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Old 12-11-2011, 04:16 PM   #16
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Metal duct from furnace leading through a basement hole


If you really have only two wires going to the fan/limit control then it can function as a fan control or a limit control, but not both. The way it seems to be wired, it will make or break the electrical current through the bridge for those two main wires so that as a...

Fan circuit it will turn on the fan at 115F and turn it off at 90F BUT it will also turn off the fan if the temp exceeds 150F when the furnace would really need the fan to be trying to keep the temp in check.

If it is solely a limit control then its bizarrly over wired since those two main wires would only need to be connected to the upper and lower RH side connection points to work properly. No bridge would be needed.

I'd wait for an oil guy to say if oil furnaces are designed to limit at a lower temp than gas. I hope he can also say where the real
fan control is since I can't see a safe way that this control could do it all without a third wire.

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Last edited by how; 12-11-2011 at 04:20 PM.
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Old 12-11-2011, 04:28 PM   #17
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Metal duct from furnace leading through a basement hole


Fan off should be 110 or so, and fn on 140 or so. Limit on oil can be set to 200, 150 is too low.
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Old 12-11-2011, 04:43 PM   #18
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Metal duct from furnace leading through a basement hole


Quote:
Originally Posted by beenthere View Post
Fan off should be 110 or so, and fn on 140 or so. Limit on oil can be set to 200, 150 is too low.
Thanks to both of you for your information. I'm sorry beenthere but are you giving this information knowing how the oil furnaces work? I just want to be sure I'm not doing something that could harm my system or my family.

Thank you both once again.

If other oil experts are able to also provide their input or confirmation, I would greatly appreciate it.
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Old 12-11-2011, 04:47 PM   #19
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Metal duct from furnace leading through a basement hole


Hey beenthere
Whats your take on a fan/limit control with only two wires hooked up to both the fan & limit output connections and connected via the lower pigtail?
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Old 12-11-2011, 04:59 PM   #20
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Metal duct from furnace leading through a basement hole


Quote:
Originally Posted by how View Post
Hey beenthere
Whats your take on a fan/limit control with only two wires hooked up to both the fan & limit output connections and connected via the lower pigtail?
sorry so much for the confusion but i took another look and did find the 3rd wire. it was well hidden under/behind the other one.

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Old 12-11-2011, 05:23 PM   #21
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Metal duct from furnace leading through a basement hole


Cool, makes everything simple and understandable. Hold the dial of the fan/limit control so that it will not move and try to slide the nib and the stop to the 200F position. If the stop will not allow the nib to be slid up to the 200F position then the fan/limit control will need to be replaced with one with the same internal bimetal arm length.
This is all 120V stuff so make sure the breaker is off before you play and write down the colored wire and their fan/limit placement positions before swapping it out. A new fan/limit control will have a small removeable link that should be left alone that takes the place of the short orange wire.

Last edited by how; 12-11-2011 at 05:27 PM.
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Old 12-11-2011, 05:51 PM   #22
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Metal duct from furnace leading through a basement hole


Quote:
Originally Posted by how View Post
Cool, makes everything simple and understandable. Hold the dial of the fan/limit control so that it will not move and try to slide the nib and the stop to the 200F position. If the stop will not allow the nib to be slid up to the 200F position then the fan/limit control will need to be replaced with one with the same internal bimetal arm length.
This is all 120V stuff so make sure the breaker is off before you play and write down the colored wire and their fan/limit placement positions before swapping it out. A new fan/limit control will have a small removeable link that should be left alone that takes the place of the short orange wire.

Although you guys are a tremendous help, I have to be honest I get nervous making some changes. Especially after reading online some websites saying its dangerous to change the limit setting - peace of mind can be a hard thing to over come lol.

We moved in to this house a couple years back and fortunatley I was able to find the manual!!! It also says the factory set limit is 200 and should not be adjusted. So someone must have adjusted it for a reason. Seeing the black mark on the front of the furnace and the fresh air line installed for combustion, I'm guessing some oil guy reduced the limit and installed the fresh air line because the flame was backdrafting.

I will for the time being increase the limit to 170 and see how that works. I guess I will adjust up more if necessary but want to be sure the system doesn't backdraft again. Is my concern legitimate you think? Would a 200 setting increase the likelyhood of a backdraft?

Thanks again!!







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Old 12-11-2011, 06:39 PM   #23
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Metal duct from furnace leading through a basement hole


I tried to slide the nib with a flat tool while holding the round piece in place with my hand and it would not slide. I tried with quite a bit of force and it still would not move.

