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Furnace does not continue to run. Cycles on and off. Troubleshooting results

Furnace does not continue to run. Cycles on and off. Troubleshooting results

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  #1  
Old 12-16-15, 01:01 AM
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Furnace does not continue to run. Cycles on and off. Troubleshooting results

International Comfort Products Model #NBF075F16A1 original equipment mounted in attic of 18 year old single story 1700sf home.

It all started when I replaced my battery powered digital TS with a Z-wave TS that requires 24Vac to operate. I connected an unused wire in my TS cable at the furnace to the C terminal of the control board and connected the same wire at the TS to the C terminal. TS worked as intended until I noticed once when walking by that the display was blank.

Upon investigation, I noticed that when the TS called for heat, the display would shut off after about 5 minutes like there was no power. After about 35-55 seconds it would power up again and the cycle would repeat. The entire time the blower is still running so everything SOUNDS normal.

At first I assumed that the TS was faulty and replaced it. The new TS did the exact same thing.

Then I hooked up a voltage meter to the R and C terminals at the TS while it was attached to the base and I noticed the voltage dropped from 26Vac to ZERO when the TS shut off. That explains the blank display.

Now since my blower still runs when this happens, I couldn't tell you if I was getting a voltage drop at R before the new TS was installed because the battery operated one always had a display. Also, the house always heated to the set temperature before and it still does even with the new TS.

So I went up into the attic to see why my voltage at the TS was dropping and this is when I discovered the heater isn't working properly.

What I observed was normal start-up: When I called for heat, the furnace vent blower would turn on, a few seconds later the burners would ignite, and after a short while the fan blower would start and everything runs like it should.

The furnace would run for about 5 minutes, but even though the set temperature has not been met, the burners would shut off. Followed shortly with the vent motor shutting down. However the fan blower continues to run.

After about a minute, the vent blower would start, the burners would ignite and the furnace would again be working. Until about 5 minutes later and the cycle would repeat.

I realized that when the furnace would shut down, so would my TS display. So the voltage loss was more than likely due to a safety feature shutting off power to the heater. Also, more than likely it was due to overheating and when it would cool down it would restart. If it was overheating, it was likely due to an airflow problem, so I began the trouble shooting:

1) I replaced the 6 month old clean filter with a new one (I even ran without a filter for a cycle) No change.
2) I checked the exhaust vent for blockage (went on the roof and removed the debris-free cap and could feel a good flow of hot air. I disconnected the lower vent pipe from the vent blower and found nothing but a few small dead insects and a small leaf)
3) I checked the inside of air intake duct from the ceiling all the way to the blower intake and it was incredibly clean with no obstructions (selfie stick on cellphone camera allowed me to see the entire distance up and over a rafter).
4) I verified that all 10 registers throughout the house are fully open
5) I cracked open the cover on the evaporator box and although I could not see the intake part of the coil, the outside was extremely clean.

So after checking all the ventilation points, I thought I could rule out airflow. I began some more troubleshooting.

1) Shorted out the R and W terminal on the circuit board to call for continuous heat. No change
2) Switched out the fan blower motor speed lead for heat from LOW (red lead) to MEDIUM (blue lead) to increase airflow (furnace would run longer and cycle off shorter, but still did it) Switched it back to LOW.
3) Tested for continuity (good) and then bypassed the Pressure switch on the vent blower. No change.
4) Tested for continuity (good) and then bypassed the high limit switch on the front panel of the heat exchanger (THE FURNACE WOULD CONTINUE TO RUN)

Not knowing if the limit switch was kicking in too soon or if indeed the temperature was too high, I only ran it for a few minutes longer than the usual cycle.

Here's the kicker. I also taped closed the safety switch on the blower motor housing door and ran the furnace with all sensors intact. With the blower door open, the furnace would run and run and not trip any sensors. I could feel the blower getting more air from the front of the door opening even though the intake duct is almost 2 feet in diameter. I actually had to turn it off after about 10 minutes because it wouldn't shut off.

So there is my problem. My furnace needs more AIR than my perfectly clean huge air intake duct can supply. How is that possible?

Again, for all I know, this is the way my heater has been operating for 18 years. I always assumed the burners were on when I heard the blower motor running because I was always feeling heat and the house would always get to the set temperature.

I only discovered this due to the new TS shutting off when the safety measures on the heater kick in, which leads me to believe it's been acting this way for a long time.

I plan to replace the limit switch to see if that cures the problem. But the actual solution sounds like more air is needed to the fan blower. However, the cycling still occurred even at a higher fan speed, so I am stumped.

Any other suggestions? (Other than calling a technician, please. This is a DIY forum)
 
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  #2  
Old 12-16-15, 05:51 AM
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If all the furnace problems started after replacing the t'stat, I would guess the new t'stat is wired wrong or is not compatible. Can you put the old t'stat back in and verify furnace operation. You don't mention why you were changing t'stat, but hopefully you can still use it. Good luck..
 
