"RP" wrote in message



BTW, according to the formula above, a PSC motor rated at 1050 RPM at full
load has a 12.5% slip. Assuming a nearly linear curve in the upper region
of the motor curve, doubling the HP would result in a slip of
approximately 6.25%, for a final RPM of approximately 1125. This is a 75
RPM increase. This may not seem like much, but the cfm increase due to
this much increase in RPM will be substantial. It more than likely won't
be an operational problem for the unit, but keeping the argument in
context, it could very well lead to a coil freezing up that wasn't
freezing before under the same ambient conditions, that is, when it was
already cool outside and the coil was already near the freezing point. Is
that understood, or would you like a bit more clarification. I can talk
about this **** all day long

hvacrmedic

OK so we put in a larger motor and now have 1125 rpm. There is a threshold
for everything and this may be enough to cause the freezup, but is it the
root cause? Given the design parameters and operating conditions of most
home ACs, do you think the oversized motor (additional cfm) is the problem
or it is just showing that there are other problems in the system? What is
the increase of cfm of the 75 rpm when compared to a very strong wind over
the condenser?

Edwin Pawlowski wrote:
"RP" wrote in message



BTW, according to the formula above, a PSC motor rated at 1050 RPM at full
load has a 12.5% slip. Assuming a nearly linear curve in the upper region
of the motor curve, doubling the HP would result in a slip of
approximately 6.25%, for a final RPM of approximately 1125. This is a 75
RPM increase. This may not seem like much, but the cfm increase due to
this much increase in RPM will be substantial. It more than likely won't
be an operational problem for the unit, but keeping the argument in
context, it could very well lead to a coil freezing up that wasn't
freezing before under the same ambient conditions, that is, when it was
already cool outside and the coil was already near the freezing point. Is
that understood, or would you like a bit more clarification. I can talk
about this **** all day long

hvacrmedic


OK so we put in a larger motor and now have 1125 rpm. There is a threshold
for everything and this may be enough to cause the freezup, but is it the
root cause? Given the design parameters and operating conditions of most
home ACs, do you think the oversized motor (additional cfm) is the problem
or it is just showing that there are other problems in the system? What is
the increase of cfm of the 75 rpm when compared to a very strong wind over
the condenser?

There is a tolerance, or should be. That doesn't mean that you should
purposely rig your system so that it's operating at one or the other
extreme. The root cause in the hypothetical set of conditions that I
referred to is that it's
1) cool outside.
2) too much cfm.
This is by definition of the conditions.
Now if I had said the unit is undercharged and running at a higher
ambient, and you then increase rpm, then the root cause would be the low
charge at a higher ambient, but not the motor, because in this case
correcting the charge will stop the freezing. OTOH, the system will now
freeze up at a higher ambient than before, so when the ambient falls
well be right back to no.1 above.
A wind will typically not affect head pressure significantly. Although
it assists draw-through on the side that it hits, it retards it on the
back side with a negative pressure. Very strong winds are a different
story. But if they cause freeze up, and they can at lower ambients, then
they would be the cause of the freeze up.

Just let me summarize the entire argument again:
Virtually anything that reduces head pressure below what it would run
with all components operating optimally, will also reduces SST, and thus
evaporator temperature. Thus any such change can lead to immediate
evaporator freezing if conditions are such that it was close to freezing
before that change was made to the system. Simply installing a more
restrictive filter can do it. Setting the t-stat down one more degree
can do it. Shutting a bedroom door can do it. The list is endless, but
all have in common that the coil skin temp must have been just above
freezing beforehand. No problems with the system are required for this
situation, since lowering either the indoor or outdoor temp tends to
shove the evaporator closer to the freezing mark. You wouldn't for
instance want to go off and leave the t-stat set to cool and 65F on a
night when the ambient is supposed to dip to 60. That is just asking for
a freeze up. HTH.

hvacrmedic

"RP" wrote in message



Just let me summarize the entire argument again:
Virtually anything that reduces head pressure below what it would run with
all components operating optimally, will also reduces SST, and thus
evaporator temperature. Thus any such change can lead to immediate
evaporator freezing if conditions are such that it was close to freezing
before that change was made to the system. Simply installing a more
restrictive filter can do it. Setting the t-stat down one more degree can
do it. Shutting a bedroom door can do it. The list is endless, but all
have in common that the coil skin temp must have been just above freezing
beforehand. No problems with the system are required for this situation,
since lowering either the indoor or outdoor temp tends to shove the
evaporator closer to the freezing mark. You wouldn't for instance want to
go off and leave the t-stat set to cool and 65F on a night when the
ambient is supposed to dip to 60. That is just asking for a freeze up.
HTH.

hvacrmedic


Thanks