Weird noise from older Lennox furnace

Discussion in 'HVAC Heating & Cooling' started by Stampede, Apr 30, 2021.

  1. Stampede

    Stampede New Member

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    Hi:
    I have an older Lennox G803-120-1 furnace that's about 38 years old. Everything is still fine - fan, motors, pilots, burners are all running smoothly. One problem I have since two years ago is that during the Spring season, there is an occasional loud rattling/grinding mechanical noise that coming from the chimney of the furnace. The noise is not coming from the actual furnace but up in the chimney/ This happens when the furnace isn't even running. I have even taken this video to the Lennox distribution warehouse/office and nobody there knows where the periodic noise is coming from. Someone there suggested maybe it's coming from a bird, but as you can hear this is a mechanical noise. I have tried disconnecting the humidifier off but the noise still comes and goes. It would go away for 1 year, several months or weeks and then coming back again. Funny thing is I have never heard the noise during winter when the furnace is on most of the time. I also noticed that when the noise say comes in for 4 to 5 reps, it would go away if I turn up the thermostat by 1 degree to activate the furnace. Heat comes on and the noise goes away. The noise is not constant. It would just comes and goes for 4 - 5 rep in between days, weeks, months. This started in 2019 periodically, none in 2020 and is now just happening again. Any idea? please help. the first noise comes on at 25 seconds of the video then at 50 seconds.
    https://photos.app.goo.gl/k2dUm7wFGLR21bxM7
     
  2. fitter30

    fitter30 Well-Known Member

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    Retired service tech
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    That noise sounds like a relay chatter or gas valve (if the valve is slow opening might not even start to open). Have you ever put your hand on different components to feel the vibrations.
    A relay that would have a load and chatter would have burnt up the contacts.
     
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  4. Stampede

    Stampede New Member

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    Thanks Fitter30 for your advise. This noise comes and goes. Last time it happened was 2 days ago, before that 13moths ago. I find that the moment I hear it and if I immediately turn up the thermostat by 1 degree to activate the furnace, it would go away. The noise doesn't seem like it's coming from the actual furnace but seems like somewhere higher up in the furnace chimney. The furnace is 38 years old. I see two manual dampers in the basement and there is no control panel on the furnace for any motorized dampers, so I am really at a lost. Other than this weird periodic chatter/rattle, the furnace works fine. Turns on and off as expected and the fan/motor run fairly smoothly. See video from yesterday while the furnace is running, it sounds pretty smooth to me.
    https://photos.app.goo.gl/8Po4iBkqS9AzbSCj7
     
  5. Stampede

    Stampede New Member

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    Also, this noise never comes on during Winter from October to March when the furnace is on most of the time. Only happens in Spring and Summer when the heat seldomly comes on.
     
  6. fitter30

    fitter30 Well-Known Member

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    Pull the 24 volt wire powering the burner control & tape the end. See if the noise stops. If it does power it back up pull the thermostat w /white wire if it noise stops. These two steps will will eliminate two possible problems. Need help locating wire for burner control take a pic of the wiring diagram inside blower door.
     
  7. Stampede

    Stampede New Member

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    I actually had a HVAC professional come out to investigate this weird strange periodic rattle/chatter. After 2 hours of checking all the dampers and connections and even went up to the chimney. He couldn't offer an explanation. So mystery continues.......
     
  8. Stampede

    Stampede New Member

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    Hi Fitter30: Thanks for your suggestion. I am not 100% sure about the wiring, so I will post 2 picture. Also the trouble is the rattle can not be duplication, the noise hasn't come on in 3 days now. Last time it didn't come on for 15 months.
    https://photos.app.goo.gl/4WZL4BW5zAZzaUhA7
    https://photos.app.goo.gl/dHtCGWjdyPJ8pbnE6
     
    Last edited: May 3, 2021 at 7:20 PM
  9. Reach4

    Reach4 Well-Known Member

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    Would turning off the AC power to the furnace do the same?
     
  10. WorthFlorida

    WorthFlorida The wife is still training me.

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    It is not quite clear, does this happen when the system is not running? Turning up the thermostat makes it run longer, gets the chimney hotter and birds will fly away. Do you have a metal chimney cap? Believe it or not, it's a woodpecker. If you have a metal cap, tape on some padding. Since it seems to be related to the seasons and very random, my money is on a woodpecker.



    Go to the 39 second mark.



    This was my first thoughts:
    There is something in the drum fan rubbing against the blades. While it is making such a noise, place your hand on the blower housing. If you feel the vibration, that is where it noise is from.
    You can turn the blower on by using the FAN position of the thermostat and let it run 24/7to wait for it to happen.
    If this is the original motor a bearing maybe rattling at times or the drum just dropped enough that it is hitting the housing.

    It isn't that difficult to remove the blower housing and give it a good cleaning anway. After 38 years the fins of the squirrel cage do fill up with dirt and dust slowing the flow of air movement.
     