  #3  
Old 12-16-15, 06:11 AM
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I wish that I could understand the fascination with programmable thermostats. There is nothing wrong with the old thermostats & there is nothing wrong with setting them at say 69 & leaving it. People think that they are saving money & the same people will crank the AC to the hilt, in the summer & never worry about how many how much they spend. I just don't get it.
 
  #4  
Old 12-16-15, 12:48 PM
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Quite a few furnace problems are caused when people change out thermostats. Quite often, people don't use a compatible thermostat or don't read and understand the instructions packed with the thermostat and don't get the new thermostat installed correctly.


Unfortunately, the thermostat sits defenseless where people can easily see it, and it LOOKS like it would be easy to change.
 
  #5  
Old 12-16-15, 01:02 PM
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I don't blame anyone for not reading the novel I wrote, but I did cover all these points:

1) The problem was "discovered" when a new TS was installed. The TS is wired properly and works. By jumping the R and W terminal at the furnace in one of my troubleshooting tests, I bypassed the TS and the heater would still cycle even when it was calling for constant heat.

2) I replaced a battery operated programmable TS with a Z-wave TS that requires power from the furnace circuit board. Because the power from the furnace shuts off, it causes the power to the TS to be lost. Because the battery powered TS always had a display, I was not aware that my furnace was cycling until I replaced the TS.

To answer the question as to why bother replacing an old TS. The programmable TS I replaced had four programmable times that you could adjust the setback. These were fixed times only and I found myself adjusting the TS still manually when a setback kicked in and I was still home wanting to be warm. My new Z-wave TS communicates with my alarm system and can automatically set back the temperature when I arm the system in AWAY mode. So now it stays at the temp I want when I'm home and sets back the temp when I'm not. I can also program it for the nightly setback when the family is warm under the covers.

The cycling can be stopped one of two ways that I have found:

1) Jump the high limit switch terminals bypassing the switch. So I know either the switch is bad and is tripping too soon, or there is an overheat situation occurring after about 5 minutes of operation at LOW speed, and after about 6 minutes of operation at MEDIUM speed. Troubleshooting results when I swapped out the power leads to control fan speed on heat. I did not run the furnace for more than 7 minutes in case the limit switch was not bad and doing it's job properly.

2) Tape closed the blower door safety switch and run heater with access panel removed. This allowed more air to enter the blower motor and the system ran for over 10 minutes without cycling until I shut it off manually.

So, I know it's either an air flow problem or a bad limit switch. I will replace the limit switch because that is a $20 item. I doubt that will fix the problem, though.

My main question was if it is indeed an air flow problem, how do I get more air than the unobstructed intake ducting provides?
 

Last edited by rkilpa; 12-16-15 at 01:21 PM.
  #6  
Old 12-16-15, 05:51 PM
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I replaced the original high limit switch with a new one with the same 200 limit. As I expected, the old one wasn't bad so the furnace still shut down after about 5 minutes even with the new one.

Out of curiosity, I connected the HIGH speed blower motor power lead to the heat port. When the blower kicked in, the fan speed was definitely like when the A/C runs, but the unit still shut down. I went ahead and re-installed the MEDIUM speed lead for heat. The system seams to run better with this selection instead of LOW.

So I appear to have an overheat situation due to inadequate air supply shutting the furnace down. The furnace will continue to run like it should if I leave the access panel to the blower open and override the safety switch on the door panel.

What I don't know is how long this situation has been happening. For all I know, it could have been like this for years. All you can really hear in the house is the blower, and the blower still runs even after the burners shut down and continues to run while the burners re-ignite, so I was oblivious to the situation until the new TS made me aware of it.

How hard on the unit is this on/off cycle situation? Is this an urgent repair, for my unit will heat to the set temperature?

I am usually able to repair things myself, but I realize it's time to call in an expensive professional instead of throwing money at individual components that apparently are still good.

Unless there are some other suggestions out there.....
 
  #7  
Old 12-16-15, 06:09 PM
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So I appear to have an overheat situation due to inadequate air supply shutting the furnace down. The furnace will continue to run like it should if I leave the access panel to the blower open and override the safety switch on the door panel.
You do have an overheat problem and it has nothing to with the stat. That system is setup so that if any safety trips.... the blower runs to cool things off.

As long as the safety is tripped.... the 24vac to the R terminal is disconnected. This is done to alert you of a problem.

You cannot run the furnace with the high heat limit bypassed under any circumstances. Allowing the furnace to bang off the high heat limit is not good. Th constant overheating of the heat exchanger is hazardous to it.

You need to determine what the air flow issue is. It can be clogged A/C coil, too many registers closed down, blocked return(s) or clogged filter.
 
  #8  
Old 12-17-15, 12:51 PM
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<<You need to determine what the air flow issue is. It can be clogged A/C coil, too many registers closed down, blocked return(s) or clogged filter.>>



I agree. This can be a good DIY project, since the main thing you need to do is examine the air flow system of the furnace, return air ducts and warm air ducts from end to end until you identify something that is plugged up or defective.

