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Old 01-20-2014, 12:23 PM   #1
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Natural gas furnace combustion air intake question.


I guess this is kind of a physics question:

Does it require more effort for the inducer fan to move colder air through the combustion chamber of a gas furnace (i.e., more effort than warmer air)?

Thanks
Nancy

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Old 01-20-2014, 05:42 PM   #2
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Natural gas furnace combustion air intake question.


I'm not sure about more effort, but it is more efficient to pull combustion air from outside instead of using conditioned air from insdie your home as combustion air.

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Old 01-20-2014, 07:05 PM   #3
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Natural gas furnace combustion air intake question.


Thank you hvactech. Guess I should explain why the question.

Furnace: 2012, 90,000 BTU, 90+ efficiency Tappan (#FG7SC-090D) gas furnace.
Symptoms: Intermittent failure to start up (only fails in very cold weather). The inducer fan comes on, runs about 20 secs, there is a click, and then it shuts down.

Combustion air intake is from basement only. Basement has at least 10,000 cubic feet of open space (40'x35'x8'), no walls, numerous cracks around windows and doors that you can see daylight through, only a gas hot water heater down there.

Installer replaced the thermostat and inspected the condensation drainage system and now appears to have given up mainly because we could never arrange for him to be there when it fails.

Through trial and error, I think I've narrowed it down to inadequate air intake or weak inducer fan. The missing piece to the puzzle is why does it only happen when it's cold. That's the reason for my original question. Air temp at furnace below about 55 degrees seems to be the cut off point when it fails.

If you leave the door to combustion chamber off, everything works fine. Shut the door and tape the cracks around the edges and it fails at higher temps. Blinking green light and steady red light indicate pressure switch fault according to the door chart.

The unit did this on the first cold weather snap after installation. This is the second year. There are very few days cold enough to test it here. I'd like to stick with the original installer if possible. But the next time I call him I'd like to have some more information. I think he's beginning to think I'm imagining things.

Air intake has a plastic louvered cover 3.5 inches across. I'm tempted to just remove the cover and put a wide screen over it, and forget it. But maybe this would only fix the symptoms, not the problem. What do you think?


Last edited by NancyNGA; 01-24-2014 at 12:30 PM. Reason: typo
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Old 01-20-2014, 07:16 PM   #4
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Natural gas furnace combustion air intake question.


Is the flue sloped towards the furnace? It may need the intake to be moved to the same pressure zone as the exhaust. The exhaust may need to be upsized to 3". You may have a pressure switch failing.
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Old 01-20-2014, 08:24 PM   #5
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Natural gas furnace combustion air intake question.


The vent pipe goes straight up 4' to a 90 degree elbow. Then the first 15 foot run is sloped toward the furnace (lower on the furnace end by about 4 inches in 15 feet), then a 45 degree elbow, then 8 feet of pipe level with bottom of floor joists. It ends with a 45 degree elbow pointed downward outside. So at least the first part of the run slopes back to the furnace.

I just assumed the installer checked the pressure switch when he was here.

Equalizing pressure zones and moving to 3" pipe are things I didn't think about. Thanks for the ideas.
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Old 01-20-2014, 09:38 PM   #6
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Natural gas furnace combustion air intake question.


It sounds as if your exhaust venting wasn't done according to manufacturer's requirements. Saying this because it's been a problem since install.

If you still have the installation manual for the furnace read through the section on venting and verify that all size of pvc pipe, number and types of elbows and slope (the pipe all needs to slope back to the furnace along it's entire length).

It is possible there is a bad part in the furnace, new out of the box, but this kind of problem is mostly due to install mistakes.
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Old 01-20-2014, 09:41 PM   #7
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Natural gas furnace combustion air intake question.


Read the install manual. Most furnaces nowadays are approved for 1 or 2 pipe install and will work both ways. I doubt that is the problem. Check the manual for the pipe size and equivalent length. 90,000 BTUs I doubt will use 2" so 2.5" which is rare and probably 3" is what is required. Just because it comes out of the furnace as 2" does make that correct. It may need a adaptor and then go to 2.5 or 3 "
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Old 01-20-2014, 10:13 PM   #8
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Natural gas furnace combustion air intake question.


Are you talking about condensate forming in the vent pipe from improper backflow, or obstructions in the pipe?

If the vent (exhaust) pipe were too small, or obstructed, then why would opening or cracking the combustion chamber door allow the furnace to go through the proper cycle and fire? It seems like opening the door would at best make no difference, or even possibly make the problem worse. Probably I don't understand how pressure switches work well enough.

Thanks for the discussion. I'm learning a lot and I appreciate it.
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Old 01-20-2014, 10:20 PM   #9
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Natural gas furnace combustion air intake question.


Condensation can occur but you would need some pretty good dips in the pipe for it to be a problem and usually that is not temp related. Pressure switches are finicky and opening the door may make just enough difference to keep it on. Make sure the venting is done properly first. He can check the suction on the switch with a manometer if he knows how and has one. Those pressure switches are cheap and made offshore so yeah it may be faulty. 8' of level pipe is not usually a problem, once it gets over 15-20 feet then it may be.
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Old 01-21-2014, 01:09 AM   #10
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Natural gas furnace combustion air intake question.


