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Old 02-17-2010, 11:45 AM   #1
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limit switch and fan control switch settings


Here's the issue with my oil forced draft furnace.
During a regular cycle for heat, (electronic thermostat set at 71, cuts in at 70 and cuts out at 72),
most often the thermostat stops the burner. Sometimes however, the limit switch activates and shuts off the burner and after a minute or two of the blower running, I hear the thermostat click indicating it has cut out. The thermostat then reads 72.
This happens more frequently at night.
Also it always happens when heating the house from 67 to 71 in the morning.

Furnace info.
Model OH4-80S-Z, serial no : G 121294.

Switch : White Rogers
Limit is set to 195
Fan set to 85/110
Fan on Auto
Filter changed this season.

I do not think it is normal.
Any suggestions on what might be the problem.
Could the blower be underperforming?
Is my conclusion (from the posts here) that sometimes the limit sensor is weak and needs replacement, correct?
Is there a way to check the temp in the plenum where the limit sensor is?
Can I check the accuracy of the sensor with hot water to 195 or would that damage the sensor?


Thanks,
CJF

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Old 02-17-2010, 11:47 AM   #2
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limit switch and fan control switch settings


I would reduce the temp change from sleep to wake and see if that helps. You should at the max only drop 5 degrees( I can see that is what you do but for some that could be too much).

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Old 02-17-2010, 02:24 PM   #3
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limit switch and fan control switch settings


If it is cutting out on the limit them you have a lack of airflow and need to correct that problem. Changing the limit won't help.
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Old 02-17-2010, 03:38 PM   #4
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If it is cutting out on the limit them you have a lack of airflow and need to correct that problem. Changing the limit won't help.
Thanks Yuri. Yes the limit won't help if the air flow is low. The only problem I can think of is if the blower unit itself is underperforming or has to be cleaned. The ducting , blower filter etc look good and is not clogged both at the inlet and outlet. Just a reminder, this is a furnace only.

The other question I have is could the limit switch itself be bad and cuttiing out early. Do they go bad with time ? I have felt the plenum area (uninsulated) and it does not feel very hot , altleat not the cut out temp (90*C or 195*F). I would never have been able to keep my hand in hot (close to boiling) water this long.
Is there a way I could test the limit switch to cut out at 195 (maybe in hot water)?

Also does it hurt if the blower is rated higher than the one present. Any tips and info on acceptable ranges of air flow would help.

Thanks once again.
CJF

Last edited by cjf1980; 02-17-2010 at 03:41 PM.
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Old 02-17-2010, 04:02 PM   #5
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limit switch and fan control switch settings


Are ALL your vents and dampers open? There should be a allowable temp rise rating by the model # info. Is it a Lennox oil furnace? Run the furnace and subtract the return air temp from the supply air temp. Poke a hole in the supply duct about a foot downstream from where it leaves the plenum and out of sight of the heat exchanger. Use a cooking/meat thermometer. Limit controls are not designed for continuos use. They are only used when the fan fails or from a lack of airflow. They never fail as they are normally never used if the furnace is working properly. Cycle them often enough and they get a bit weaker and sometimes fail open. No way to test them for accuracy. Hard on the furnace heat exchanger if it is overheated and can cause premature failure.
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Last edited by yuri; 02-17-2010 at 04:13 PM.
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Old 02-17-2010, 05:16 PM   #6
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Has it always done this? or did it just start?

Limits get weak if they have been opening and closing constantly because the ir temp was too high.
And if the system is operated too close to trip temp.
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Old 02-17-2010, 06:18 PM   #7
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Quote:
Originally Posted by yuri View Post
Are ALL your vents and dampers open? There should be a allowable temp rise rating by the model # info. Is it a Lennox oil furnace? Poke a hole in the supply duct about a foot downstream from where it leaves the plenum and out of sight of the heat exchanger. Use a cooking/meat thermometer.
Thanks Yuri. This makes a lot of sense.
All the vents and dampers are open and yes the furnace is a Lennox,
Well noted that this condition is hard on the heat exchanger. This is why I want to be sure the temp is approx 195 when it cuts out. Should I poke the hole in the same area where the limit switch is for measurement consistency? The limit switch is currently mounted on the plenum about 12 inches above the top of the furnce.
Also what is the general range of allowable temp rise rating?
What's the possible breakdown if the burner cycles often?

