"Home Guy" wrote in message ...
Steve wrote:


I will interject my $.02 here...

How exactly do you improperly install a furnace?

By having too high of a TESP (total external static pressure.


If you are replacing an existing furnace, one that has been running for
years in a given house and presumably giving satisfactory service, then
how possible can you remove it and "improperly" install a new one in
it's place?


Older furnaces had a lower CFM rating, hence they have a higher temp rise
for a given output BTU rating. Newer equipment for effeciency's sake have
thinner heat exchangers (less metal thickness to push the heat through) that
can't tolerate the heat without cracking hence they have a higher CFM
rating, hence more TESP by trying to shove more CFM through the existing
ductwork.

Would these be the same ductwork designed and installed by licensed
contractors?


Sometimes, just go to the hvac-talk.com wall of shame and see all the bad
ductwork installations, many being a "ductopus" using flex duct.

Would these be the same ductwork that was original to the homes in
question - the same ductwork that somehow didn't manage to dammage or
burn out the motor in the previous furnace - presumably an AC motor?

Yes, and here is why. Your average PSC or split-phase induction blower
motor on high runs at a fairly constant speed (a 4 pole motor can only speed
up from its rated speed, usually 1725 RPM to just under 1800 RPM @ 60Hz).
With a centrifugal blower (squirrel cage) the torque load on the motor is
directly controlled by the amount of air flowing through it (ande vice
versa), hence as you restrict the airflow (increase the SP) say with
undersized ductwork the blower unloads. Less torque at the same speed means
less HP (HP=torque in ft-lbs x RPM / 5252) hence less motor watts. An
underloaded motor is less effecient but lasts longer. Too little TESP on a
system with an induction motor can actually overload the motor, hence why
old systems that had belt drive blowers usually has a variable pitch sheave
on the motor. The belt ratio hence wheel speed was adjusted to run the
motor at full load with a new system. As the ductwork and/or filter got
dirty the TESP went up and the motor unloaded some.

Now here is where it gets tricky, ECMs as used on indoor blowers are
constant torque NOT constant speed. The shaft torque is held constant hence
the airflow is held mostly constant. Increase the TESP on these systems and
the blower speeds up either till the torque/airflow goes back to rated or
till the motor hits its top speed limit. More RPM X same torque / 5252 =
More HP = more watts. More watts x same airflow means hotter electronics
hence shorter life. Add in a plugged filter and the poor little motor runs
its little heart out at max speed with little cooling airflow till it burns
up.

I'm sorry, but if my 36 year-old AC motor didn't burn out because of the
size of my existing ductwork, then it's a crock of **** that the same
ductwork is the reason why a new ECM motor burns up.

Explained above.

Blame the ductwork. When you have to explain to the customer why his
new $4000 furnace is costing so much repair hassles, blame the ductwork.


You need to show you're a man by pointing out exactly which of my
statements above are wrong.

I'm right when I say that:

1) ECM motor uses 100 less watts when running full speed compared to 1/4
hp AC motor running at full speed.

As said this depends on TESP. At high TESP the ECM can use more watts than
the PSC.

2) The extra 100 watts used by AC motor is dumped into the house as heat
during the heating months, so it isin't exactly wasted energy from the
point of view of the home owner.

Electric resistance heat is usually more expensive than gas heat and in the
summer it is just more sensible heat load on the evaporator hence more watts
still loses.