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Thread: why when cold?

  1. #1

    why when cold?

    From reading a bunch of on-point threads on this forum, I have learned that my Lennox 80 MFG goes into "watchguard mode" probably because it requires a control-ignition board and/or dual rod flame sensor upgrade, and that I'm at a point where I need to call in the pros do decide which. (I already tried cleaning off the flame sensor rods; didn't help.)

    But I've found a recurring observation in people's description of the problem that's true for me too: It happens more in cold weather, and the system works fine in milder weather.

    Seems to me the pattern should, theoretically, be the same in warm or cold weather; when activated, the system either works or doesn't. But if it's 40 degrees outside, my system runs fine -- full cycles every time the thermostat calls on it. When it's 10 degrees outside, it goes into watchguard mode all the time. Why would outside temp affect the communication between flame sensors and the board?

    I don't plan to DIY this, it's just driving me crazy not knowing why the outside temperature matters. And it's not just simple odds, like "it happens more because the furnace is called on more"; this isn't a stats question, because it NEVER goes into watchguard mode when it's more than about 32 degrees outside. Why does this error occur more when it's colder?

  2. #2
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    At 40* OD temp. It doesn't run as long or as often.
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  3. #3
    Well, sure. I understand that the heat doesn't need to run as long or as often in warmer weather. But that doesn't explain the observation:

    Warmer (but cold enough to require heat) = 100% success rate.
    Full cycles. Never goes to watchguard mode. Something like 40* outside, but a toasty 68* inside. At this temperature, the flame sensors and the board communicate perfectly.

    Colder = 90% failure rate.
    Almost ALWAYS goes into watchguard mode before ever running even a portion of the cycle. I can sometimes trick it into a full cycle by resetting the board by turning power off then on.

    There must be a logical explanation beyond "it runs more so it happens more," because it never fails above 32 or so degrees. In other words, it doesn't happen more when cold than when warm, because it *never* happens when warm. It's not an acceleration of a pattern because the pattern does not otherwise exist. It's a change in behavior based on one observable variable: OD temp. So it seems OD temp must have some effect on the sensor/board dynamic beyond frequency of use. (At least to my sense of logic... using the term very, very loosely.)

    Thanks for any insights. I really appreciate this board. Like I said above, I've read many helpful posts and have been trolling for months prior to posting myself. I'm thankful to experts willing to help us homeowners become more informed consumers of products and services.

  4. #4
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    Could be a lot of different things effecting it. Have a professional come take a look at it because it is really hard to tell from here whats wrong.

  5. #5
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    Where in the house is the furnace located? Does it draw combustion air from outdoors, or from around the area it is located?

    And, being that I'm not familiar with that piece of equipment, what does the furnace do when it is in "watchguard mode"? Does it cease to operate?

  6. #6
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    Even as a Lennox dealer, I too am not familiar with that equipment. If it's like most other flame sense technology, the flame senor sends a signal to ground through the flame. I guess in theory, the more frequent furnace cycles would not allow the flame sensor to get cold and with increased heat, you get increased resistance, maybe not allowing the flame sensor to send the appropriate microamps. I am not @ all certain this is your answer, but it's the best I can come up with over the internet.
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  7. #7
    Thanks for the feedback, everyone. To answer some questions/comments:

    To Buttwheat, I have a pro on the way. I'll be curious to see if he has a theory.

    Shophound: The furnace is in the basement. I can see the burners through the cover grate, and I see no venting coming in, so the combustion area seems to be using air from the area in which it is located. There's about three feet of clearance on all sides (in case you were thinking about intake obstruction). "Watchguard mode" is the term from the manual. After activation by the thermostat, the flame goes on, but if the flame isn't getting sensed by the board after some short duration (30 sec?), it shuts down and a clearing fan ventilates the combustion area. It will try the ignition/flame sensor sequence four more times for a total of five tries before going into "watchguard mode" for an hour -- basically the vent fan runs for another minute and then the whole system shuts down. In an hour, the whole process starts again.

    As for the product ID, I got "Lennox 80 MFG" from another thread, and then searched for and found the owner's manual as a pdf online. What's actually written on the outside of the unit is "Lennox Value Series," if that helps at all. It came with the house, and I'm not sure how old it is.

    Finally, Ira: Your theory about the flame sensor rods not having a chance to cool down enough to send proper amperage to the board between cycles when it's colder and therefore running more frequently is very interesting! It certainly seems to give some logic to what I'm observing. Thanks for taking the time to think that through and pass it along.

    I wonder if that's what the "dual rod sensor" and "control-ignition board" upgrade kits (each recommended for this problem on separate threads) somehow address. And, if so, how? Perhaps Lennox adjusted the settings in the sensors and/or board. (BTW, right now the unit has one dual-rod sensor, and one single-rod sensor, leading me to believe that only one has ever been replaced since the replacement kits I've seen are all dual-rod.)

    I know the question I'm posing in this thread probably has no practical application, but I'm just really just curious and appreciate that people with knowledge are willing to share. Thanks to all of you!

  8. #8
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    The 80 MGF is an 80% multi-position gas furnace. It is a builders model. Due to the 80% efficiency, it is not a two pipe system. I wonder if you have a grounding problem that becomes aggravated in colder weather? Just a thought. Since the furnace draws combustion air from the house, do you have some sort of make-up air being brought into the house? Do you have an air to air heat exchanger that maybe isn't balanced properly causing a positive or negative pressure in the house? I'm grasping for ideas here because technically, outdoor temp shouldn't affect your power vented 80% furnace.
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  9. #9
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    Does the basement receive any heat from the furnace?

  10. #10
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    Mar 2005
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    burlington county n.j.
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    gas pressure in area dropping in cold weather???

    that style ignition needs proper pressure or it does not like to fire off properly.

  11. #11
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    For some reason i tend to thing you have a differant problem having nothing to do with the flame senser.

    as to what it could be exatly is just guess work

    you should call and have it looked at when it is cold out

    see is they can do a draft test and combustion analizer test on the unit

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