Thanks once again for the info. I just thought I'd let you know you guys are the best.

CJF
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Old 02-17-2010, 06:24 PM   #8
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Quote:
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Has it always done this? or did it just start?
Thanks BeenThere. I can't say I remember if this has happened before.
But this is an older furnace which was very well maintained - tune up every year for the last 20 years (records hanging by the furnace).

CJF.
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Old 02-17-2010, 06:25 PM   #9
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limit switch and fan control switch settings


That is a very old furnace as I don't have technical info for it ( I have a portal for that). The newer oil furnaces have the limit switch inside and not in the ductwork so I suspect it is quite an old unit. Not very efficient either. The modern units sense the temp near the heat exchanger to protect it. The old school like yours are not that fussy/critical. Measuring the temp by it won't prove anything or do any good anyway. You need more airflow. The heat exchanger on an oil furnace will eventually fail and oil can produce CO Carbon Monoxide just like gas. You may want to have a Pro assess the unit and check the ductwork etc.
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Old 02-17-2010, 06:55 PM   #10
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Quote:
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You need more airflow. You may want to have a Pro assess the unit and check the ductwork etc.
Thanks once again. It was serviced this year before heating season. Eff : 79%.
Will excessive cycling also cause damage to electrodes/ transformer?
I'll be looking at my service contract to have a tech come in. Not sure if this diagnostic is covered since it hasn't stopped working.
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Old 02-17-2010, 07:02 PM   #11
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Yep. Starting and stopping is hard on everything, just like a car. And inefficient as it purges heat/air thru the chamber and that does not show up in the efficiency test but in the real life facts.
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Old 02-17-2010, 07:08 PM   #12
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You did say that the manual for this furnace was unavailable. Which portal were you talking about - is there a website?
Also what would be the closest available manual for this furnace? for the sake of specs.

Thanks,
CJF
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Old 02-17-2010, 07:20 PM   #13
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I am a Lennox dealer and we have a private portal (DaveNet) with almost all the technical info for their units. If there was no manual written for it then we have no access to it. Looks like there was none. The old school units were pretty basic and setup by qualified oil techs. The specs came into play when A/C got popular and we needed to know fan specs and capacity etc. Does it have a belt drive fan. Usually we just increase the motor size to 1/3 or 1/2 and add a 3 3/4" or 4" pulley to it and increase the airflow, not rocket science.
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Last edited by yuri; 02-17-2010 at 07:28 PM.
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Old 02-17-2010, 08:57 PM   #14
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Quote:
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The old school units were pretty basic and setup by qualified oil techs. The specs came into play when A/C got popular and we needed to know fan specs and capacity etc. Does it have a belt drive fan. Usually we just increase the motor size to 1/3 or 1/2 and add a 3 3/4" or 4" pulley to it and increase the airflow, not rocket science.
Yuri, I cannot thank you enough for the info you provided. You were bang on that the air flow was low. Yes it is a belt driven blower and the furnace design is just great in it's simplicity. These things were built to last.
All I had to do was tighten the tensioning bolt for the motor which must have been causing the blower to slip. I have seen a couple of bolt marks along the tension arm which leads me to believe the techs did it earlier. The good old days.
Reading the posts here, got me all technical. In this case, it was plain old first principles (not rocket science).
It being heating season, I did not want to tinker with the blower. I guess I wanted it to be the sensor.

Thanks once again,

CJF.
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Old 02-17-2010, 09:02 PM   #15
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limit switch and fan control switch settings


Check your belts condition.
May be time for a new one.

Belt R us may not be open if it breaks in the middle of the night.

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