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  1. #27
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    Apr 2002
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    11,808
    Actually I hope that is a net stack temp
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

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  2. #28
    Join Date
    Feb 2005
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    1,475
    I should have been more clear you want to it where it would not be effected by any wind currents blowing in the direction of your door, what you are trying to accomplish with the kleenex is to determine a slight in difference in pressure between your home and outside you should expect to see the kleenex blow towards the outside if it is blowing into the house without being affected by wind currents then your house is in a negative pressure condition which can cause a lot of comfort issues including the short cycling of a furnace and insufficeint combustion air for your gas appliances, its just something to check it may or may not mean a whole lot in the final analysis its just that your problem is a little odd what we do know as fact is that flue gases condese at 140 degrees so at some point between where you took the stack temp and where it starts to condensate your flue pipe is getting cold enough to bring the temperature of the flue gases down, you said that the gas pressure was turned down I can only assume that the gas pressure was either to high or it was set right and then turned down to compensate for short cycling if it was done for the short cycling your furnace just got a lot smaller as far as firing rate is concerned and now your flue is probably really undersized. I'm going to go with the post that your vent may be to large or to long for your furnace.

  3. #29
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    Apr 2002
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    11,808
    So if it is too long then it modulates the furnace?
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

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  4. #30
    Join Date
    May 2001
    Location
    Kansas City
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    2,729

    Talking

    Make sure when doing the kleenex test to hold it half way up so the warm air leaving and the cold air coming in doesn't move it!
    The furnace is underfired, High O2-low stack temp-too much excess air. The furnace is getting too much cumbustion air for the amount of gas being burned. The gas pressure needs to be turned up, you have to use a test gauge to set it at proper pressure. Might even be able to check for negative or posative pressure with a gauge like that. I would think if the tech had a combustion analizer he should have a magnehelic or manometer to set gas pressure.

    [quote]So if it is too long then it modulates the furnace?

    Only on a Rheem.

    [Edited by rsmith46 on 02-17-2005 at 01:01 AM]

  5. #31
    Join Date
    Nov 2000
    Location
    Indiana
    Posts
    318
    After I posted this, I realized it was intended to be inserted into the conversation much earlier. Oh well.

    Noel addressed what temp rise is. The temp rise can be adjusted by changing blower speed. If the blower is moving too much air and the temp rise is at the lower end of the nameplate rating, that is contributing to the condensation problem.

    Noel also addressed the high O2. But there could be more to this. Vent size and configuration can play into both high O2 and condensation problems. This problem can be a little tough to address via this venue as you could easily have two problems that share symptoms but require different fixes. Fixing a short cycling problem by underfiring the furnace is not the way to go. I might consider addressing the short cycling problem and then the condensation problem if it is still there. Find someone who can look at the problem in a more holistic fashion.

    [Edited by 7X on 02-17-2005 at 09:22 AM]

  6. #32
    Join Date
    Dec 2004
    Posts
    21
    thank you all for overwhelming responses from all of you.
    From all your analysis, I have the following.

    1. High O2 clearly shows that it is underfiring. Underfiring could result in low stack tempearture and condensation. Right?

    2. I am not sure whether I am still having short cycling. I set my stat at 66F and When I monitered it, the fire stays on for only 3 minutes and the blower continues to run for 2 minutes after the fire. so the total cycle is 5 minutes to keep the 66F. Is this normal or still short cycling?

    3. Eddy mentioned that the furnace might be short of combustion air. But, rsmith46 observed that "The furnace is underfired, High O2-low stack temp-too much excess air. The furnace is getting too much cumbustion air for the amount of gas being burned".
    Does too much excess air mean that the furnace is getting more combustion air? Could someone clarify this?

    Thanks again for your support.


  7. #33
    Join Date
    Sep 2002
    Posts
    367
    What we have seen in the past if you had a crack in the heat exchanger usually you could tell because the CO levels are elevated when the blower comes on. I agree that the oxygen levels will also be elevated.

  8. #34
    Join Date
    Apr 2002
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    11,808
    Originally posted by sanrishi
    thank you all for overwhelming responses from all of you.
    From all your analysis, I have the following.

    1. High O2 clearly shows that it is underfiring. Underfiring could result in low stack tempearture and condensation. Right?

    2. I am not sure whether I am still having short cycling. I set my stat at 66F and When I monitered it, the fire stays on for only 3 minutes and the blower continues to run for 2 minutes after the fire. so the total cycle is 5 minutes to keep the 66F. Is this normal or still short cycling?

    3. Eddy mentioned that the furnace might be short of combustion air. But, rsmith46 observed that "The furnace is underfired, High O2-low stack temp-too much excess air. The furnace is getting too much cumbustion air for the amount of gas being burned".
    Does too much excess air mean that the furnace is getting more combustion air? Could someone clarify this?

    Thanks again for your support.

    1) Have the furnace clocked. This means have the tech temporarily shut of the gas to all other gas burning appliances, then run the furnace and time how fast the meter spins, to see how much gas the furnace is actually burning.

    The furnace is induced draft. This means a fan sucks heated products of combustion through the heat exchanger and blows it up the chimney. This fan draws in the combustion air. So the amount of air is fixed.

