I have an old furnace with a one-speed-at-a-time-based on the wiring
I once rewired it, using the schematic, to see if I could find a lower
speed, so I know the schematic is correct, but in my dotage, I can't
It calls it a "direct drive fan motor" and it has 4 wires coming out
of it. It appears that either 2 or 3 wires are to be connected at a
given time, although that doesn't correspond to my vague memory.
The wires are:
White Common, connected at all times
Black High, perhaps connected at all times, if you go by the
The schematic shows 2 wires connected to the motor and a iiuc a 3rd
On Monday, December 7, 2015 at 3:33:07 AM UTC-5, Micky wrote:
What's unclear? It has three possible speeds. If there are only
two wires going to it, pick you speed. If there are three wires
going to it, then one of those is for heat and one for cooling.
Typically cooling is on high, heat would be on medium.
If that were the case, that would be certainly make it unclear to
someone who didn't expect that, who had never seen a fan motor hard
wired for two different speeds.
But, no. There is no separate heating and cooling speed.
The schematic is unclear, perhaps incorrect in this one small part,
but I did't post this when I wrote it because I figured out the rest
of the fan relays and enough of my memory came back that I lost
interest in the details of the fan.
On Tuesday, December 8, 2015 at 3:24:47 PM UTC-5, Micky wrote:
You posted this:
"I have an old furnace with a one-speed-at-a-time-based on the wiring
fan motor. "
"The schematic shows 2 wires connected to the motor and a iiuc a 3rd
optional wire. "
That would imply that it has only two wires and runs at only one speed.
IDK what "one speed at a time" means, otherwise. Fans with 3 speeds can
only run at one speed at a time.
"But, no. There is no separate heating and cooling speed. "
Are you as sure about that as you are the number of wires? It would
seem odd to run wires for two speeds to the fan motor and not use
You only quoted part of what I wrote.
And no, it implies that the schematic shows it has 3 wires, but that I
don't understand how it can work with three wires.
Why don't you just skip this thread and go on to something else.
On Wednesday, December 9, 2015 at 2:10:29 PM UTC-5, Micky wrote:
I quoted what defined the number of wires and connection.
What the schematic shows is one thing. How it's actually wired
is another. It wouldn't be the first time a schematic was for various
models and the particular model you have doesn't use all the possible
wires. So, what is actually there?
Why don't you learn how to make one complete post with all the
information? Many people here have tried to explain it to you
based on the info that you provided, but apparently it's not
sinking in. With three wires you get two speeds.
"And no, it implies that the schematic shows it has 3 wires, but that I
don't understand how it can work with three wires. "
One for high speed for cooling, one for low or med speed for heat,
one neutral. 1+1+1 = 3 Got it now?
My old furnace has a 3 wire motor. Onewas common, one was "constant
run" (1/6 HP, 600RPM more or less, and the other was full fan - for
heating/cooling. On most furnaces they would be set up that when the
furnace calls for the blower to come on it switched from low to high.
On mine, the low was run full time, and the high came on with the low
for normal heating/cooling. It worked that way for years, even though
it was not the proper way.
A lot of newer furnaces have multi-speed direct drive motors. The HVAC
guy selects the speed that produces the specified heat rise for
heating, and the specified temp drop for AC. That will virtually
always be 2 different speeds, and will deprnd on the static pressure
in the ducts (in other words how much restriction in the system)
The motor in yours is likely designed for use in a furnace / central
air handler combination that allows for separate heating and cooling
speeds, or a constant run fan. If there are 4 wires it could support
My new one has a 3 phase brushless DC motor with variable speed drive
It's been a year or so since I wired one of those.
The one I did, the circuit board had terminals
labelled HEAT, COOL, and PARK, if memory serves.
The one I did, had red on heat, blue on cool,
and black was on park. The family wasn't getting
enough AC in the second floor. I changed the
AC blower from medium to high.
In your case, sounds like you may wish to take
blue off HEAT, and put it on PARK. Take red off
park, and put it on heat.
Remit $74.50 for internet consult, please.
On Mon, 07 Dec 2015 08:41:12 -0500, email@example.com wrote:
I'm sure that's right. Long ago I tried the other speeds, on the
faint hope that one would be slower than what I had, but I had the
slowest, as the color indicated. (So I started closing the bedroom
door when the AC was on. Strangely, I've never wanted to close the
door when the heat is on, even though it's the same fan speed. Old
furnace.) The schematic seems to show that 3 wires from the furnace
are connected to the fan, but I don't think that's so.
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