multi-speed furnace blower motor

 First off, I have no idea if this is an inappropriate forum for this
question, so please redirect me if there is a better group for this.

I've got an old (1970's) gas-fired forced-air furnace in my basement.  It
has, as one might expect, a single-speed fan motor.  The motor is a 1725rpm
1/4hp.  During the winter we heat primarily with wood.  We have a woodstove
in the livingroom, and for the past few years have turned the furnace fan on
during the day to circulate the heat to the rest of the house.  We plug the
cold-air intake during the winter, because otherwise we get outside-temp air
mixed with the recirculated air which can have more of a cooling than
heating effect when it's 0F outside.  The house is >60 years old and has
plenty of air leaks already, so please don't bother warning me about
plugging the cold air intake... we've been doing it for years and nothing's
broken down and everybody's still alive.

I'd like to switch my fan motor to a two-speed for a number of reasons.  It
will allow me to re-connect the cold air intake to the furnace, it will
lower my electricity costs as I don't need the furnace fan on full speed
just to keep heat ciruclating, etc.  The problem is that the only motors I
see available are 1725/1140rpm motors.  1140rpm is too fast for me, and will
definitely still pull in too much cold air, plus I doubt I'll save that much
$$ using it.  I was thinking more like 500rpm.

So, first off I'd like to know if anyone knows of a manufacturer (and even
better a retailer) who sells such a motor?  I have been perusing the
internet for an hour and while I see references to multi-speed FURNACES I
don't see any mutli-speed motors for sale (other than the aforementioned
1725/1140 motors).

Secondly, how do the multi-speed newer furnaces which run at a low speed all
or most of the time deal with the cold-air intake issue?

I don't want to invest a pile of money in this ancient furnace, which would
preclude any type of microprocessor-controlled, sensor-driven,
cfm-regulated, variable-speed ECM motor system... I just want a two-speed
fan to circulate the air and not pull cold air into the house and which
switches to high speed when the thermostat kicks the furnace on.

Thanks in advance for any replies.

Dave
British Columbia, Canada

 


A centrifugal blower is a square law device.  That is the flow is proportional
to the square of the speed.  1140 RPM will probably drop the flow by half.

Second thing that is important to know is the a centrifugal blower is LEAST
loaded when the flow is zero.  That is, either the intake or exhaust is
blocked.  The implication is that you can effectively imitate a slower fan by
simply blocking one port.  Usually blocking the intake is less noisy.  The
power draw will drop accordingly.  You probably will NOT see much of a shift
in current draw.  As an induction motor becomes less loaded, it's power factor
drops.  The current moves out of phase.  The result is that though the current
is about the same, the real power (what you pay for) drops.  If this is a 120
volt fan, you can get a Kill-A-Watt and see the effect.

The only thing to be careful of is that most fan motors are rated as "air
over".  That means that they depend on a certain minimum air velocity for
cooling.  You'll need to monitor the temperature and make sure the motor
doesn't overheat.


The usual place for a replacement motor is a motor repair shop or appliance
parts warehouse or last resort, an HVAC wholesaler.  The latter will charge
you the 'walk-in' price which is double the trade price.

The most speeds I've seen is 5.  Most general purpose replacement fan motors
are 4 speed.   The installer chooses the speed that best fits the air handler.
The idea is to only have to carry a few part numbers on the service truck.

If you can't find anyone else, you can call the place where I occasionally
moonlight, Jerry's Electric Motor Service in Cleveland, TN, 423 472-5959.
Jerry Brown (Sr and Jr) operate the place.  They ship UPS all over the
country.  You'll need the following when you call:  ALL the nameplate data.
the shaft diameter.  Whether the shaft is round or has a flat machined on it.
How it mounts (by the frame bolts or via clamp rings.)  Figure about $80.

