Very informative presentation! Usually not a technician strong point in troubleshooting. Thank you very much for publishing. I should add that this now emphasizes the importance of proper duct sizing when replacing/retrofitting ducts. Drop in replacements and retrofits modify the ducting and airflow. This presentation emphasizes the importance of proper engineering of the ducting.
Can anyone explain why the fan blades on walk in cooler/freezer have the shaft hub and lock-screws facing the motor sides? It’s so hard to remove those fan blades. Why those fan blades are not made the same as the condenser fan blades?
Think of a vacuum cleaner - block off inlet, motor speeds up - not moving any air so current goes down - try it with vacuum cleaner plugged into a kill-a-watt device. Same thing with a vacuum pump - draws more current when initially pumping air - current on motor goes down as air volume goes down as vacuum is reached.
Best video I've ever seen on this subject and explains a lot. As far as amp draw when static pressure increases, I have the documentation for my Ruud AHU, which has an ECM X13 constant torque motor, and when static increases, RPMs go up and so does the wattage, so there is clearly a difference between a PSC and ECM motor.
I finally get the inverse relationship between fans and blowers. For blowers the best way to think about it is with a straw. If say you want to move a piece of paper that is 3 feet away from you, you can blow try really hard and exert a lot of power (amps) to move the paper. You even purse your lips to increase static pressure. Now if you blow through the straw, you have static pressure and you can easily puff into the straw with less power and direct that to the paper with ease.
It has been my experience, when restricting supply air to a blower or fan it cavitates, has has less load, and amps decrease. The motor may actually increase in temperature, and system pressure will increase, causing higher than normal amps on the compressor and decreased efficiency overall.
This is true and can be shown on the blower curve, but this is only when the cfms are cut way back. In fact, the lowest load is against a fully closed system.
So with blowers (vs. fans) higher static pressure leads to lower amps (but higher speed). Counter intuitive. But the lower amps are actually bad for motor longevity because the lower CFM isn't cooling the motor enough. That blew my brain windings.
QUESTION: what are the mostly likely issues that would cause the delta T at one register to be only 8 F, but 16 F at another?? Background info: the ac unit is in the attic, there are two intake registers in the ceiling and 14 outlet registers. the 16 F register is closer than the 8 F register. the ducting in the attic are R8, so i dont see how the ducts would be so hot as to heat up the air more than 2 F. I though maybe there is a leak, but the air flow from the 8 F seems ok and because it is on the upstream side of the blower any cooled air would go into the attic, not the other way around.
Hi, related question I hope........... I have a fuji aru24 RGLX medium duct system. As you may or may not know the two ton unit comes with the default setting of 31 for Function 26 -> Static pressure which = .18 inWG. Settings go from 03 to 20, ranging from 03 = .12 inWG by increments of 1, up to 20 = .8 inWG There is also a setting of 32 = Automatic Air Flow Adjustment. Does this vary the static pressure or something else. Is it reasonable to use this if the installers clearly have not made any calculations other than the system is designed for that unit and thus the default setting is fine ( no measurements of lengths of ductwork type of duct, number of turns, height of and length of copper etc….). I am assuming that this setting basically looks at the supply fan load ( amperage ) to decide the optimal setting..........??
A bit confused: Question, If you remove the blower door shouldn’t the amount of available air( boxes of air) increase thus less static pressure( less air restriction ) ,thus higher amps (blower works harder) and therefore more air moved across the blower?
Depends on the manufacturer,. I have a Ruud AHU, that lists specs for statics as high as .70. I also have seen specs from other manufacturers that list specs up to 1.0 static. Your design goals for duct should try to keep static as low as possible.
Hoping to pick your brain. I have a 100,000 (input) btu 80% Gas furnace in my 2 story home that is 2500 sq. ft. but there’s a 3 ton AC unit here as well but both units do great heating and cooling all season(s). Since this furnace specs are 5ton, 2000 cfm ECM motor, should my fan be set at 1200 cfm for the cooling speed? I realize the AC is undersized for whatever reason but it works great. Anything here that you would see as being an issue? Thank you for your time.
Most manufacturers air handler tables for total CFM at various static pressures are listed in blower motor watts. To calculate watts from measured motor current you must convert measured V*A to watts by multiplying VA by motor's power factor. Best if you measure PF directly as it changes with motor load, but if unable assume a PF of 0.85 for PSC motor and 0.65 for ECM motor. For ECM motors you should use a true rms clamp-on amp meter. If you have a meter to measure PF it will usually give you watts readout directly.
Very, very important presentation. Thx Brian
Good to see that there is some educational videos on UA-cam. We need more videos like this as this is a real world value video.
Blowers are blowing my mind with how counter intuitive they are
Very informative presentation! Usually not a technician strong point in troubleshooting. Thank you very much for publishing. I should add that this now emphasizes the importance of proper duct sizing when replacing/retrofitting ducts. Drop in replacements and retrofits modify the ducting and airflow. This presentation emphasizes the importance of proper engineering of the ducting.
lots of this is more engineering than a tech on the field , replace what you took out !
thank you for the class
Can anyone explain why the fan blades on walk in cooler/freezer have the shaft hub and lock-screws facing the motor sides? It’s so hard to remove those fan blades. Why those fan blades are not made the same as the condenser fan blades?
