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  1. #1

    Oil furnace, limit switch triggering, poor air flow?

    Hi,

    I have a Lennox 023Q5-140 oil furnace, with forced air, which is approx. 10 years old. Lennox told me that it is a 5 Ton 2,000 CFM unit.

    Last year the heat stopped and the mechanic found the high-limit switch was open. He removed the switch and it reset itself. He then found that the limit switch was frequently tripping and a few minutes later resetting itself. This is apparently unusual as the limit switch should only trip in rare cases as a safety valve. Is this correct?

    The mechanic found the total static pressure was 0.84-0.9" WC. He felt that poor air flow at the heat exchanger was causing the limit switch to trip so frequently.

    The mechanic told me that I should replace my limit switch as it failed once and is overused by tripping so frequently. If it failed again in the open position, I could have no heat again. If it failed in the closed position, I could have a major problem. Is this correct?

    As I understand, the furnace is suffocating and overheating due to insufficient air flow. The mechanic told me that I should modify the return air duct so more air gets to the burner. Lennox agreed and said this is to protect the limit switch, heat exchanger, etc.
    The "cut-in" between the main air return trunk and the furnace is 22" x 14" and I use standard (NOT high efficiency) 16" x 25" disposable filters.

    This sounds expensive, including a larger hole into the furnace, larger air filter, and maybe a larger main air return "trunk". Could you please clarify the need for this and what should be done?

    These repairs make sense, however the furnace has worked fine for the last year, although I don't know if the limit switch is tripping frequently or not. It also has been working for 10 years with no problems other than this and an unrelated circuit board problem. So why should I do anything? Maybe just let it be?

    Thank you very much.

  2. #2
    Join Date
    Jun 2006
    Location
    New Jersey
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    4,340
    Quote Originally Posted by FredHVAC View Post

    These repairs make sense, however the furnace has worked fine for the last year, although I don't know if the limit switch is tripping frequently or not. It also has been working for 10 years with no problems other than this and an unrelated circuit board problem. So why should I do anything? Maybe just let it be?
    It hasn't worked fine for years. It might have made heat, but it's not run properly and safely.

    The furnace is suppose to have return air pulled from both sides of the furnaces. It was actually shipped with 2 16x25 air filters tracks, for that reason.

    With the frequent tripping on limit, has also put alot of strain on the heat exchanger. I would first find out if you really need a 140kbtu furnace....and if so, have the heat exchanger inspected for any cracks or nasty stress points. And if that all checks, then proceed with the proper duct modifications to get the air proper.

    Safety should have no price tag.

  3. #3
    Join Date
    Dec 2001
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    Ocean Pines, MD
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    7,011
    The payback of this should answer that for you. Better efficiency, more reliable, less chance of damaging other components, probably better comfort too.

  4. #4
    Join Date
    Sep 2002
    Location
    Northeast Ohio
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    4,854
    That is a LARGE oil furnace. Was a proper heat loss calc done when it was installed? It may be over-fired for your needs. Downsizing the firing rate and properly adjusting the temperature rise is the first remedy that comes to mind. Let’s be realistic, the actual btu output of the unit is compromised by this type of cycling anyway. At any rate, bouncing off the limit is not acceptable and will eventually ruin the furnace not to mention the potential for other dangerous issues.
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  5. #5
    Thanks for your replies. I don't know what calculations were done to determine the correct size furnace for the house. It seems like it would need a decent size furnace, since it heats a 3,500 square foot house in Connecticut. What type of duct modifications does it need? Do I just need a larger hole between the main air return trunk and the furnace (where the air filter sits) or do I need two holes (for a second air filter?) and/or a larger main return trunk? Or should I just nothing and let it be - nothing bad has happened in 10 years except a one-time stuck limit switch. Thanks for all your help - I am a novice homeowner and don't want to spend a lot of money if I don't have to.

  6. #6
    Join Date
    Jan 2011
    Posts
    7

    Question

    Quote Originally Posted by heaterman View Post
    ..... At any rate, bouncing off the limit is not acceptable and will eventually ruin the furnace not to mention the potential for other dangerous issues.
    General question;
    Why aren't oil furnace controls commonly designed to automatically maintain optimal heat exchanger temperatures through altering blower speed (ECM blower) and intentional burner cycling? While, of course, maintaining an independent high limit safety? One reason for this might be that a frugal homeowner might shut the registers in unused rooms of a perfectly designed duct work system. Just curious.

