shown in Figure 2.

Your troubleshooting question: What is the next step you need to take in order to achieve proper airflow in this system?

Compare your answer with ours by clicking on the PDF link below.

Publication date: 9/3/2012

PDF - September Troubleshooting Answer

For information on Jim Johnson’s HVACR technician training DVDs, go to

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