    Last edited: May 4, 2021 at 10:28 AM
  11. fitter30

    fitter30 Well-Known Member

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    Furnace doesn't have a burner control. To basic with a standing pilot. Might want to consider replacing it pilot light cost $15 a month in gas alone, basic furnace now is 85% efficient, yours 70-75%. Condensing furnaces 95%.
     
    Dana likes this.
  12. Stampede

    Stampede New Member

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    Thanks so much Gentlemen, I too believe it's a woodpecker now after watching the videos. Both My wife and I feel the sound we hear virtually sound identical to what you are posting. Yes the sound comes when the furnace is not on. In fact last week, I just realized the sound (bird flies away) the minute I turn up the thermostat to activate the furnace (heat exhaust makes the bird go away). Thank you so much for solving this mystery sound I've had periodically since 2019. Time to put up a plastic owl on the roof. Thanks again
     
    Last edited: May 4, 2021 at 8:54 PM
  13. Stampede

    Stampede New Member

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    Thanks so much WorthFlorida, I think you are right, I too believe it's a woodpecker now after watching your videos. Both My wife and I feel they virtually sound identical to what we hear from time to time. Yes the sound comes when the furnace is not on. In fact last week, I just realized the sound (bird flies away) the minute I turn up the thermostat to activate the furnace (heat exhaust makes the bird go away). Thank you so much for solving this mystery sound I've had periodically since 2019. Time to put up a plastic owl on the roof. Thanks again. We do have a lot of large trees where we live. A family of squirrels live in one of my neighbour's tree, I guess a family of woodpeckers moved in the last couple of years too. Never happens in winter, when they fly south to escape the Canadian winter.
     
  14. Dana

    Dana In the trades

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    Natural gas is pretty cheap in Calgary- not sure if it's quite $15/month (maybe CDN$5/month).

    But getting swapping out a senescent low efficiency furnace for a new condensing and right sized replacement is worthwhile for comfort reasons alone. A Lennox G803-120-1 is sub-optimally oversized for about 99% of the homes in Calgary, even at Calgary's somewhat cool -14F/-26C 99% outside design temp. A somewhat air-leaky 2500 square foot 2x4/R13 type house with clear glass double panes (no low-E) or clear storms over single panes and NO foundation insulation would still have a design load of only ~60,000 BTU/hr @ -26C. With reasonably tight 2x6/R19 construction, better glass and insulation on the basement walls you'd be looking at <40,000 BTU/hr @ -26C.

    Which better describes your house? How big is this house?

    Yes, it gets colder than that in Calgary (for only 87 hours out of a typical winter), so some upsizing is called for, but not 2x (unless you're anticipating -50C during a polar vortex disturbance event- not too likely). Following ASHRAE oversizing guideline of 1.4x, with a 99% load of 60KBTU/hr you'd want a furnace no bigger than (60K x 1.4=) ~84,000 BTU/hr . That would be a 90K-in condensing furnace (and the G803-120-1 would only slightly oversized at 91K of output). For a 99% design load of 40K you'd want a furnace no bigger than (40K x 1.4= ) 56,000 BTU/hr which would be a 60K-in condensing furnace (and your current furnace is way sub-optimally oversized.)

    So how do you know what your design load really is? Don't just guess, and don't use some idiot's " Lessee, 50 BTU a foot times 2500 feet gives ya 125,000 BTU/hr" rule of thumb, which is probably how they selected the equipment the first time around. Use the existing furnace as a measuring instrument, comparing fuel use against wintertime weather data from a nearby weather station as outlined in this bit o' bloggery. Be sure to use ONLY wintertime gas usage- December through January or half-past February will be the most accurate, since that's when the average heat load is highest, and the error from solar gains lowest.

    Don't be surprised if the actual fuel-use measured 99% load comes in at only ~30K ( ~1/3 the 91K specified output of your current furnace)- 3x oversizing is very common. If your measured heat load is actually the ASHRAE recommended (91,000/1.4 =) 65,000 BTU/hr @ -26C it would be a 2-sigma outlier.

    What happens with 3x oversizing is that even when it's pretty cold outside the duty cycle is less than 35%, which means the heat is delivered in 5- 10 minute chunks of hot-blast & noise followed by 10-20 minutes of drafty chill. With right-sized condensing furnace it's running 5-10 minute burns with only 5-10 minutes off during average cold weather and when it's -26C outside it's running a (1/1.4=) 71% duty cycle (7 minutes on/3 minutes off) with the "warm summer breeze" going on more than 2/3 of the time, increasing to over 90% of the time during Polar Vortex disturbance extremes.