  7. #7

    Oil furnace, limit switch triggering, poor air flow? (Continued)

    I appreciate all the replies so far, however I'm still confused and would appreciate anyone who can clarify the situation. I am using a professional HVAC contractor and was hoping to get a second opinion from this expert group. Thanks for any clarification or additional advice that you could provide.

    I summarized the situation below, my 2 questions, and the answers so far. The answers to question #1 (do I need to fix this) are fairly consistent, however there are many different professional opinions regarding question #2 (what specifically needs to be done).

    Thanks again for all of your expert advice with this!

    Situation (summary of description in post 1 and 5)Lennox 023Q5-140 oil furnace, with forced air, which is approx. 10 years old.
    Lennox told me that it is a 5 Ton 2,000 CFM unit.
    3,500 square foot home in Connecticut
    3 zones
    High limit switch was stuck in open position 1 year ago.
    The mechanic found the total static pressure was 0.84-0.9" WC.
    The "cut-in" between the main air return trunk and the furnace is 14" x 22"
    I use standard (NOT high efficiency) 16" x 25" disposable filters
    The main return air trunk is 12" x 24"

    Question #1. Do I need to do anything? It has been working for 10 years with no problems other than a one-time stuck high limit switch and an unrelated circuit board problem. So why should I do anything? Maybe just let it be? Will it work for another 10 or 20 years if I do nothing?

    Summary of answers so far:It hasn't worked fine for years. It might have made heat, but it's not run properly and safely.

    Better efficiency, more reliable, less chance of damaging other components, probably better comfort too.

    Bouncing off the limit is not acceptable and will eventually ruin the furnace not to mention the potential for other dangerous issues.

    By running this unit with inadequate ducting it will dramatically reduce the life of the furnace

    Its been working for 10 years, but has had the same problem for those 10 years, just its catching up to you now. If the heat exchanger fails. You'll spend more getting a new furnace, since warranty doesn't cover failure from low air flow.

    The need is gigantic!

    Question #2. What specifically needs to be done?I understand the concept of "more air" or get the static pressure to 0.5" WC, but I want to know specifically what the HVAC professional needs to do to achieve this, e.g. enlarge cut if for a 20" x 25" filter, add another cut in for two filters (perhaps by wrapping the return duct around the furnace so I can have a filter on the side AND the back, enlarge the main return trunk, replace the high-limit switch, etc.

    Summary of answers so far:
    The furnace is supposed to have return air pulled from both sides of the furnaces. It was actually shipped with 2 16x25 air filters tracks, for that reason.

    I would first find out if you really need a 140kbtu furnace....and if so, have the heat exchanger inspected for any cracks or nasty stress points. And if that all checks, then proceed with the proper duct modifications to get the air proper.

    Downsizing the firing rate and properly adjusting the temperature rise is the first remedy that comes to mind.

    Typically, when you get in the 1600 cfm range and above, a single 16x25 side return won't cut it. Either double intake from both sides or the bottom

    You do need more return air but you may also need more supply ductwork as well to get your system working properly. Also the chimney and flue compartment in furnace is probably severely clogged and needs to be cleaned by a pro that could be why the limit tripped

    You could have a new duct system designed and installed to accommodate the furnace. Or you could replace the furnace to match the duct system. Or you could have a knowledgeable oil technician, who also knows about airflow and duct design, to adjust the burner to more closely match the blower and duct system.

    Oil furnaces usually have a very large heat-exchanger very near the top of the furnace; if the evaporator coil is not installed at least 6 inches above the top of that kind of oil furnace it will cause a bad airflow restriction with a lot of back-pressure.

    Is the cooling coil installed at least 6" above the furnace? Also, if cooling coil is directly on top of furnace, he probably didn't get a static reading between the blower outlet & the bottom of the cooling coil. Therefore, that high restriction static area - would NOT have been read!

  8. #8
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,025

    Major Heating & Cooling AIRFLOW & SIZING Problems...

    Quote Originally Posted by FredHVAC View Post
    I appreciate all the replies so far, however I'm still confused and would appreciate anyone who can clarify the situation. I am using a professional HVAC contractor and was hoping to get a second opinion from this expert group. Thanks for any clarification or additional advice that you could provide.

