Warm Air Lowboy operation seems odd to me..
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  1. #1
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    May 2008
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    Question Warm Air Lowboy operation seems odd to me..

    I was in a small greenhouse the other day that had a warm air lowboy oil fired furnace. When it fired up the oil burner lit and started warming the heat exchanger as I'm used to seeing in residential furnaces. After a short time the fan kicked on and started blowing heat down the duct - all good. But then what I found odd was the fan shut off but the burner kept running until the heat exchanger warmed back up, then the fan kicked back on again. This cycle continued for half an hour till the greenhouse was up to temp (55 F). I'm just used to seeing both the fan and the burner run till the t-stat no longer calls for heat. Is that lowboy hooked up wrong and running up against a high limit or something or is that possibly normal operation. The owner said all warm air furnaces run like that... I've never seen one do it.

    TIA

    Neo

  2. #2
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    it sounds like it is working ok the thing is a low indoor temp causing the fan switch to open
    the one caution to that is most or all mfgs. wont back the warranty unless a min. indoor temp is maintained I believe (60')
    because the flue gases will condense while still in the hx when running at a constant low indoor (r/a) temp.

  3. #3
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    Ok, Thanks for the reply. It just seemed odd that the fan was shutting off. Interesting about the condensing flu gas. The owner said he has to replace the flu pipes every other year. I guess the condensed flu gas is pretty corrosive.
    Thanks again.

    Neo

  4. #4
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    ah yes it is
    carbonic acid is produced in combustion

  5. #5
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    I dont no your relationship though the hx should be tested asap

  6. #6
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    Opertion is normal. That fan control is operating by temperature, not a timed cycle. The return air is cool enough to cause the on off cycling you observed. As far as the chimney and flue pipe. Corrosion is very common in greenhouse environments and we see it around here all the time. Greenhouse oil furnaces are some of the nastiest oil units I work on.
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  7. #7
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    No matter the installation, the air flow across the furnace should be adjusted to maintain a minimum temperature rise across the furnace. Not that it would protect the furnace as hoped 100% by the manufacturer, but it would definately help the whole situation.
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  8. #8
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    Quote Originally Posted by firecontrol View Post
    No matter the installation, the air flow across the furnace should be adjusted to maintain a minimum temperature rise across the furnace. Not that it would protect the furnace as hoped 100% by the manufacturer, but it would definately help the whole situation.
    So, in other words the fan would be slowed down to reduce or stop the cycling and thereby keeping the heat exchanger hot and less likely to condense the flu gases?

  9. #9
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    Quote Originally Posted by neosec View Post
    So, in other words the fan would be slowed down to reduce or stop the cycling and thereby keeping the heat exchanger hot and less likely to condense the flu gases?
    Under ideal conditions in a more controlled typical application. In this instance, my quess is there is no real duct system to speak of, maybe a plenum with possibly some ducting or possibly greenhouse poly tube air distribution, not return duct. Odds are, the air coming back to the furnace is considerably colder than the actual discharge air and irregardless of what is done the condition will persist. Slowing the blower to a degree that will allow for constant operation will decrease the airflow to a point that the throw and diffusion will be ineffective.
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  10. #10
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    Quote Originally Posted by neosec View Post
    So, in other words the fan would be slowed down to reduce or stop the cycling and thereby keeping the heat exchanger hot and less likely to condense the flu gases?
    It may already be operating within factory temp rise spec. 55F return air heated up 55F is only 110, the fan limit may be set to shut the fan off when the air temp drops to 115 or 120F.
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  11. #11
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    I think I would also ask if the Fan/limit switch as been replaced lately. If the F/L is the wrong insertion length your going to get erratic cycling

    OR

    If the F/L is very old, the bi-metalic strip (which makes the thing work) could be weak, that would also give erratic operation
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  12. #12
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    Quote Originally Posted by jmac00 View Post
    I think I would also ask if the Fan/limit switch as been replaced lately. If the F/L is the wrong insertion length your going to get erratic cycling

    OR

    If the F/L is very old, the bi-metalic strip (which makes the thing work) could be weak, that would also give erratic operation

    What he is describing is common for oil furnaces without a blower on timer, in a green house application. Its not really erratic operation when the return air is 55F or less.
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  13. #13
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    Quote Originally Posted by beenthere View Post
    What he is describing is common for oil furnaces without a blower on timer, in a green house application. Its not really erratic operation when the return air is 55F or less.
    sorry, I blew right pass the "oil furnace" and who keeps a greenhouse at 55 I thought greenhouses were suppose to be warm
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