Most permanent capacitor run fan motors (most likely what you have) can be
slowed with an SCR light dimmer.  The dimmer must be a commercial version that
can handle inductive loads.  I use the commercial grade Lutron dimmers that
Home Depot sells.  I have a squirrel cage fan mounted on a tripod that I use
for comfort cooling in my shop.  The dimmer lets me vary the speed.  I use a
combination of speed and intake blocking to get the amount of air I need.

Another way to slow the motor is to place an appropriate value of motor run
capacitor in series with one power lead.  One can compute the size but it's
easier to just try a few until you get the speed you want.  If this is a 120
volt fan, I'd guesstimate about a 40uF unit to start.

I suggest first throttling the fan and seeing how that works.  The throttle
can be as simple as some cardboard duct-taped to partially block the intake.
Or you can partially block the air filter housing.  That will cost you almost
nothing and will almost surely do what you want.

Realize that the amount of saving isn't going to be that much.  A 1/4 HP motor
draws about 250 watt if it is fully loaded which it probably isn't.  If it
runs 24/7 for a month, that's 168kWh.  At 10 cents a kWh, that's $16.  If you
throttle it back to half power, that's only an $8 saving.

Even that is not a real saving because every watt the motor dissipates
contributes to heating your house.  It is not wasted.

It's not the cold air intake that is an issue.  The issue is pushing heavier,
denser cold air through the duct work in the summer time.  That's why dual
speed fans run faster on AC than when heating.

John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
To have doubted one’s own first principles is the mark of a civilized man
-Oliver Wendell Holmes Jr.

 

Thanks a lot for all of the information.  I went to my local motor repair
shop, and they tried to tell me that an 1140/1725 was all that was
available.  Maybe I'll try another shop.


My fan is mounted inside the back of the furnace, and there is a filter just
above the fan.  I could very easily cut a piece of cardboard the same size
as the filter and make a hole in it to only let a portion of air through.
This will do what I want as far as restricting the air flow, but may present
problems when the furnace turns on as I'd conceivably have much less
efficient distribution of the heated air and more wasted heat up the chimney
if I'm not getting my flow through the heat exchanger.  In addition, as you
note I may run into a motor overtemp situation.

I like the SCR dimmer idea... does the dimmer dissipate the dropped power
via heat?  I am thinking that I could wire the dimmer in via a relay, so
that when the furnace kicks on, the relay coil energizes and cuts the dimmer
out of the circuit resulting in high speed.  Do you see anything wrong with
this approach?

Why would anyone buy a new 2-speed motor if they could use a cheap dimmer
and relay?


Same concerns as above.  It  Would be pretty easy though.  Is the idea that
the capacitor introduces phase shift which makes the motor less efficient?
My understanding of capacitors is that they pass AC with minimal power loss,
otherwise they'd be hot all the time, which they are not.


Just to clarify (you probably already figured this, but I'll state it just
to be clear) my house has a duct system for heated air, and a separate
system for cold air return.  Each room in the house has two ducts, on
opposite sides of the room in most cases.  The cold air return duct system
leads to the furnace and connects right above the filter and (below that)
the fan and motor.  There is a duct which runs from the outside of the house
and connects to the cold air return ducting just prior to the furnace.

When I run JUST THE FAN in the winter time, with the outside air duct open,
it blows COLD AIR in my house, at least a whole lot colder than the ambient
air inside the house.  I don't know why this would be.  The house is old,
and the doors and windows probably do leak A BIT, but I'd think a fair
amount of cold air needs to be being pulled in to feel as cold as it does.

 


That should work.

Cost and complexity.  The Lutron dimmer will run you about $30.  Pulling an
extra speed tap out of a motor costs almost nothing.

There is very little power dissipated in a phase angle controller.  It simply
turns the power on for less than a full cycle 120 times a second.


No, the cap doesn't affect the motor's efficiency much. No more than slowing
it down by any other means.  It simply inserts some reactive impedance in the
circuit, limiting the current.  It actually makes the power factor seen at the
incoming power leads better.