It seems that Nordyne includes several brands (Tappan, Frigidaire, Westinghouse,...) and each has a furnace with the same model number (FG7SC 090D). After *MUCH* web searching I found a generic Nordyne installation manual, downloaded it and now can't find the link to post.

Anyway either 2 or 3 inch vent pipe is allowed. The specs for 2" are max 60' run with one long radius elbow. Subtract 2.5 feet for every additional long radius elbow, 5 feet for every short radius elbow and every pair of 45 degree elbows. That leaves at least 45 feet.

It also says: "Always use the same size or a larger pipe for combustion air that is used for the exhaust vent."

That louvered cover on the intake surely restricts the flow to less than what a 2" pipe would have. I'm taking it off unless someone tells me not to.
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Old 01-21-2014, 08:15 AM   #11
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Natural gas furnace combustion air intake question.


I forced the furnace to fail at 34 degrees (57 in the basement) this morning by taping the cracks around the door again last night. Blinking lights code said open pressure switch fault. Removed the inlet cover and it fired up the next cycle. It's supposed to get down to 19 Thursday. If it still works at that temp, I'll call the installer, see what he thinks (and maybe ask if they can make a less restrictive cover for that hole).

Thanks for forcing me to find the installation manual and to keep trying.

If someone thinks it could be something else please chime in. I know this could just have been a coincidence.

Last edited by NancyNGA; 01-21-2014 at 09:08 AM. Reason: Added some info
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Old 01-21-2014, 02:56 PM   #12
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Natural gas furnace combustion air intake question.


The entire flue pipe must slant back toward the furnace.

As your basement gets colder, you are getting more condensate in the level flue pipe section. It restricts the flue gasses and the pressure switch opens.

By removing the burner cover, you are making it easier for the inducer to pull more air. So it has a stronger push.
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Old 01-21-2014, 03:25 PM   #13
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Natural gas furnace combustion air intake question.


You may be able to loosen the straps on the level piece and shove a shim in there to give it some slope. I would.
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Old 01-22-2014, 08:42 PM   #14
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Natural gas furnace combustion air intake question.


Quote:
Originally Posted by beenthere View Post
.....As your basement gets colder, you are getting more condensate in the level flue pipe section. It restricts the flue gasses and the pressure switch opens.

By removing the burner cover, you are making it easier for the inducer to pull more air. So it has a stronger push.
That makes sense, Beenthere. Tomorrow I think I will try to run an electrician's fish tape through the level section of the vent pipe to see if I can detect condensate or other debris.

A little more information. Tried to repeat the test for insufficient intake volume last night by taping the furnace door and replacing the louvered cap again. This morning the condensate tubing had frozen outside. Pump lights blinking. This never happened before. Thawed out the tube and about a gallon of water ran out. Furnace kicked on immediately and has been working fine for 18 hours in spite of temps below 35 outside all day.
 
I just viewed a video where a Nordyne tech said if your condensate tubing extends below the level of water inside the pump, it constitutes double-trapping, which can cause the pressure switch to open either at start up or during a cycle. Not clear what that means. I'll have to think some more about it.

Coincidence, or another piece of the puzzle?

I suppose it could be that little experiment caused the furnace to fail during the night and that's why the tube froze. Or maybe not. Guess I'm going to have to get a cot and sleep next to the furnace to see what goes on during the night.

Last edited by NancyNGA; 01-22-2014 at 08:52 PM.
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Old 01-22-2014, 11:50 PM   #15
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Natural gas furnace combustion air intake question.


Quote:
Originally Posted by NancyNGA View Post

That makes sense, Beenthere. Tomorrow I think I will try to run an electrician's fish tape through the level section of the vent pipe to see if I can detect condensate or other debris.

A little more information. Tried to repeat the test for insufficient intake volume last night by taping the furnace door and replacing the louvered cap again. This morning the condensate tubing had frozen outside. Pump lights blinking. This never happened before. Thawed out the tube and about a gallon of water ran out. Furnace kicked on immediately and has been working fine for 18 hours in spite of temps below 35 outside all day.
 
I just viewed a video where a Nordyne tech said if your condensate tubing extends below the level of water inside the pump, it constitutes double-trapping, which can cause the pressure switch to open either at start up or during a cycle. Not clear what that means. I'll have to think some more about it.

Coincidence, or another piece of the puzzle?

I suppose it could be that little experiment caused the furnace to fail during the night and that's why the tube froze. Or maybe not. Guess I'm going to have to get a cot and sleep next to the furnace to see what goes on during the night.
If I was the servicing /trouble shooting it id probably open the basement and just try firing it up for a minute with the flue pipe off to see if it would run, then you could eliminate it from the puzzle.
If it still acts up with the flue off id check inducer motor for dead birds, also check pressure switch if he didnt already.
As long as the hole on top of the furnace is open for combustion air I dont imagine it should be a problem on that side of it anyway, like has been said previously people have been running 1 pipe systems on 90 plus furnaces for years.

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