Think of a vacuum cleaner - block off inlet, motor speeds up - not moving any air so current goes down - try it with vacuum cleaner plugged into a kill-a-watt device.
Same thing with a vacuum pump - draws more current when initially pumping air - current on motor goes down as air volume goes down as vacuum is reached.
nice analogy!
Best video I've ever seen on this subject and explains a lot. As far as amp draw when static pressure increases, I have the documentation for my Ruud AHU, which has an ECM X13 constant torque motor, and when static increases, RPMs go up and so does the wattage, so there is clearly a difference between a PSC and ECM motor.
extremely important presentation. keep it up
I finally get the inverse relationship between fans and blowers. For blowers the best way to think about it is with a straw. If say you want to move a piece of paper that is 3 feet away from you, you can blow try really hard and exert a lot of power (amps) to move the paper. You even purse your lips to increase static pressure. Now if you blow through the straw, you have static pressure and you can easily puff into the straw with less power and direct that to the paper with ease.
This was a great video
It has been my experience, when restricting supply air to a blower or fan it cavitates, has has less load, and amps decrease. The motor may actually increase in temperature, and system pressure will increase, causing higher than normal amps on the compressor and decreased efficiency overall.
This is true and can be shown on the blower curve, but this is only when the cfms are cut way back. In fact, the lowest load is against a fully closed system.
So with blowers (vs. fans) higher static pressure leads to lower amps (but higher speed). Counter intuitive. But the lower amps are actually bad for motor longevity because the lower CFM isn't cooling the motor enough. That blew my brain windings.
For real life isn't what it seems anymore.
He ruined my feeling of having common sense.
This is true for PSC motors, but with something like a ECM X13 Constant Torque motor, both RPM and Amps go up with increased static.
Excellent 👌👌
QUESTION: what are the mostly likely issues that would cause the delta T at one register to be only 8 F, but 16 F at another??
Background info: the ac unit is in the attic, there are two intake registers in the ceiling and 14 outlet registers.
the 16 F register is closer than the 8 F register. the ducting in the attic are R8, so i dont see how the ducts would be so hot as to heat up the air more than 2 F.
I though maybe there is a leak, but the air flow from the 8 F seems ok and because it is on the upstream side of the blower any cooled air would go into the attic, not the other way around.
Check your CFMs at both registers. The 8F my be significantly lower than the 16F.
Also check your “wet bulb” at both return register locations also.
thnks u somuch papa hvac
Hi, related question I hope...........
I have a fuji aru24 RGLX medium duct system.
As you may or may not know the two ton unit comes with the default setting of 31 for Function 26 -> Static pressure which = .18 inWG.
Settings go from 03 to 20, ranging from 03 = .12 inWG by increments of 1, up to 20 = .8 inWG
There is also a setting of 32 = Automatic Air Flow Adjustment.
Does this vary the static pressure or something else. Is it reasonable to use this if the installers clearly have not made any calculations other than the system is designed for that unit and thus the default setting is fine ( no measurements of lengths of ductwork type of duct, number of turns, height of and length of copper etc….). I am assuming that this setting basically looks at the supply fan load ( amperage ) to decide the optimal setting..........??
A bit confused: Question,
If you remove the blower door shouldn’t the amount of available air( boxes of air) increase thus less static pressure( less air restriction ) ,thus higher amps (blower works harder) and therefore more air moved across the blower?
I would imagine it would drop the static pressure. If it is a PSC motor it would lower the amp.
@@mackenziegray2090 No, with PSC, the amps go up because there is more air to move.
very good video loveit
Why would a blower motor's external static pressure regularily be 0.4 to 0.8 W.C ? dont most furnaces call for 0.5 W.C max?
Depends on the manufacturer,. I have a Ruud AHU, that lists specs for statics as high as .70. I also have seen specs from other manufacturers that list specs up to 1.0 static. Your design goals for duct should try to keep static as low as possible.
Hoping to pick your brain.
I have a 100,000 (input) btu 80% Gas furnace in my 2 story home that is 2500 sq. ft. but there’s a 3 ton AC unit here as well but both units do great heating and cooling all season(s). Since this furnace specs are 5ton, 2000 cfm ECM motor, should my fan be set at 1200 cfm for the cooling speed? I realize the AC is undersized for whatever reason but it works great. Anything here that you would see as being an issue?
Thank you for your time.
Yes, you need less CFM for cooling. You are picking up very little superheat with that oversized blower, and my guess too much subcooling.
Most manufacturers air handler tables for total CFM at various static pressures are listed in blower motor watts.
To calculate watts from measured motor current you must convert measured V*A to watts by multiplying VA by motor's power factor. Best if you measure PF directly as it changes with motor load, but if unable assume a PF of 0.85 for PSC motor and 0.65 for ECM motor. For ECM motors you should use a true rms clamp-on amp meter. If you have a meter to measure PF it will usually give you watts readout directly.
0.8" w.c. I call it "Oh Shit Mode"
bye bye ecm blower bye bye