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Old 07-22-2018, 05:13 AM   #1
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HVAC vent system: Air flow and pressure


I was wondering if someone could help me better understand this topic. I have a few questions. I will also make a few statements. If they are wrong, please correct.

- (1) Round corner are so much better compare to 90* angle corner when it comes to air flow and air resistant. Why are ducts design with 90* angles? Even new homes built now days have HVAC duct system with 90* angle bend! If the industry understand that round angles are way more efficient, then why bother with using 90* angle all together?

- (2a) Given that the blower is properly design for a HVAC duct system. Does closing air registers and/or air duct flaps bad for the system? If doing those two things are bad for the system, then what is the point of putting in air registers and air duct flaps in the first place?

- (2b) A HVAC duct system has three supply ducts. Each of those ducts has a duct flap that the homeowner can close off. By closing one of the duct off, does it necessarily force more [b]volume[b] of air to the other two supply ducts? Or does closing one of the duct off, only do harm to the HVAC duct system? It's creating more air pressure on the other two ducts. Because of the increase air pressure, now there are more air resistance, which leads to more air leakage.

- (3) Closing off return air openings or reducing the amount of air going through return air openings via filters. Say the air flow for an HVAC duct system was design properly and has three return air openings. The CFM rating of the HVAC system is 1500. The CFM rating of those return air opening is 500 CFM each. So let's say what happen if I choose to completely close off the one of the return air opening. So I'm essentially limiting the amount of air getting back to the HVAC duct and blower by 1/3. 1000 CFM is now returning instead of 1500 CFM.

- (3a) What will happen now in term of air flow back to the home? Is it 1000 CFM back to the home? Or the CFM will be somewhere between 1000 and 1500 due to air being pulled through the leaks section of the HVAC ducts to compensate for the loss of 500 CFM? Will more air be pulled back to the system via the other two air return openings to compensate for the loss of 500 CFM?

- (3b) If there are damage to the system, what kind of damage or damaging behavior will happen short term?

- (3c) If there are damage to the system, what kind of damage or damaging behavior will happen long term?

- (3d) How will the blower fan be effected? What other components of the system will be effected by this?

--

Thanks.
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Old 07-22-2018, 05:53 AM   #2
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Re: HVAC vent system: Air flow and pressure


Quote:
Originally Posted by 012abc View Post
I was wondering if someone could help me better understand this topic. I have a few questions. I will also make a few statements. If they are wrong, please correct.

- (1) Round corner are so much better compare to 90* angle corner when it comes to air flow and air resistant. Why are ducts design with 90* angles? Even new homes built now days have HVAC duct system with 90* angle bend! If the industry understand that round angles are way more efficient, then why bother with using 90* angle all together?


Cost.


- (2a) Given that the blower is properly design for a HVAC duct system. Does closing air registers and/or air duct flaps bad for the system? If doing those two things are bad for the system, then what is the point of putting in air registers and air duct flaps in the first place?


To throttle them for balancing, not close off completely.


- (2b) A HVAC duct system has three supply ducts. Each of those ducts has a duct flap that the homeowner can close off. By closing one of the duct off, does it necessarily force more [b]volume[b] of air to the other two supply ducts? Or does closing one of the duct off, only do harm to the HVAC duct system? It's creating more air pressure on the other two ducts. Because of the increase air pressure, now there are more air resistance, which leads to more air leakage.


Some of the 500 CFM will go to the other 2 ducts, but not all of it. As you will have increased the resistance to air flow.



- (3) Closing off return air openings or reducing the amount of air going through return air openings via filters. Say the air flow for an HVAC duct system was design properly and has three return air openings. The CFM rating of the HVAC system is 1500. The CFM rating of those return air opening is 500 CFM each. So let's say what happen if I choose to completely close off the one of the return air opening. So I'm essentially limiting the amount of air getting back to the HVAC duct and blower by 1/3. 1000 CFM is now returning instead of 1500 CFM.

- (3a) What will happen now in term of air flow back to the home? Is it 1000 CFM back to the home? Or the CFM will be somewhere between 1000 and 1500 due to air being pulled through the leaks section of the HVAC ducts to compensate for the loss of 500 CFM? Will more air be pulled back to the system via the other two air return openings to compensate for the loss of 500 CFM?


The other 2 returns will draw in more air, but not the full 500 CFM.


- (3b) If there are damage to the system, what kind of damage or damaging behavior will happen short term?


Possible coil freezing ane heat exchanger over heating in heating season.


- (3c) If there are damage to the system, what kind of damage or damaging behavior will happen long term?


Shortened compressor life and furnace heat exchanger life span.


- (3d) How will the blower fan be effected? What other components of the system will be effected by this?


