ACHRNEWS

Btu Buddy 84: Condensing Furnace Problems

March 22, 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 first call of the day already had Bob confused. It started with a call from the dispatcher who described a gas heat customer who had no heat. It was a new installation and the customer was upset because the new system was not heating. It was up to Bob to make the customer as happy as possible. The weather was really cold, 10°F at night and only 20° during the day.

Bob arrived to find an angry housewife. It was hard to be polite, but he had to. He assured the woman that he would do all in his power to get the system started quickly. Meanwhile, he recommended that she take the two children to the kitchen and put a pan of water on each stove eye to boil. This would keep the electric cooking elements from getting too hot and put some humidity in the air. He then went to the furnace in the garage.

When he got to the garage and the furnace, the first thing that he noticed was that the furnace was cold. It had not been operating. He decided to check the power supply and found that he had 24 volts at the low voltage power supply. This proved that both the low and high voltage power was on. He then checked from the common terminal to the “W” terminal to see if the thermostat was calling for heat and it was not. Now Bob was sure that the thermostat was defective.

Bob then went to the thermostat and removed it from the sub-base. He then jumped from the “R” terminal, which is the hot terminal, to the “W” terminal for heat. He was expecting the heat to come on so he went to the furnace and was surprised the furnace was not running. He was standing there confused when Btu Buddy arrived and said, “What is the problem, Bob?”

Figure 1. This wiring diagram shows where a jumper can be applied to temporarily operate the furnace. Don’t forget to remove the jumper. It is recommenced to use a long jumper that will be obvious so you won’t forget it. (From Refrigeration & Air Conditioning Technology, 5th Edition, by William Whitman, William Johnson, and John Tomczyk, published by Delmar Cengage Learning.) (©Delmar Cengage Learning.)

Bob explained what he had done and Btu Buddy said, “Why don’t you jump from ‘R’ to ‘W’ on the furnace (Figure 1)?”

Bob then said, “When I get confused, my thinking ability seems to stop.”

Bob jumped the terminals and the furnace started.

“What does that tell you, Bob?” asked Btu Buddy.

Bob said, “It tells me that the furnace will operate and the signal telling it to operate is not getting through. It has to be in the field wiring somewhere. I am going to let the furnace run while I try to figure it out.”

Bob then said, “I know the problem is in the field wiring. I think I will trace it out and try to find the problem by just observing.”

Bob then started with the wire that connected to the “W” terminal and followed it. It went for a ways and then went up in the wall towards the room thermostat. There was no visible problem.

He then started following the wire on the “R” terminal. It went to the other side of the furnace to a terminal on the condensate pump. He turned to Btu Buddy and asked, “Why would the wire go to the condensate pump? Also, the pump seems to be running. What is that all about? I thought that pump was for the air conditioning only.”

Btu Buddy then explained, “I thought you would hear that pump running when we walked in. Probably you only had your mind on why the furnace was not running and did not notice what else was happening.

Figure 2. This is a cutaway of a condensing furnace. Notice the last heat exchanger is in the return air so it can remove the most heat from the flue gases. This will condense water out of the flue gases that must be piped to a termination point. (From Refrigeration & Air Conditioning Technology, 5th Edition, by William Whitman, William Johnson, and John Tomczyk, published by Delmar Cengage Learning.) (©Delmar Cengage Learning.)

“This is a condensing furnace and it actually generates condensate. It is a 96 percent efficient furnace. It gets some of that extra efficiency by running the flue gases through an extra coil that is mounted where the cool return air passes over it (Figure 2). This cools the flue gases enough that the water in the flue gas condenses. This furnace makes enough condensate that it must be dealt with like you would with air conditioning condensate. This system has to pipe the condensate outside since the furnace is below ground level. Now you need to find out where the condensate pump line terminates.”

Bob followed the condensate line to the end and said, “The line terminates about four feet from the house under a bush.”

Btu Buddy then asked, “Is there any water on the ground?”

Bob said, “No, everything is frozen including the condensate line. I am going to cut it off closer to the house and see what happens.”

Bob then said, “Wow, water is rushing out of the line.”

Btu Buddy said, “Now take your jumper off of the furnace terminal board and see what happens.”

Bob removed the jumper and said, “The furnace is still running. I don’t get it.”

Figure 3. This shows a condensate pump that may be used for air conditioning and heating to move the condensate to a proper drain. (From Practical Cooling Technology, by William Johnson, published by Delmar Cengage Learning.) (©Delmar Cengage Learning.)

Btu Buddy explained, “The condensate pump has two float switches in it. The first one is supposed to start the pump. The second switch is wired into the hot side of the control wiring and interrupts the signal to the thermostat and neither heating or cooling will start. That is why the pump was running but the furnace would not start. It also prevented condensate from getting all over the floor. When the outlet froze solid, the next time it called for pumping out the pump reservoir, the pump started and just ran, but did not pump. It is a centrifugal pump so no damage occurred (Figure 3).”

Bob told the homeowner what had happened and she was relieved and thankful that it wasn’t a whole system failure.

As they were leaving, Bob said, “That was another brain twister. What should I have noticed first to shorten the call?”

Btu Buddy said, “The pump running was the first sign. It doesn’t make much noise, but it shouldn’t have been running all of the time. You really did the right thing by keeping the furnace running while figuring out the problem. It helped make the homeowner more comfortable as the service procedure was figured out.”

Bob then said, “Thanks again. You can’t teach me how to think, but your guidance sure helps.”

Publication date: 03/22/2010