    I summarized the situation below, my 2 questions, and the answers so far. The answers to question #1 (do I need to fix this) are fairly consistent, however there are many different professional opinions regarding question #2 (what specifically needs to be done).

    Thanks again for all of your expert advice with this!

    Situation (summary of description in post 1 and 5)Lennox 023Q5-140 oil furnace, with forced air, which is approx. 10 years old.
    Lennox told me that it is a 5 Ton 2,000 CFM unit.
    3,500 square foot home in Connecticut
    3 zones
    High limit switch was stuck in open position 1 year ago.
    The mechanic found the total static pressure was 0.84-0.9" WC.
    The "cut-in" between the main air return trunk and the furnace is 14" x 22"
    I use standard (NOT high efficiency) 16" x 25" disposable filters
    The main return air trunk is 12" x 24"

    Question #1. Do I need to do anything? It has been working for 10 years with no problems other than a one-time stuck high limit switch and an unrelated circuit board problem. So why should I do anything? Maybe just let it be? Will it work for another 10 or 20 years if I do nothing?

    Summary of answers so far:It hasn't worked fine for years. It might have made heat, but it's not run properly and safely.

    Better efficiency, more reliable, less chance of damaging other components, probably better comfort too.

    Bouncing off the limit is not acceptable and will eventually ruin the furnace not to mention the potential for other dangerous issues.

    By running this unit with inadequate ducting it will dramatically reduce the life of the furnace

    Its been working for 10 years, but has had the same problem for those 10 years, just its catching up to you now. If the heat exchanger fails. You'll spend more getting a new furnace, since warranty doesn't cover failure from low air flow.

    The need is gigantic!

    Question #2. What specifically needs to be done?I understand the concept of "more air" or get the static pressure to 0.5" WC, but I want to know specifically what the HVAC professional needs to do to achieve this, e.g. enlarge cut if for a 20" x 25" filter, add another cut in for two filters (perhaps by wrapping the return duct around the furnace so I can have a filter on the side AND the back, enlarge the main return trunk, replace the high-limit switch, etc.

    Summary of answers so far:
    The furnace is supposed to have return air pulled from both sides of the furnaces. It was actually shipped with 2 16x25 air filters tracks, for that reason.

    I would first find out if you really need a 140kbtu furnace....and if so, have the heat exchanger inspected for any cracks or nasty stress points. And if that all checks, then proceed with the proper duct modifications to get the air proper.

    Downsizing the firing rate and properly adjusting the temperature rise is the first remedy that comes to mind.

    Typically, when you get in the 1600 cfm range and above, a single 16x25 side return won't cut it. Either double intake from both sides or the bottom

    You do need more return air but you may also need more supply ductwork as well to get your system working properly. Also the chimney and flue compartment in furnace is probably severely clogged and needs to be cleaned by a pro that could be why the limit tripped

    You could have a new duct system designed and installed to accommodate the furnace. Or you could replace the furnace to match the duct system. Or you could have a knowledgeable oil technician, who also knows about airflow and duct design, to adjust the burner to more closely match the blower and duct system.

    Oil furnaces usually have a very large heat-exchanger very near the top of the furnace; if the evaporator coil is not installed at least 6 inches above the top of that kind of oil furnace it will cause a bad airflow restriction with a lot of back-pressure.

    Is the cooling coil installed at least 6" above the furnace? Also, if cooling coil is directly on top of furnace, he probably didn't get a static reading between the blower outlet & the bottom of the cooling coil. Therefore, that high restriction static area - would NOT have been read; IN FACT THERE WILL BE A HUGE STATIC LOSS WHEN READ AFTER THE COIL!

    The answers above are good...
    I know this is an older thread, however, it wasn't completely answered & there will be a lot of Oil heating situations that will have similar problems!
    It is October 25, 2012 & the OIL Heating season is beginning...


    Due to severe carpal tunnel in both hands; I can't do enough typing to provide all the details regarding what needs to be done to correct both heating & cooling AIRFLOW & SIZING Problems. (Several Extra large RA Filter areas are very important for Mid to larger equipment.)

    First, were I in your shoes, I'd have a Home Energy Efficiency Audit performed; then a room by room heat-gain heat-loss calc done.