<<How hard on the unit is this on/off cycle situation? Is this an urgent repair, for my unit will heat to the set temperature?
>>


Heh, heh! This problem usually becomes critical ON THE COLDEST DAY OF THE YEAR when you need heat the most!

In moderate temperatures, or when the system hasn't become too plugged up, the house can be heated even though the limit switch is shutting off the burner for significant amounts of time.

In VERY COLD weather, the furnace burners need to run most or all the time. That's when the house will get cold because the burners can't operate enough to keep the house cold.

I used to see this quite often during unusually cold weather.
 
  #9  
Old 01-05-16, 10:51 PM
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I have inspected the inside of the intake duct and exhaust vent and have found no blockage and everything is surprisingly clean. I have inspected the outside of the flexible return ducts and have found no kinks or collapsed ducting. I am not able to view the inside of the returns without tearing open the connections where the ducts come out of the junction boxes but with no damage visible from the outside I can assume nothing has gotten inside.

I don't think my overheat problem is from blockage.

I have a technician coming out Thursday to run some diagnostics on the gas pressure to the manifold and to test my inducer and blower motors for proper performance.

My thinking is either my flame is too hot from too much gas pressure or the motors aren't running at the correct speed to circulate the air properly.

I hate to admit DIY defeat, but I don't have the proper tools or specs to make adjustments myself.

I do have some questions from some observations I made during my inspections:

1) The high limit switch my furnace uses looks like a big watch battery that is mounted to a plate that sits flush against the heat exchanger housing. I have seen videos of other styles that are on the end of long extensions. I would think that the ones on extensions measure AIR temperature as it passes through the exchanger, and the ones I've seen have like 160 limits. The end of my sensor actually touches the metal of the heat exchanger when it is in place, as evident by a mark on the sensor that lines up with the edge of the metal it touches. Is my 200 sensor supposed to touch metal? I know I can put my finger close to a hot pot and it would be warm, but If I touched the pot I would get burned. I would think that metal that a flame is going through would easily exceed 200 normally. Is this correct?

2) When I inspected the evaporator coils to see if I could find blockage, I discovered what looks like cardboard sitting loose underneath the unit that was dry but looked water damaged. I would assume that the coils would drip some water from condensation and there are dry drain pipes connected to this section of the unit for that reason. Is the cardboard supposed to be there to catch small amounts of moisture? I would think that this could be a problem if pieces of the cardboard broke off due to dampness and got pushed downstream into the junction box causing blockage (which is one of the airflow troubleshooting areas I couldn't inspect)
 
  #10  
Old 01-05-16, 10:59 PM
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I'm not aware of any cardboard that is near the evaporator coil. Could it be part of original packing and left behind ? Can you take a picture and post it ? http://www.doityourself.com/forum/el...-pictures.html

The high limit control is not mounted on metal that is in direct contact with the flame. The 200 location is more than likely the heated air just out of the exchanger.
 
  #11  
Old 01-05-16, 11:40 PM
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Here is a shot of the cardboard I mentioned. I could only crack open the access panel to see inside, but I could reach in and feel that it was sitting loose. I was going to remove it, but I figured it was there for a reason so I left it in place and then resealed the panel.

So if the limit switch is touching metal, wouldn't that cause it to trip earlier than it should? I would think that metal would reach 200 a lot quicker than the air around the metal. However, there is no adjustment for mounting depth and if I added spacers the mounting plate would not sit flush and seal the opening. The mounting hole is right in front of a piece of metal just inside the hole so I assume it was designed this way.
 
  #12  
Old 01-08-16, 01:56 PM
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Solved

The technician didn't even need to test the gas pressure.

When I explained my two concerns from my observations, he was able to confirm:

1) The high limit switch is NOT ever supposed to TOUCH the heat exchanger.
2) There should be NO cardboard under the evaporator coils.

Problem #1):
The high limit switch is mounted directly in front of the heat exchanger. The panel it's attached to reaches just over 100 when the furnace runs and the heat exchanger reaches almost 400. When the heat exchanger temperature rises it expands. The sensor was so close to the exchanger that it would touch the heat exchanger as evident by the mark on the sensor that lined up with the heat exchanger. As soon as the 200 limit had 350+ metal touching it the switch would trip. This is a design flaw that evidently has been a problem since day one (18 years).

Solution to problem #1):
Replaced the old limit switch with the new one I already purchased. The old one still functioned, but since it had been opening an closing for 18 years it was bound to fail soon. Tweaked the metal plate where the switch is mounted to allow more clearance between the limit switch and the heat exchanger.

Problem #2):
In addition to the cardboard that I had seen under the coil, the technician discovered a plastic bag that contained the installation manual and extra installation parts that the original installer had decided to leave inside my unit. Both the cardboard and bag were actually browned from the heat of 18 years!

Solution to problem #2) Remove the trash. Don't buy another tract home.

Fortunately, since I already had the part, it was fixed for the price of a service call.
 
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