Capacitive reactance is used to limit current and drop voltage in a wide
variety of consumer electronic gadgets.  An example is something we talk about
a lot here - the Kill-A-Watt.  There is a large cap in series with a low value
resistor that supplies power directly from the line to the voltage regulator.

Another example is the Bosch cordless drill battery charger from a few years
ago.  The charger seems too light weight to have anything inside.  I had one
of those drills and when I opened the charger case, found little more than a
large cylindrical polypropylene cap in series with the line.  IT worked well,
as I had that drill for almost a decade before the batteries finally crapped
out.


Let's clarify terminology.  The ducting that brings air back to the air
handler is called the return ducts.  During AC season, it returns hotter air
and during heating season, it returns cooler air than what is discharged. And,
of course, when you're running the fan only, the air temperatures in the
return and supply ducts are about the same.

Separate from those, some systems have an intake for outside air, variously
called a fresh air intake or tempering air intake.  In the past a fresh air
intake was used to supply what turned out to be an overabundance of fresh air.
Research showed that the interior could be maintained comfortable with much
fewer air exchanges than previously believed.  Your system is old enough to
have been installed during that era.

Another use is tempering.  Many modern systems have a little computer module
that measures the outside air temperature and humidity and the return duct
temperature and humidity and operates dampers to select the more efficient of
the two.  In summer, if the outside air is cooler and dryer than the return
air, the unit will use that air rather than running the compressor.  There is
usually another damper that directs return air outside.  In winter, the
opposite is true.  If the outside air is hotter than the return air (nice
sunny day on the roof) then outside air is used instead of running the heat
pump.

For modern very air-tight houses where air infiltration can't be relied on to
achieve the necessary air exchange rate, it is becoming common to install an
air to air heat exchange.  The air headed outside gives up its heat or coolth
to the corresponding air being drawn in.

Anyway, back to your situation.  A house that old probably does NOT need the
fresh air intake that you have.  Unless you get a CO buildup from the wood
furnace, you can safely block it during heating season.  You could manually
cut it in during mild weather when the outside air is more comfortable than
inside.  Or you could get one of those little computer modules and a couple of
damper drive motors and automate the process.

John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
If the letters PhD appear after a person's name, that person will
remain outdoors even after it's started raining.

 
 into a motor overtemp situation.

Capacitive reactance is one of those terms I find tough to get a handle on.
I repair electronic equipment as a hobby, and have (started to) read many
books on electronics theory.  Phase angle and capacitive reactance have
never been clear though.  You know the old first law of thermodynamics:
energy is neither created nor destroyed, simply made to change forms.
Resistors drop voltage, they get hot.  (linear) voltage regulators drop
voltage, they get hot.  I can see where logic-based switching circuits, like
PWM's, can vary voltage by switching, similar to an SCR so that less power
is let through the circuit rather than having to dissipate excess.  But
capacitive reactance dropping voltage... where does the extra power go?  I
look at capacitors like voltage reservoirs, but I guess they are current
limiters as well seen from a different angle.


Yah, I used the term "cold air" instead of "fresh air".


I've got wood-frame single-pane windows, for which I have built single-pane
wood-frame storm windows.  I seal the be-jesus out of them in the fall,
using a product called "draft stop" which is a removable (doesn't exactly
come off like they show in the commercial but...) silicone sealant around
the outside of the inner windows and closed-cell foam around the edges of
the storms.  But I still get condensation between the storm and inner
windows in the winter.  It's been suggested to me that if I ran the fan 24/7
on slow speed, with a fresh air intake connected, that I'd create a positive
pressure inside the house which would tend to push dry air into the airspace
between the windows rather than drawing cooler wet air in from outside.  I
have no idea if a) this is true or b) if this is attainable.  Right now my
fresh air intake is a 4" duct.  Maybe if I damped this down (manual-style,
stick a bat of insulation into it or something like that) it would help
mitigate the seemingly excess amount of cold air drawn in to the return
system.

Again, many thanks for your time and expertise.

Dave