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Old 10-12-2013, 06:25 AM   #1
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Retrofit furnace fan rewiring - help


Hi all,

We have an old (1989) oil fired furnace and a heat pump. The previous owner of the home replaced the single speed fan with a 3 speed fan (hard wired on HI). I have since added a 3 way switch to control the fan speed (MED for heat in the winter, HI for AC in the summer).

An Aube TH146 thermostat controls the system. It has an option to keep the fan on continuously (and it works) but it can control only a single speed.

There is also a cool down sensor at the front of the furnace (with the dial, etc). And it has a white pull out pin for manual or automatic fan (and it works) for the one speed.

Finally there is a grey steel box just below the cool down sensor which is about 3 x 6 and it has a relay with many connections and a SPST switch labeled "FAN". Most of the box is filled with wire... there are many. Some go into the furnace, some go into cool down sensor. The switch does not appear to do anything as far as I can tell. Anyone?

The fan is an Emerson 115 Volt 3 speed. There are 3 wires for speed and a common. You can activate only one at a time otherwise kaboom. However the LO speed wire (and common, and the MED/HI wire from my switch) are connected to the 3-wire cable encased in steel that disappears into the furnace under the heat exchanger. I wonder about the low speed connection... how to activate??

What I would like to do is have the fan run at low speed when no heat, ac, or cool down required.

Thanks for any info!
Regards,
Terry

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Old 10-12-2013, 09:50 AM   #2
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Retrofit furnace fan rewiring - help


Is all of your duct work in conditioned space. If its in a basement or attic. The air will become warmer in the summer, and colder in the winter causing your A/c and heat to have to come on faster. Using more electric in both summer and winter. And more oil during your colder temp weeks.

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Old 10-12-2013, 10:43 AM   #3
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Retrofit furnace fan rewiring - help


all duct work in conditioned spaces... just want to keep the fan on low speed to try to maintain even temperatures. It's a split level with 4 half floors... very difficult...
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Old 10-13-2013, 05:20 AM   #4
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Retrofit furnace fan rewiring - help


You would need to add a relay to use the low speed tap and still have the fan go to med for heating and high for cooling.

Plus fish the wire down to the blower motor.
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Old 10-13-2013, 05:47 AM   #5
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Retrofit furnace fan rewiring - help


I have a 3 way switch to control med and hi near the fan, so that's good!
The power cable connected to the motor has 3 wires. One is the common; one is connected to the 3 way switch for med/hi; and the last is connected to the lo speed tab. So I think that's good too!

Assuming I can find the other end of the wire which is connected to the blower...

what kind of relay do I need?
what will I use to control the electric magnet in the relay? What will I connect them too?
the "normally open" (open when no power) side of the switch in the relay
will be used to control the med/hi.
the "normally closed" (closed when no power) side will control the LO.

keeping in mind I have that mystery box with a switch labeled ""fan" and a relay inside. I can remove the cover and take a picture and attach if it helps?

in summary.... what model/type of relay? where would I connect the relay electric magnet side?

thanks!
Terry
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Old 10-13-2013, 06:05 AM   #6
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Retrofit furnace fan rewiring - help


The relay in the box is what brings on the fan in cooling mode. It should be a single pole double throw, or double pole double throw relay. And it should be controlling which speed is used for cooling and which for heating.

You could use 1- R8222D relays(need 2 instead of 1 like I originally said). Both relay coils would need to wire to common. Then the 1 for high speed the opposite coil terminal wires to Y, and the heating wires to W. So each relay activates on the respective call.

The low fan speed is wired in series through both relays, so that both relays being non energized the fan is in low, but as soon as either one is energized the fan goes to that respective speed.
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Old 10-13-2013, 06:19 AM   #7
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Retrofit furnace fan rewiring - help


it looks like a double... more than 5 connections it might be capable of controlling which speed but it does not

those relays are DPDT...yes?
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Old 10-13-2013, 06:32 AM   #8
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Retrofit furnace fan rewiring - help


see attached wiring image
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Retrofit furnace fan rewiring - help-fan-wiring..jpg  
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Old 10-13-2013, 06:35 AM   #9
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Retrofit furnace fan rewiring - help


I guess I should not touch the existing relays, etc...
and should get two like in the image... and just run a fresh cable from the fan to the relay box.

How would I do this if I had a two speed fan? let's say I wanted to keep my throw switch for MED/HI and not invest a lot?
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Old 10-13-2013, 06:41 AM   #10
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Retrofit furnace fan rewiring - help


just thought of something...where does the cool down sensor fit in the picture?
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Old 10-13-2013, 06:55 AM   #11
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Retrofit furnace fan rewiring - help


The cool down, is the fan limit. it should bring the fan on when it senses the air temp reach X degrees, and then shut the fan off when the air reaches x degrees.

Here is a rudimentary drawing of how you can wire up the R8222D relays.
Attached Files
File Type: pdf Dual R8222D relays.pdf (23.1 KB, 24 views)
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Old 10-13-2013, 06:58 AM   #12
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Retrofit furnace fan rewiring - help


great thanks! Do you have an image/PDF for a 2 speed fan?
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Old 10-13-2013, 07:19 AM   #13
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Retrofit furnace fan rewiring - help


May or may not be what your looking for.
Attached Files
File Type: pdf Single R8222D relay.pdf (21.0 KB, 20 views)
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Old 10-13-2013, 07:43 AM   #14
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Retrofit furnace fan rewiring - help


it's pretty close... it has a Y for AC, but no W for heat ... it gets pretty darn cold up here... do you any other drawings similar with AC and heat for 2 speed fan?

AC and heat on HI spend, idle on MED speed
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Old 10-13-2013, 08:13 AM   #15
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Retrofit furnace fan rewiring - help


The fan limit brings on the blower in that drawing. The fan limit is not allowed to be bypassed.

You can use 2 relays like this though.
Attached Files
File Type: pdf Dual R8222D relays 2 speed fan.pdf (22.1 KB, 12 views)

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