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Old 12-18-2011, 08:08 PM   #16
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Furnace wiring


Control unit on side of furnace is a Honeywell R857D.
customer.honeywell.com/techlit/pdf/60-0000s/60-2171.pdf

I have a 4 wire setup.
W on my thermostat is connected to C on this unit.
G to G, R to R, Y to Y1.

There's a manual switch on the side of the plenum and and W really goes from the thermostat into this switch then back to C on the control box.

There is about 25 V between C and R with the thermostat not connected.

Now I must be dense but it looks to me like shorting together T and "W" does exactly that. Creates a short on the secondary windings of the transformer. I must be missing something.

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Old 12-18-2011, 08:18 PM   #17
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R and W (not T and W)
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Old 12-18-2011, 08:24 PM   #18
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Quote:
Originally Posted by raleighthings View Post
Control unit on side of furnace is a Honeywell R857D.
customer.honeywell.com/techlit/pdf/60-0000s/60-2171.pdf

I have a 4 wire setup.
W on my thermostat is connected to C on this unit.
G to G, R to R, Y to Y1.

There's a manual switch on the side of the plenum and and W really goes from the thermostat into this switch then back to C on the control box.

There is about 25 V between C and R with the thermostat not connected.

Now I must be dense but it looks to me like shorting together T and "W" does exactly that. Creates a short on the secondary windings of the transformer. I must be missing something.
can you post a picture of the low voltage at furnace for us?
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Old 12-18-2011, 08:41 PM   #19
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Furnace wiring


Pics are hard. From my iPhone to another computer just now is a PITA due to several factors. Figure 6 is similar to my setup. Now I realize that what is called Heating Control on that figure is what I have but wired in ahead of the control box. (In a very ugly splice I might add.) I need to go look at it and see what's up inside of it. It is a push button box that is labeled push for manual, pull for automatic. If I push the fan runs but no flames light. I'll be back.
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Old 12-18-2011, 09:21 PM   #20
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This "box" is the limit switch. So how can I bypass or test it to see it this is the issue?
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Old 12-18-2011, 09:24 PM   #21
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Side question. What gauge wire should be used on the low voltage side of things? What's there now looks like it was done with scrap found in the back of a pickup truck.
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Old 12-18-2011, 10:38 PM   #22
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18 guage wire should always be used for low voltage wiring
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Old 12-18-2011, 10:40 PM   #23
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OK. When thermostat is set to call for heat voltage on transformer output is 24.x volts give or take. Honeywell document referenced above says transformer is a 48 volt unit. And when I try and manually engage the system either via the limit switch or by engaging the contactor the voltage drops to 0.

I'm guessing the transformer is bad? And no there were not any transformers in the old stuff not being used.

Does this make sense?

Thanks
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Old 12-18-2011, 10:42 PM   #24
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ive never seen a 48v transformer,are you sure its not 48va or something like that?
transformer should read 24-26v as long as there is power to furnace
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Old 12-18-2011, 10:48 PM   #25
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it is 48 va, not volts.

http://customer.honeywell.com/techli...0s/60-2171.pdf
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Old 12-18-2011, 10:53 PM   #26
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A volt-ampere (VA) is the unit used for the apparent power in an electrical circuit, equal to the product of root-mean-square (RMS) voltage and RMS current.[1] In direct current (DC) circuits, this product is equal to the real power (active power) [2] in watts. Volt-amperes are useful only in the context of alternating current (AC) circuits (sinusoidal voltages and currents of the same frequency).

-----------------------------------------------------------------------

Volt-amps or power demand. Think of it as watts for most purposes.
This device is producing 24 volts (alternating current) and has the ability to provide 50 watts.
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Old 12-18-2011, 10:54 PM   #27
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Assuming the furnace is 30 years+.
Do you have a meter and know how to use it?
There are ways to bypass the upper right hand side and marked (Limit) of the fan/limit control but it is usually 120V and I'm not comfortable explaining it where someone else might get hurt duplicating it. Meter measurement though is possible.
The reason I took Harleys bet is because the older fan/limit controls seldom suffer a failed limit unlike the newer ones. There is a good chance however that you have a secondary limit because your furnace is in a crawlspace and is a horizontal style furnace? Is the furnace's longest side vertical or horizontal?
What you are looking for is another seperate limit control with a reset button on the back of it. It will be between 1 - 3 " across have two wires attached to it.
The fan compartment is the most common location to find it.
Let me know if you find it?
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Old 12-18-2011, 11:02 PM   #28
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Quote:
Originally Posted by how View Post
Assuming the furnace is 30 years+.
Do you have a meter and know how to use it?
There are ways to bypass the upper right hand side and marked (Limit) of the fan/limit control but it is usually 120V and I'm not comfortable explaining it where someone else might get hurt duplicating it. Meter measurement though is possible.
The reason I took Harleys bet is because the older fan/limit controls seldom suffer a failed limit unlike the newer ones. There is a good chance however that you have a secondary limit because your furnace is in a crawlspace and is a horizontal style furnace? Is the furnace's longest side vertical or horizontal?
What you are looking for is another seperate limit control with a reset button on the back of it. It will be between 1 - 3 " across have two wires attached to it.
The fan compartment is the most common location to find it.
Let me know if you find it?

Jeez man, what did I do to you to insult me like that? Whatever it is, I'm sorry.
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Old 12-18-2011, 11:09 PM   #29
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Old 12-18-2011, 11:10 PM   #30
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