It will move less air. A PSC blower will use less electric as it isn't doing as much work. An ECM motor will use more electric as it tries to move the air flow it was set to, by increasing its speed.


--

Thanks.

Your welcome.
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Old 07-22-2018, 11:42 AM   #3
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Re: HVAC vent system: Air flow and pressure


Quote:
Originally Posted by beenthere View Post
...
"Cost."

What's the cost increase percentage wise going from a 90* angle corner HVAC system to a round corner HVAC system?

--

"The other 2 returns will draw in more air, but not the full 500 CFM."

So if the other two return air openings can draw more air when the 3rd return air opening is closed off, does it mean that the HVAC system was not design to be 100% efficient to begin with?

--

"An ECM motor will use more electric as it tries to move the air flow it was set to, by increasing its speed. "

So the way ECM motor blower works is that it will dynamically self adjust to a set/specified air pressure? Since there is less return air feeding the blower, the blower must increase its speed to suck more air in to maintain the air pressure?

With a PSC motor, there's no such thing as a sensor for the PSC motor to adjust itself to? Why does the PSC motor do less work? Wouldn't the PSC motor do the same amount work but not able to product the same amount of output air because the amount of input air is less? I'm a bit confused to why a PSC motor blower will do less work.
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Old 07-22-2018, 12:42 PM   #4
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Re: HVAC vent system: Air flow and pressure


Quote:
So the way ECM motor blower works is that it will dynamically self adjust to a set/specified air pressure? Since there is less return air feeding the blower, the blower must increase its speed to suck more air in to maintain the air pressure?
It adjusts itself to maintain proper airflow.

When you close vents, the duct pressure goes up and if it adjusted itself to maintain a certain pressure, airflow would tank as you closed them.

It does the opposite, as duct pressure increases and airflow at it's current rpm drops, it actually raises the rpm to try and compensate.

This only applies to the fully variable speed ecm.

For the x13 version, as the duct pressure increases and the amount of work (force being applied to the blower wheel to keep it turning) drops at the rpm it's running at, it'll increase the rpm to maintain the same amount of force/torque being applied.

This offers some compensation, but because the amount of power needed to move x cfm increases exponentially as duct pressure goes up, airflow still drops off.

Amp draw of induction motors drops as the load declines.

Quote:
With a PSC motor, there's no such thing as a sensor for the PSC motor to adjust itself to? Why does the PSC motor do less work? Wouldn't the PSC motor do the same amount work but not able to product the same amount of output air because the amount of input air is less? I'm a bit confused to why a PSC motor blower will do less work.
The blowers used in furnaces are negative displacement.

They don't directly "push" against the air to get it to move like axial fans do. They spin and the air in between the blades gets forced out via centrifugal force.

The blades have a curve to them and scoop air up, into the spaces between the blades.

For this type of fan, the greater the pressure difference is upstream and downstream of the blower wheel, the less air the blades can scoop up and move per revolution. So less power is needed to spin the blades at a given rpm and amperage draw drops a little bit.

About the psc motor:

It's a very passive device. No electronics.

The rpm is fairly fixed (based on number of polls - https://www.coburns.com/blog/Tech-Ti...r-Poles-vs-RPM), it only drops if the motor is overloaded.
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I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.

Last edited by user_12345a; 07-22-2018 at 12:45 PM.
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Old 07-22-2018, 04:09 PM   #5
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Re: HVAC vent system: Air flow and pressure


Quote:
Originally Posted by 012abc View Post
"Cost."

What's the cost increase percentage wise going from a 90* angle corner HVAC system to a round corner HVAC system?


Don't know. But its more labor time.

--

"The other 2 returns will draw in more air, but not the full 500 CFM."

So if the other two return air openings can draw more air when the 3rd return air opening is closed off, does it mean that the HVAC system was not design to be 100% efficient to begin with?


No, it means that closing one off partially makes it less efficient. Each return would be designed to draw X amount of air at X resistance. To draw more air, increases resistance.

--

"An ECM motor will use more electric as it tries to move the air flow it was set to, by increasing its speed. "

So the way ECM motor blower works is that it will dynamically self adjust to a set/specified air pressure? Since there is less return air feeding the blower, the blower must increase its speed to suck more air in to maintain the air pressure?


To maintain set work load.


With a PSC motor, there's no such thing as a sensor for the PSC motor to adjust itself to? Why does the PSC motor do less work? Wouldn't the PSC motor do the same amount work but not able to product the same amount of output air because the amount of input air is less? I'm a bit confused to why a PSC motor blower will do less work.

Because a PSC motor simply spins based on the slip of its magnetic field.
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