Proper wiring inside a furnace?


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Old 02-03-07, 01:41 AM
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Question Proper wiring inside a furnace?

My parents moved into an old house (circa 1915ish) this past summer, and I have had some concerns about the HVAC system since the beginning. My first concern (and the only one in THIS post) is that the blower control is a light switch (hearafter refered to as THE light switch) mounted on the side of the furnace. The blower is either on or off regardless of whether the burner is firing! When they ran out of heating oil the first time (they are used to gas) they found that space heaters actually keep the house fairly comfortable IF you turn the blower off. It seems that letting the air sit around and stay warm would be more efficient than having it circulate all the time. I'm not an expert, but I understand the concepts fairly well, so I opened up the front panel on their oil furnace to see what I could do.

Here is how it was set up:
(I hope you don't mind a long post. I'm going to try to be as descriptive as possible. Also, I'm doing this from memory so bear with me.)

First, I'm not sure what brand the furnace is or anything, the model number is LUF80C112/125D20-1A.

The thermostat is an ancient looking two conductor type with an alchohol thermometer in the front.

Inside the furnace we have a fan-&-limit switch, a transformer (I finaly came to the conclusion that the secondary is just the terminals marked R and C(?) and the other three are just joining points for other lines from the thermostat), a DPST relay with one NC and one NO pole and of course, the burner assembly.

The two lines from the thermostat are directly connected to two terminals on the burner assembly, when they short the burner comes on.

Next we have the relay that controls the blower. The coil is connect by two violet lines to the transformer, one to the C and one to the G. The light switch is connected to the R 'hot' terminal and the G terminal--switch is on = relay activated.
The NO contact connects the black, 'high' line from the blower to the black mains line from the main power switch.
The NC contact connects the blue, 'low' line from the blower to the fan-&-limit switch.

The fan-&-limit switch connects the buner assembly to mains power, and will connect the blower terminal to mains when the heat exchanger reaches a certain temperature.

Except for the thermistat and the light switch connections, this is consistent with the wiring diagram from the furnace manual.


My dad and I monkeyed around with it for a while and this is how we left it (and what I hope is not really screwed up):

We left the blue, 'low' line from the blower disconnected because it does not seem to work. We tried connecting it directly to the mains but nothing happened (they recently had to replace the motor, I don't know who installed it). We also left the mains line that was attached to the relay disconnected because we didn't need it.

We connected the black, 'high' line from the blower to one side of the NC pole of the relay (so we wouldn't have to rig a jumper), and the fan switch to the other side. (We probably should have disconected the relay from the transformer so no one ca accidentaly disconnect the fan)

Now when the thermistat calls for heat, the burner comes on and, when the exchanger reaches about 80, the blower comes on. When the burner shuts off, the blower continues to run untill the exchanger is about 70.

It looks like the factory set wiring is arranged to force the blower on, if at a low speed, in the event that the burner comes on (and the exchanger gets too hot) without the blower being activated for some reason. It is this apparent 'Backup' system that makes me think we may have done something wrong, even though it seems to work much better now (no more constant wind ).

Since my parents seem to have adopted an attitude of take-what-you-have-and-make-it-work, I have also worked out a way to use the light switch to manualy turn on the blower for... like... air circulation in the summer (heaven forbid they get a decent thermistat before then!) To do this I would make a jumper and connect the 'High' line to both poles of the relay and the mains line to the NO pole. I would leave the light switch controling the relay so that the blower comes on when the light switch is on OR when the exchanger heats up.

Thank you for your input.
 

Last edited by PhoenixofMT; 02-03-07 at 12:03 PM. Reason: spelling
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Old 02-03-07, 01:58 PM
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Manual

Here is a URL for the instalation manual for the furnace. The wiring diagram is on page 14.

http://www.alpinehomeair.com/_viewresource.cfm/ID/161/p/American%20Aire%20Oil%20Furnace%20Installation%20Manual

I also have a question about the capacitor on the blower's brown wires. It says "(if used)". Why would it be used if it will work without it, and why would you leave it off if you do need it? What purpose does it serve?

Thanks again.
 
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Old 02-03-07, 09:28 PM
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Furnace wiring

That model # looks like an Armstrong number. The only reason a capacitor would not be wired in would be if it were a split phase motor such as usually used on a belt drive motor. Most direct drives are of the split capacitor type.
Disconnect the wires to the "switch" & wire the furnace according to the diagram. Once done, fire the furnace & allow to heat. When the fan/limit dial reads 130-140 you should hear a slight click. Measure voltage between where the blue wire connects to the fan/limit and white. You should get 120volts. If other than 120 is measured, replace the fan/limit. If 120 is present, move to the terminals on the relay marked 4 & 5. You should get zero volts between those terminals. If you get voltage (especially 120) replace the relay.
 
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Old 02-06-07, 06:44 PM
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Figured it out

Thanks for the reply.

I took a look inside the blower compartment today and lo and behold, the blue wire was not even connected. It was just sort of wrapped up in the bundle of wires hanging off the blower. I cut off the push-on terminal, stripped the wires, and used the wire-nut that was capping off the blue blower wire to connect them. I set it back up like the diagram and it works just fine now.

Apparently, the guy who replaced the blower for my parents didn't know what he was doing. They tell me he was convinced the fuse was blown, due to his special furnace-repair-man-sense, and then spent a whole day looking for one. When he returned the next day, a new fuse didn't help. I guess he didn't realize you can just look at a fuse and see if it's blown.

Another question I have now, though, is: what temperatures should the thermal switch be set to? I forgot what the settings were when I started screwing around. I've seen elsewhere in the forums where people have been advised not to set it too low because you can crack your heat exchanger.
 

Last edited by PhoenixofMT; 02-06-07 at 06:55 PM. Reason: Forgot to add a thought.
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Old 02-07-07, 12:31 PM
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Post Found it.

http://forum.doityourself.com/showthread.php?t=290803

140F seems awfully hot though. Seems like the burner would shut itself off if it got up to around 115 when we were messing with it.
 
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Old 02-07-07, 07:24 PM
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Fan/limit settings

Fan off (first pointer)= 100; fan on (middle pointer)= 130-140; Limit (last pointer) 180-200. Make sure to hold the dial while moving the pointers.
This is a good starting point. You may have to fine tune the on/off settings.
 
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Old 02-08-07, 06:25 PM
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What am I looking for as I tweak. Proper temp rise?
 
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Old 02-08-07, 06:34 PM
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Settings

Usually the biggest concern is air temperature coming out of the registers by the time the fan cuts off. Anything less than about 95 is can feel cool & drafty. The other concerns are how long it takes to bring the fan on & does the fan stay on the whole time the burner is on. The latter as well as temperature rise is generally a function of fan speed. On a three speed fan motor, red is low, blue is medium, & black is high.
 
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Old 02-08-07, 11:02 PM
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Sweet!

Thanks for all the help.
 
 

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