Oil furnace, limit switch triggering, poor air flow?

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Oil furnace, limit switch triggering, poor air flow?

Postby Fred » Sat Feb 12, 2011 9:16 am

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.
Fred
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- Oil furnace, limit switch triggering, poor air flow?

Postby nomadpeo » Sun Feb 13, 2011 10:17 am

a limit switch is a safety device. if it trips once, there is a problem. if it is automatic reset and trips frequently, you may not know just how often it is tripping. it is always an indication of poor heat transfer, usually an airflow problem. get input from another professional and correct the problem. a tripping limit switch shouldn't be viewed as a nuisance, but an indicator that you have a problem that needs to be addressed.
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- Oil furnace, limit switch triggering, poor air flow?

Postby 4ftBanger » Sat Feb 19, 2011 9:29 am

I agree with nomadpeo. Get a second opinion. The furnace has a rated temperature rise - The technician should measure the temperature of the return air and supply air to determine if the furnace has enough airflow to operate within the design limits. If the furnace is overheating then something should be done to improve the airflow.
(Or alternately, reduce the firing rate. Oil furnaces are much more adjustable than gas. An experienced oil tech with combustion analysis equipment could lower the firing rate of the furnace to reduce the chamber temperatures)

If the furnace operates within its rating then you might just have a bad limit switch. The switch has been exposed to 10 years of temperature extremes (and maybe moisture in the summer) and, like all mechanical parts, sometimes they just fail.
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