Btu Buddy 144: Fan Motor Problem for a Gas Furnace
Bob is a service technician who is well trained and nationally certified. However, he has sometimes suffered from the same confusion that all technicians occasionally do — the facts that he gathers may or may not point to the obvious cause of the problem or the best solution. But Bob has had something that no one else has. He recalled his long-time HVACR mentor and imagines him accompanying him as “Btu Buddy,” someone who reminded him to take time to stop and think before rushing to judgment, helping keep him on the right track, even with facts that are confusing.
Now, Bob’s company has promoted him to help train a new employee, right out of a school specializing in HVAC, just like Bob was. Bob is now Tim’s Btu Buddy. Tim is anxious to travel with Bob. Tim realizes that he is right out of school, with the theory and lab work that he accomplished in school, but still needs help. He knows that he worked with many of the components of the systems in the school, under ideal conditions with good light and air conditioning. Now it is into the field, sometimes under the house with poor lighting, or out on the rooftop in the sun, where the real action is. He is naturally and normally reluctant, but he has Bob to help guide him.
Bob and Tim were on their way to a no heat call for a gas furnace. Tim asked, “What do you think we need to look out for on this call?”
Bob said, “It could be any number of things. We will start by talking to the homeowner and see what she knows.”
They arrived and were listening to the housewife and she said, “We were gone on vacation all last week and when we came home the house was cold. We had turned the thermostat down to 55°F while we were gone, and didn’t expect the house to be warm. After about two hours last night we didn’t feel the house getting any warmer, and we started looking around. We felt some heat rising from the air outlet in the den, but it didn’t seem like the fan was blowing. My husband turned the heat off. He said that he was afraid there was a problem.”
The house was equipped with air conditioning with a gas furnace. Bob said, “Let’s go to the thermostat and turn the fan to the ‘on’ position and see if the fan will run.”
They tried the fan and it didn’t come on so Bob said, “Well that tells us that we must have a fan problem. Let’s go to the furnace. It is in the basement under the house (Figure 1).”
They set the thermostat to call for heat and went under the house when Tim said, “I hear the furnace burner, but no fan.”
Bob asked, “What do you think the problem could be?”
Tim said, “It sounds like a defective fan motor to me. If the ‘fan on’ switch didn’t bring the furnace on and the furnace fan switch didn’t start the fan, I believe the fan motor must be the problem.”
When they got to the furnace, Bob turned off the furnace at the disconnect switch and said, “The high-limit switch is going to turn the furnace off, so I am just turning it off to keep the furnace from overheating.”
Tim asked, “The owners were out of town and the furnace must have been cycling on and off for no telling how long. How long will a furnace high-limit control last under those conditions?”
Bob responded, “High-limit switches are made to be able to cycle thousands of times without failing. It is called a duty cycle. The design of the switch is made to be reliable and their duty cycle is tested to ensure long life and safety.”
Tim then said, “We have determined that the fan motor will not run, but we have not determined why.”
Bob said, “Let’s remove the fan door and check the fan. We’ll turn it by hand and see if the fan motor will turn freely.”
Tim removed the door and tried the fan and it turned easily.
Bob then said, “Let’s check the voltage to the fan motor leads to make sure voltage is getting there.”
Tim said, “There are a lot of wires going to the motor. Which ones should we check (Figure 2)?”
Bob said, “All of the wires have a purpose. There are:
1. Two brown wires — those attach to the capacitor.
2. A white wire that is the neutral power wire on a gas furnace, because it is 115 volt.
3. A black power wire that will attach to the circuit board or relay that will distribute the hot power wire to one of the other colored wires to choose the fan speed, summer or winter.
4. The other colored wires run the motor in Hi-Med-Lo speeds. Typically, the high-speed wire operates the fan in the cool mode. That leaves two choices for heat, medium speed or low speed. One of those wires will be used and the other wire will be tied off and not in use.
“This is winter so the wire that is most likely energized is the low or medium speed wire.
“Place one meter lead on the common white wire using alligator clips and the other meter lead on the low or medium speed wire terminal that operates the fan for winter.”
Tim said, “I am beginning to get the picture.”
He fastened the other meter lead to the wire on the circuit board that said “lo-fan.”
Bob turned on the furnace switch and the burner started and the furnace began to heat up. After some time passed, the fan didn’t start and Bob asked Tim, “Did power ever come through the circuit board to the fan motor leads?”
Tim said, “No, I believe we have a defective circuit board.”
Bob said “It sure looks like it. Let’s tell the owner we need to go to the supply house and we’ll go get one.”
They returned from the supply house with a new circuit board and installed it.
Bob said, “Tim, turn the power on and let’s see what happens.”
They turned the power on and the burner started and after about three minutes the fan started.
They went upstairs and the homeowner was really pleased to have heat.
While riding away, Tim commented, “That was another learning experience. That keeps up my interest in this career. There is always something new.”
Publication date: 3/23/2015