This troubleshooting situation has a history. In the six years that the up-flow gas furnace in this three bedroom house has been in service, several attempts have been made to solve the problem of one bedroom that, according to the customer, is always too cool during the heating season. This residence is a brick home on a slab, and the air handling system, which located in the crawl space above the ceiling, is shown in Figure One.
The problem bedroom is served by the supply system branch at the lower right side of our illustration. Each room has its own return grille and the return duct system is under the slab. In your evaluation of the equipment, you note that on a call for heat, the ignition cycle operates properly, and the air handler starts on a normal time delay. You also note that some of the supply registers in other rooms have been adjusted to almost totally closed position in an effort to provide adequate air flow to the last branch of the duct system.
After adjusting the supply registers properly to accomplish a better balance of air flow in the system, you check the equipment tag, and it shows that the manufacturer has specified a 65-degree temperature rise through the furnace. When you perform a temperature rise test you note that the return air temperature is 70-degrees and the supply temperature is 140-degrees. You also check the wiring for the indoor blower motor and find the wiring configuration shown in Figure Two.
Your troubleshooting question: What is the next step you need to take before re-evaluating the temperature rise of this system?
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Publication date: 1/8/2018