HeyBub wrote:
Most conventional induction motors don't get that warm, so how
much electricity are they wasting?
They don't get warm because in a furnace, they're constantly being
cooled by the return airflow hitting them.
In general, from what I've read, a 1/4 to 1/2 hp PSC furnace motor is
shedding 150 to 300 watts of heat energy when in use.
Also; let's say that it is a 200 watt (quarter horsepower motor?)
and you can 'save' one quarter of that consumption.
At 100% efficiency, 1 hp = 746 watts. Since efficiency falls with
motor size, a 1/4 hp motor is at best 50% efficient (see reference
below). So it's consuming 373 watts (and shedding 186 watts as heat).
I pay 10.6 cents per kw-hour (about 60% of that is for the electricity
itself, and the other 40% is for other charges related to it's
delivery to me, as well as gov't taxes). In the summer, I'm running
my furnace fan constantly. For the average month, that's 730 hours.
For a motor that's using 373 watts for 730 hours, that's 272
kw-hours. At 10.6 cents per kw-hour, that's $28.83 per month.
Given a usage profile that has me running my fan for 100% during July
and August, and tailing off to 20% in February, I project an average
monthly usage of 49% for the entire year. That's about 4300 hours,
which works out to 1600 kw-hours, which works out to $170.
So if I had a conventional 1/4 hp PSC fan motor (50% efficient)
running for 4300 hours per year, I'm going to pay $170 per year.
If the motor was 100% efficient, I'd be paying $85 - saving $85 per
year.
At best, for an ECM motor, I'm going to increase my efficiency by 25%
(to 75%), so I'm going to save $42 per year.
I'd be saving more if I had a bigger motor, possibly saving $85 per
year if this was a comparison using 1/2 hp motors.
So what is the "over-the-counter" cost of a single speed 1/4 or 1/2 hp
ECM motor?
Are there any HVAC jobbers here that can answer that?
------------------------
http://www.eere.energy.gov/buildings...s_analysis.pdf
Good information really starts on page 13.
Capacitor-start, 4-pole motors have efficiencies from 50% to 65% (1/4,
1/3 and 1/2 hp) with efficiency increasing along with motor hp.
------------------------
On other hand that 'wasted' electricity is dissipated within
the house as heat so the furnace would have to run a few more
minutes, burning a little more oil/gas each time to compensate?
Most fan usage happens in conjunction with household cooling - not
heating.
During the non-heating seasons you are going to be running the fan -
sometimes without the A/C going.
I estimate that out of my hypothetical 4300 hours of fan use per year,
1640 hours will happen with the furnace on (about 38% of the time).
Where I live, the climate is similar to Detroit, Buffalo or Toronto.
Further south, more fan use will be for cooling than my projection of
62%. Based on this, the heat shed by the fan motor is not beneficial
or desirable most of the time.
But the motor is outside the conditioned-air area, so it won't
affect the heating/cooling.
The motor is *inside* the house. And it's placed directly in the
circulating air stream. So of course 100% of the heat it's shedding
will be transfered to the interior of the living space of the house.
Assuming the motor costs $200, according to your calculations
you would break even in only eight years.
Assuming my savings are between $40 and $80 per year (say, $60) and if
the motor costs $180 then it will take 3 years for payback. If the
motor is $240, then payback will be 4 years. If the motor is $480,
then payback will be 8 years.
The idea of earning interest on the money is nice, except that most
likely electricity costs will likely also rise, perhaps in a similar
way as your proposed 6%, so the opportunity cost factor will be a
wash. Especially given the volitile nature of the stock markets, and
declining interest rates. The S&P is up only 4.5% now vs the start of
the year.
Then there's the labor to install the motor,
That was going to be done by yours truly, so that's not a factor.
increased taxes,
Not sure what you mean by that.