ACHRNEWS

Btu Buddy 82: Electric Heat Furnace Confusion

January 18, 2010

Bob is a service technician who is well trained and nationally certified. However, he sometimes suffers 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 something that no one else has. He recalls his long-time HVACR mentor and imagines him accompanying him as “Btu Buddy,” someone who reminds 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.

The weather was really cold and a call came in from a new customer that the system in a small office building was not heating properly. It was a new customer.

Bob started by talking to the office manager who said, “The building was nice and warm yesterday but when we came in this morning, we noticed the room temperature was down to 62°F, even though the air at the registers was warm.”

Bob went to the furnace room in a closet to have a look. He measured the air temperature at one of the registers and it was about 105°, so he went out to see what the heat pump outdoor unit was doing when he discovered it was not a heat pump, just a straight air conditioner. The system was electric heat only.

Bob went back to the furnace room for another look. He decided to open the panel to the electric heat furnace and see what was going on in the control panel. When he removed the panel, he was amazed at how many wires and terminals there were. He was becoming confused when Btu Buddy showed up.

“You look confused, Bob,” Btu Buddy said.

Figure 1. These two heater connections terminate through ceramic insulators to the control box. (Figures are from Practical Heating Technology, by William Johnson, published by Delmar Cengage Learning.) (©Delmar Cengage Learning.)

Bob said, “I don’t think that I have ever seen so many wires and terminals in a control panel. I really don’t know where to start.”

Btu Buddy then said, “When you have this much going on in a wiring panel, you just have to start with what you can understand and learn the rest as you work your way through the circuits. Can you spot the heater circuits in the panel?”

Bob said, “Yes, those are the ones with the white ceramic terminals. There seems to be five heating elements, and 10 terminals (Figure 1).”

Btu Buddy then said, “Take your ammeter and check for amperage at each side of each heater to see which ones are working.”

Bob checked on each side of each heater and found three of the five heaters were drawing current; therefore they were heating. He said, “I can see why the system was not heating the building. It is very cold outside and will probably require all of the heaters to keep the building warm. Now, I need to find out why two of the elements are not working, and there are so many wires that I don’t really know where to start.”

Figure 2. This is a ladder type diagram for the electric heating system in the article. (©Delmar Cengage Learning.)

Btu Buddy then suggested, “Let’s take it a circuit at a time. Look at the ladder portion of the wiring diagram and see what the thermostat energizes first (Figure 2).”

Bob looked at the diagram and said, “I can see that the thermostat starts the timed fan control and sequencer #1. Now what happens?”

Btu Buddy then said, “You tell me. I know that if you will look at it closely, you can see what happens next.”

Bob then said, “It looks like that sequencer #1 closes the contacts A1 to A2, which will then energize sequencer #2. Then sequencer #2 contacts A1 to A2 energizes sequencer # 3. I get it, each sequencer in turn energizes the next sequencer. If one of them does not work, it interrupts power to the remaining sequencers. Since heaters 4 and 5 are not operating, the control signal must be stopping somewhere in sequencer #3 or #4.”

“Good deductions, Bob. Now let’s see where the signal stops,” said Btu Buddy.

Bob got out his voltmeter and found that sequencer #4 had voltage going to the coil, but the sequencer was not energizing the heater.

Btu Buddy suggested that he shut the system down with the room thermostat and check the sequencer coil circuit.

Bob then asked, “Why not just shut off the breaker to the system?”

Btu Buddy explained, “If you shut off the breaker, there will be a lot of residual heat left in the heater. It is better to let the system shut down in an orderly fashion. Shut the thermostat off and let the system run until the fan stops running. The system will be cooled down normally.”

When the system shut down, Bob shut off the unit disconnect and locked it out. He then removed one of the coil wires on sequencer #3 and found the coil circuit to be open.

Bob remarked, “Here is the problem. This coil is burned out.”

Figure 3. Notice the jumper across the A1 to A2 contacts on sequencer #4, which allows power to go on to the sequencer #5 coil circuit. (©Delmar Cengage Learning.)

Bob had to go to the parts house to purchase a new sequencer so he turned to leave when Btu Buddy remarked, “Shouldn’t you leave the heat operating while you are gone? It will keep some heat in the building until you can get a part to finish the repair.”

Bob replied, “That is a great idea. When I come back, there will be at least some heat that will aid the system. Come to think of it, all of this is low voltage control wiring. I think I could put a jumper on the A1 to A2 contact on sequencer #4 and sequencer #5 would also be heating the building. If the thermostat were to satisfy, the #2 sequencer would shut off the #5 sequencer and the system would be safe (Figure 3).”

“Good thinking, Bob,” said Btu Buddy. “It always pays to give the customer all that you can for the service call.”

Bob fastened a jumper across the A1 to A2 low voltage terminals on sequencer #4 and went to the parts supply store after turning the system back on. When he returned, he used the room thermostat to shut the system down and then shut off the power and locked it out and replaced the sequencer. He then turned the power back on and used the thermostat to restart the system. All of the heaters were now working.

Bob then said as they were leaving the job, “Thanks for all of the help and suggestions. I have a couple of questions. Why did you have me check the amperage on both sides of each heater? Why is the sequencer coil circuit often called a heater circuit? Why is a sequencer used instead of a contactor?”

Btu Buddy then said, “I think we have covered enough for today. Let’s meet for coffee in the morning and I’ll answer your questions.”

Publication date: 01/18/2010