    When it is underfired then, the normal amount of combustion air is too high.

    2) Short cycling. Your furnace does not sound like it is operating normally. In cold weather it should run for longer period of time.

    If a furnace is grossly oversized it does not run long to warm your house up. I doubt this is the situation in your home as your furnace is small compared to what most American HOs post about.

    If it was mild outside there would not be much heat demand, so the furnace would run for a relatively short time, then be off for a relatively long time.

    If the furnace comes on runs for a few minutes, shuts off for a few, fires back up for a few minutes, then off again then there is most likely a problem with the thermostat.

    Possible causes

    A) Wrong anticipator setting. If you have just installed a new digital thermostat and this short cycling started, read the manual and see if you are supposed to flip some switches in the back if you have electric or gas heat.

    B)Thermostat is being influenced by cold drafts. If the home is under negative pressure then cold outside air is being drawn into the home and contacting the thermostat.

    Is there an unsealed hole for the wires to pass through the wall to the thermostat? If so, perhaps there are holes in the top plate for wires to pass into the attic as well. Cold air could be drawn down the attic and then out through the hole in the wall behind the thermostat.

    It could be cold drafts in the room as well.

    C) Thermostat mounted on an exterior wall. Exterior walls are cool and will trick the thermostat.

    E) I think you said it was a digital thermostat so you should not have a problem if it is not level.

    3) See number 1

    4) What is the diameter of the chimney, and how high is it from the vent connection on the furnace to the chimney cap. What other appliances and BTU ratings are going into this chimney. Is there a 38,000 Btu/hr Water Heater Connected.

    What is the actual BTU input of your Furnace is it 80,000 input, 85,000 input.

    If the chminey is larger than 5.5 inches on the outside then it is most likely oversized, has too much surface area making it easy for it to lose heat and condense acid rain.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

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  9. #35
    Join Date
    Dec 2004
    Posts
    21
    Carnak:
    Thanks a lot for your detailed response.
    I have checked my thermostat(Honeywell programmable model:3500/3595) user manual and found nothing to be adjusted. The cycle rate is preset for gas furnace(6 cycles). Also, There is a fan switch only for electric furnace.

    I have to check for cold draft influencing the thermostat.

    You are right. I have 38000 BTU water heater connected to the same flue pipe. The furnace input is 67000 BTU. The b-vent pipe is 5 inch in diameter. The vent connection is probably around 25 ft from furnace to cap

    Thank you.

  10. #36
    Join Date
    Apr 2002
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    67,000 is an odd input, I thought it was a full fire output. If, in fact, the rated input of this furnace is 67,000 then the combined input going up the 5 inch B-Vent is then 67,000 + 38,000 = 105,000.

    Looking at Table 11-7 of 1996 NFPA 54 says that the furnace input is too small to use a single wall vent connector from the furnace to the B-Vent chimney.

    Therefore the connection from the furnace to the chimney must be double walled.

    Table 11-6 of the 1996 NFPA 54, and for a 105,000 combined input, and 25 ft high it should be a 4 inch B-Vent.

    The vent connector from the furnace to the B-Vent Chimney should be 4" B-vent as well.

    Perhaps you have a single wall vent connector. I noted an air temp of 51F in the tech's readings so it is possible that significant heat is being lost through a single wall connector in the 'cool' basement.

    Maybe you could use the old 5 inch B-vent as a chase and pull a four inch liner or B-Vent down it. Call the local gas authority to see what you can do. Have some one familiar with the codes enforced in your area take a look, in particular some one current on the venting tables.

    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

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  11. #37
    Join Date
    Apr 2002
    Posts
    11,808
    Originally posted by sanrishi
    Carnak:
    I have checked my thermostat(Honeywell programmable model:3500/3595) user manual and found nothing to be adjusted.Thank you.
    Check System Type Setting. Feature Number 4, page 12 of the following link

    http://hbctechlit.honeywell.com/requ...m?form=69-1199

    Should be set for "6". If all else fails, as an experiment try "3"
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

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  12. #38
    Join Date
    Nov 2003
    Posts
    138

    sizing

    I agree with Carnak on the smaller vent and use B-vent for the connector. Change the T'stat location for a longer cycle. Check the indoor Rh%--soggy house =soggy flue.
    consider building an isulated chase in the attic to keep the pipe warmer. If they don't have a gas DHW heater, I'd consider putting one in. This will but down on "wet time" and add a standing pilot flame to keep the stack warmer. He should easily be able to handle a 40K BTU/ 40 gal. water heater with this 5" B-vent.

  13. #39
    Join Date
    Dec 2004
    Posts
    21
    Carnak:
    thank you for your replies. Right now, the stat is set to 6 cycles. I will try changing it to 3. I am positve that the vent connector from the furnace is double wall. When the furnace runs, I can't touch the flue pipe from the furnace with my bare hands since it is always very hot. Does it say anything? If it is double wall, Should the pipe becomes very hot or not?

    chimney doc:
    Do you want me to add a heater in my attic to keep the attic warmer?

    thank you all

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