If the fan draws twenty amps at fourteen volts, that's 280 watts, which
would give it an equivalent resistance while running of Ohms = 14 volts /
20 amps = 0.7 ohms.

Adding another 0.7 ohms in series would reduce the power by half, to 140
amps -- 70 each from motor and resistor.   Using the rule of thumb of
employing a resistor of twice the rating you will actually use, that would
be a 140 watt resistor.

Also note that at the instant of startup, the fan resistance will be very
low and the resistor will be carrying perhaps 200 watts or more briefly.

Yrs,
d

On Thu, Oct 19, 2017 at 10:04 AM, David Beierl <dbeierl@attglobal.net>
wrote:

>  7ohm 50watt resistor I found one on eBay.  You may be hearing from
>
>
> That would be zero point seven ohms, and fifty watts is the sort of size
> you might use for your heater blower.  I'd suggest looking for at least 100
> watts.  Dennis says the stock fan resistor works well to give three speeds
> total.
>
> Yrs,
> d
>