So I guess I need to replace the part. When I buy a new one, how do I know which one to buy? And where to buy from? I'm guessing if someplace had it locally it would be quicker than shipping it.
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Old 12-11-2011, 06:41 PM   #24
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Metal duct from furnace leading through a basement hole


Your concerns are reasonable.
Another reason techs adjust the settings on the fan/ limit controls is to compensate for the fan/limit control aging and wearing out which can show up as the control not responding properly to the real temp of the furnace.
The proper way to deal with the difference between actual furnace air stream temp and what a fan/limit shows, is to replace the fan/limit control.
The detail to watch for in replacing the fan control is to pull the old one and match the length of the spiral bimetal arm that protrudes into the furnace. You are talking about a pretty common $100.00 buck part from a general HVAC supply house. Many will balk at selling gas control parts to folks without gas licenses but will probably be OK with the part you are after. The arm of the fan/limit will probably range between 5 and 12 inches but chose the identical one to your old one unless you can find out what your furnace was originally suposed to have.

Last edited by how; 12-11-2011 at 06:54 PM.
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Old 12-11-2011, 07:05 PM   #25
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Metal duct from furnace leading through a basement hole


Thanks for all the info.

Do you think that adjusting the blower speed for the heat could also solve the problem? The yellow wire (heat) is set to low currently.

The manual says the Purple in High and the Yellow in Low would be the best for "normal applications"

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Old 12-11-2011, 07:19 PM   #26
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Metal duct from furnace leading through a basement hole


I work on a lot of oil furnaces. So yes, my advice is from knowing how they work.

Fan off at 100 tends to give a somewhat cool air at the supplies that are the furtherest from the furnace, so 110 works better. good chance someone put in a fan limit that was used for olde gas furnaces, and that is why the limit is set to 150 max.

Standard oil furnace fan limit has its high limit set at 200.
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Old 12-12-2011, 02:13 PM   #27
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Metal duct from furnace leading through a basement hole


I installed a new fan limit switch from grainger. (I left the orange wire out as instructed by "How")

http://www.grainger.com/Grainger/HON...820?Pid=search

3 nibs are as follows:
100
125
200

the fan on temp 125 was reached and the fan would turned on but in about 15 seconds the dial went to the fan off setting (100) and the fan goes off. The burner continues going on and this keeps repeating to create 15 second fan on times.

I adjusted it to 150 and the fan off setting to 90. The fan stayed on about 30 seconds before turning off at temp 90. It repeat getting to 150 turn on and 30 seconds back to 90 and turn off and so on since the house thermostat temp hadn't been reached.

What is the normal behavior for a furnace?
Should the fan coming on bring it to the fan off limit that fast?
What do I need to do? Please help. Thank you!

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Old 12-12-2011, 02:45 PM   #28
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Metal duct from furnace leading through a basement hole


What was the probe length of the old fan/limit compared with the new one?
I would keep turning down the fan off nib on your fan/limit control until the temp coming out of your heating vents at the end of the heating cycle was barely luke warm.
The "on" temp nib can also be adjusted. Too high and you are losing that extra heat that is going up your chimney vent before the fan comes on. Too low and your unit can start short cycling. I'd leave that "on" nib where it is for now.
Once you got the furnace fan to run continuously through the heating cycle by adjusting the "off" nib down, then watch what the normal operating temp on the fan control is and tell us.
Your original set up for the humidifyer would have been helping to preheat the return air (as a direct bypass) and the original fan/limit control would have be adjusted accordingly. The options on preventing your fan from shutting down in the middle of the heating run are,
(1) lower the fan off nib temp. (a higher "on" won't help, its high enough already)
(2) lower the fan speed (probably will make it less fuel efficient).
(3) reconnect the humidifyer line from the supply plenum to the return.

I would also use the wireholder that came with your fan/limit control on all three wires. The picture to these old eyes looks like the 120V lines are unsecured where they transit the control case.

Last edited by how; 12-12-2011 at 03:05 PM.
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Old 12-12-2011, 02:54 PM   #29
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Metal duct from furnace leading through a basement hole


They were both 11.5 inches

I guess I will take out the new fan limit control - replace the old one and hope grainger will let me return the one i just bought.

Replace the connection between the humidifier and the return.

I was hoping to increase the air flow in the house by removing that humidiifer connection and it did do that but it created more problems. It seems messing with the settings in this new setup would be less efficient.

I guess as the old saying goes...... if it ain't broken....


(thank you for all your help though - I definitely wanted to test out my theory and I couldn't have done it without you! Thank you How and beenthere!)

Last edited by Arjun007; 12-12-2011 at 02:59 PM.
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Old 12-12-2011, 03:26 PM   #30
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Metal duct from furnace leading through a basement hole


Hold your horses there Bro.
If you are getting a better air flow through the house with the humidifyer set up removed, why not just try turning the nib on your new unit down to a lower temp as well as leaving off the humidifyer set up? .

I think you'd get to keep that better flow, it would probably be more fuel efficient with a lower operating temp going through your furnace, and it's not good to activate a limit control like you were on an ongoing basis. Those limits are only designed for occaisional usage.

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Last edited by how; 12-12-2011 at 03:29 PM.
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