    IMO, the heating & cooling appear to be way oversized which is creating needless problems.

    The Thermo-Pride OL6 Low Boy upflow has a LISTED BTUH OUTPUT ranging from +60,000 to +89,000-Btuh depending on which of 3 nozzle flow-rates is selected, either the 0.50, 0.60, or 0.75 GPH rate.

    A 0.50 GPH rate, #2 diesel fuel at 140,000-Btuh a gallon * .50 is 70,000 Input *.865 efficiency is 60,550-Btuh Output.

    Well, 140,000-Btuh a gallon using the .75 nozzle is 105,000 Input * .865 efficiency is 90,825-Btuh Output. Here in SW WI I heat my farm home with 56,000-Btuh; it has plenty of reserve left-over.

    It uses a direct-drive 5-spd blower which at 0.50" static will get the proper heating temp-rise & tonnage airflow increments from 1.5 to 4-ton of cooling.

    That OL6 furnace only weighs 250 lbs & the height of the casing is only 34&3/4 inches; considerably lower than most Thermo Pride Low-boys which will provide more room for more than 6-inches of transition between the E-coil & the top of the Oil furnace - reducing back-pressure toward trying to get to an efficient 0.5" of ESP.

    If you live in the northern half of the U.S. & into many southern areas of Canada this furnace should cover the majority of those heating & cooling conditions; of course there are heating exceptions.

    Tell us more about where you live & how energy efficient you think your home is; that is something you need to pretty accurately know.

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,586
    Quote Originally Posted by akbrian View Post
    General question;
    Why aren't oil furnace controls commonly designed to automatically maintain optimal heat exchanger temperatures through altering blower speed (ECM blower) and intentional burner cycling? While, of course, maintaining an independent high limit safety? One reason for this might be that a frugal homeowner might shut the registers in unused rooms of a perfectly designed duct work system. Just curious.

    Same reason gas furnaces aren't. $$$$$ Additional controls cost additional money.
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  10. #10
    Join Date
    Sep 2002
    Location
    Northeast Ohio
    Posts
    4,854
    You have to look at the entire duct system to determine the proper solution. More air in won't necessarily equal more air out. Typically, when you get in the 1600 cfm range and above, a single 16x25 side return won't cut it. Either double intake from both sides or the bottom but the manufacturer outlines this in their specs and installation manual along with the other system blower parameters that need to be met. This generally falls way outside the normal DIY range so you most likely are going to need the services of a qualified HVAC company.
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  11. #11
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
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    Sounds like you do need more return air but you may also need more supply ductwork as well to get your system working properly. By running this unit with inadequate ducting it will dramatically reduce the life of the furnace. I would get a pro in to tell you what needs to be done to get the furnace working right if not you could be looking at a replacement in the near future. Also the chimney and flue compartment in furnace is probably severely clogged and needs to be cleaned by a pro that could be why the limit tripped

  12. #12
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,841
    I know you're not shopping for a replacement oil furnace, at least intentionally but I've attached a couple of documents that are a little more in depth answers to the questions you're posing. You see, it all goes back to the day the new furnace was installed. It the installing company didn't do their homework before the furnace size was chosen, then you're now faced with the problems you've mentioned.

    For you own edification you should know that the manufactuers of warm air furnaces do NOT install high limit controls as operating controls. That is to say, they are not there to normally cycle the burner off while the t-stat is still calling and the blower still operating, trying to deliver Btus to the living space. So the furnace should be properly sized and then when the t-stat calls, the temperature rise on the furnace is appropriately controlled by the blower speed and the duct system is sized to allow the proper airflow. So you could have a new duct system designed and installed to accommodate the furnace. Or you could replace the furnace to match the duct system. Or you could have a knowledgable oil technician, who also knows about airflow and duct design, to try an adjust the burner to more closely match the blower and duct system.

    But no matter your intended solution, to do it correctly mean undoing a lot of wrong work and that's going to cost handsomely. The results will be stunning if done properly but...getting it done properly may be the first major hurdle.
    Attached Images Attached Images
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  13. #13
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    Jan 2004
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    Lancaster PA
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    Its been working for 10 years. But has had the same problem for those 10 years, just its catching up to you now. If the heat exchanger fails. You'll spend more getting a new furnace, since warranty doesn't cover failure from low air flow.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

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