On Mon, Oct 6, 2008 at 7:24 AM, Randall <tr3driver@ca.rr.com> wrote:
>> > Probably what you are hearing is the control transformer in
>> the furnace.
>> > Your thermostat and controls runs on 24V. There will be a small
>> > amount of eddy current loss in the transformer, but that's about it.
>>
>> Probably 10 watts or so. (If it's old, maybe lots more.)
>
> That seems way high to me. Whatever it dissipates is converted almost
> entirely to heat; and 10 watts into something the size of a control
> transformer would cause it to get quite warm even in free air. Mine is some
> 40 years old, and just barely gets warm to the touch. Might be 1 watt, but
> I doubt even that.
You'd be surprised. Transformers waste an incredible amount of power
when not in use. I didn't just pull the number out of a hat. I've
measured a bunch of similar things, and 10 watts is what I'd guess.
You're an engineer, as I recall, so lets do some math:
Warm to the touch means it's above body temperature, 37 C, 98.6F.
Let's say our transformer is 10cm a side. That gives it an area of
1,000 cm^2, or 0.1 m^2. If it's at 40c (just slightly more than body
temp), it'll radiate (area * Stefan's constant* T^4, or 0.1 m^2 * 5.67
X10-8 * 313^4 ~= 54 W) about 54 watts, were it a perfect blackbody
(it's not, of course, but close enough.) It's in a room at about 20C,
so it will absorb (0.1 * 5.67x10^-8 * 293^4) 41 W. net output is
about 13W.
In reality, it's not a perfect black body, and so it's going to
radiate less. On the other hand, it's probably got one side mounted
to something where it can shed heat through conduction, which is more
efficient.
--
David Scheidt
dmscheidt@gmail.com
_______________________________________________
Support Team.Net http://www.team.net/donate.html
Shop-talk mailing list
http://autox.team.net/mailman/listinfo/shop-talk
http://www.team.net/archive
|