    To those who have never lived with right-sized equipment (which is most of us) that might seem like it's "struggling to keep up", but in fact the high duty cycle is much more comfortable, since it's moving air at a lower rate with less wind chill, less noise, and significantly lower & shorter cold drafts building up between on cycles. Right sizing also evens-out the room-to-room temperature differences, whereas the furthest rooms at the end of the duct runs are often short changed with oversized equipment, never running long enough for those rooms to approach steady-state.

    So while it's possible to keep running a 35+ year old furnace until the heat exchangers start leaking exhaust into the conditioned space air or the controls/motors fail catastrophically, it's usually better to retire them at age 20-25, especially when they're grossly oversized and delivering lower comfort & efficiency than a right sized furnace would. It's always good to track the fuel use derived load numbers and have a contingency plan. That beast could already have a leaking heat exchanger, and at that age it could fail at any time during the next heating season. By tracking the real heat load you'll know what replacement equipment makes sense, and what DOESN'T make sense. A panic replacement when it has suddenly crapped out and -10C outside usually results in replacing it with something the same size or larger, condemning the house to another 25 years or more of lower comfort.
     
  15. Stampede

    Stampede New Member

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    Hi Dana:
    Thank you for your detailed explanation. My home is 2100 sq ft built in 1982. It actually has 2 Lennox furnace (probably both from 1982). A large one at 96000 BTU for the family room and 3 bedrooms/2 bath. The smaller one at 68400 BTU for the main floor and basement. At this point both seem to be running smoothly. Is it necessary to change them out now? Thanks again for your advise.
     
  16. Dana

    Dana In the trades

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    Jan 14, 2009
    Location:
    01609
    In 1982 some houses were still built with 2x4 framing & R13 batts, others were 2x6/R19. How about yours?

    In 1982 some houses had single pane windows, others had clear glass double panes (no low-E). If the windows have been upgraded since Y2K they're likely to be U0.30 - U0.35 low-E type windows.

    Are the basement foundation walls insulated?

    Assuming 2x4/R13, clear glass double panes, no foundation insulation you're looking at a whole house load of something like 50-60,000BTU/hr @ 0- in which case the combined output of the G803-120 + G803-90 furnaces is 91.2K + 68.4K= 159.6K is on the order of 3x oversized, which is ridiculous. If it's 2x6/R19 with low-E glass & basement insulation it's more than 4x more heat than needed, which would be LUDICROUS.

    As long as the furnaces still work and aren't a maintenance nightmare for finding replacement parts when they fail there isn't an efficiency argument for replacing them. Even though they have pretty crummy ~76% raw efficiency the high oversize factor for the actual loads doesn't have much further impact on operational efficiency (the way oversizing a cast iron boiler by 3-4x would.) But there is no way to rationalize 159,000 BTU/hr of furnace for this house, even if it had NO insulation.

    Sure, the equipment will still heat the place to 20C - 22C indoors (even at -100C and beyond outdoors), but the duty cycle is super low at average winter temps. At Calgary's average mid winter temp of about -4C the whole house load is <30,000 BTU/hr which means the furnaces are blowing warm air < 20% of the time (12 minutes per hour) which way sub-optimal for comfort. Depending on the duct designs, a duty cycle that low is often unable to fully heat the rooms at the furthest rooms at the end of the ducts before the thermostats get satisfied.

    Since the two furnaces are heating separate zones you'd need to run a room by room load calculation on each zone to size the replacement equipment accurately. A fuel-use based load calculation can only tell you the whole house load, not the individual zone loads. Run the fuel use load calculation, but also run room-by-room load calculations for the individual zones using online Manual-J(-ish) tools such as LoadCalc or CoolCalc. Be aggressive in all of the air-tightness & duct tightness & R-value assumptions- don't assume the worst case, assume the BEST case (as if the house were built perfectly). Those tools all overestimate reality even when using aggressive assumptions- often by 30% or more. If you're conservative, assuming it's super-leaky with the crummiest possible windows that blows up rapidly to 2x+ the real loads. Using the fuel use load numbers for the total it's possible to scale the calculation tools' loads appropriately. eg:

    Say the fuel use based whole house load numbers comes in at 42,000 BTU/hr, the LoadCalc says 26,000 for one of the zones, 31,000 BTU/hr for the other, for a total of 57,000 BTU/hr, which is 57/42= 1.36 bigger than furnace-measured reality. (That amount of overshoot is pretty common using the freebie online tools.) The real design loads can be corrected by scaling them to 26K/1.36= 19K for the one zone, 31K/1.36= 23K for the other.

    Then using ASHRAE's 1.4x recommended oversize factor you'd replace the smaller furnace with something that puts out about 1.4 x 19K= ~27,000 BTU/hr (like a 30,000 BTU/hr-in condensing furnace) and replace the larger one with a furnace that delivers 1.4 x 23K=32,000 BTU/hr (like a 35,000 BTU/hr-in condensing furnace, or a 2-stage 40K-in condensing furnace.)

    Getting to the right solution starts with running some load numbers, then checking them twice.
     
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