Hello,
I am currently considering the appropriate size for the exhaust duct of the range hood.
Based on my research, I found a table from Miele that recommends duct sizes depending on the airflow capacity of the hood:
Ø 100 mm (4 inches) up to 300 m³/h (176 CFM)
Ø 125 mm (5 inches) up to 500 m³/h (294 CFM)
Ø 150 mm (6 inches) up to 800 m³/h (470 CFM)
Ø 200 mm (8 inches) up to 1800 m³/h (1060 CFM)
Our range hood operates at 270 m³/h (159 CFM) in normal mode and 420 m³/h (247 CFM) in intensive mode.
Accordingly, we probably need a 125 mm (5 inches) duct.
If the duct is too small, the airflow noise is said to be higher because too much air is forced through too narrow a duct, which causes noise.
Now I wonder what the effects would be if I installed a 150 mm (6 inches) duct anyway:
- Would the noise level be even lower?
- Would it be less efficient in terms of energy? (We are installing the Weibel "Mauerkasten," which is supposedly 100% airtight against air, wind, and water.)
- Any other negative impacts?
Thank you very much for your advice.
I am currently considering the appropriate size for the exhaust duct of the range hood.
Based on my research, I found a table from Miele that recommends duct sizes depending on the airflow capacity of the hood:
Ø 100 mm (4 inches) up to 300 m³/h (176 CFM)
Ø 125 mm (5 inches) up to 500 m³/h (294 CFM)
Ø 150 mm (6 inches) up to 800 m³/h (470 CFM)
Ø 200 mm (8 inches) up to 1800 m³/h (1060 CFM)
Our range hood operates at 270 m³/h (159 CFM) in normal mode and 420 m³/h (247 CFM) in intensive mode.
Accordingly, we probably need a 125 mm (5 inches) duct.
If the duct is too small, the airflow noise is said to be higher because too much air is forced through too narrow a duct, which causes noise.
Now I wonder what the effects would be if I installed a 150 mm (6 inches) duct anyway:
- Would the noise level be even lower?
- Would it be less efficient in terms of energy? (We are installing the Weibel "Mauerkasten," which is supposedly 100% airtight against air, wind, and water.)
- Any other negative impacts?
Thank you very much for your advice.
Knallkörper schrieb:
... If installed correctly (follow the assembly instructions), nothing can get between the panel and the facade. The duct extends up to the flap, so it protrudes beyond the base plate. The space between the duct and the base plate opening is then sealed with silicone. Exhaust air cannot reach this sealing area because of the protruding duct. Only condensation moisture can move there.
And for the other units, where the mechanism is in the wind tunnel, condensation can form just the same. A look at the construction drawings shows quite a few edges and corners where the air has to flow past. Likewise, the other duct collars also protrude from the sides of the pipe and therefore have contact surfaces with the exterior wall, where condensation could form as well.
Furthermore, an incoming pipe must be installed for the other units. How is it ensured there that the connection from the pipe to the collar is sealed and no condensation forms? With the Weibel model, the duct can run all the way through to the outside.
But as I said, the Weibel is already chosen.
I only wanted to point out the disadvantage of heat loss. On the topic of airflow performance depending on duct size, you gave me a very clear and convincing answer. Many thanks for that. I’m not concerned about “a few cents,” although over the course of a year it might add up if you calculate it theoretically. But the exhaust system is also fixed.
If the difference in exhaust airflow is only slight and the exhaust setting does not change, then a smaller duct seems more reasonable to me, as it generally means less heat loss.
A smaller duct means that 125mm (5 inches) is sufficient according to Miele’s table.
Then the only remaining “question” is whether I have chosen the right size.
K
Knallkörper22 May 2017 14:33Kaspatoo schrieb:
If installed correctly (follow the assembly instructions), nothing can get between the plate and the facade. The pipe extends up to the flap, meaning it goes beyond the base plate. The gap between the pipe and the base plate opening is then sealed with silicone.How do you want to seal a gap several centimeters (inches) wide with silicone?
Kaspatoo schrieb:
And in the others, where the mechanism lies inside the wind duct, condensation can form in the same way.The dew point inside the pipe will definitely not be reached during operation, so no.
Kaspatoo schrieb:
Moreover, an incoming pipe must be inserted into the other boxes. How is it ensured here that the transition from pipe to box is sealed tight and that no condensation forms?For the other wall boxes, sealing between the base plate and the pipe is planned. Condensation always forms, but I definitely wouldn’t want it between the base plate and the facade, as that area does not dry out quickly.
Kaspatoo schrieb:
If the difference in exhaust performance is only minor and the exhaust level doesn’t change because of it, then a smaller pipe seems more reasonable to me because, in general, it results in less heat loss.I honestly don’t understand the logic behind that. But I recommend using a 50mm (2 inch) pipe if a lower performance and higher noise level don’t seem to bother you.
Kaspatoo schrieb:
A smaller pipe means 125mm (5 inch) is sufficient according to Miele’s table.
Then the only remaining "question" is whether I have chosen the right size.There’s hardly a right or wrong here. Over 200mm (8 inch) diameter makes little sense for a typical domestic system; below that it’s basically “bigger is better.” Just take a look at a fan characteristic curve. Pressure loss is roughly inversely proportional to the square of the pipe diameter. Usually, the goal is to have as high a volume flow as possible, meaning a larger pipe.
If the gap is really that large, it is not necessary to seal the entire gap; sealing the pipe and the slab is sufficient, as there is masonry in between.
Alternatively, a 150mm (6 inch) pipe can be installed and still reduced to 125mm (5 inch) on the inside if desired or required.
Sealing between the base slab and the wall is also planned for the Weibel system. Therefore, it is just as watertight as others at this location.
The misunderstood logic is explained in the following quote, stating that 125mm (5 inch) is the smallest reasonable pipe size. The mention of the 50mm (2 inch) pipe was inappropriate.
Bigger = better -> 150mm (6 inch) pipe and no 125mm (5 inch). Okay, thanks, then we’ll go with that.
Alternatively, a 150mm (6 inch) pipe can be installed and still reduced to 125mm (5 inch) on the inside if desired or required.
Sealing between the base slab and the wall is also planned for the Weibel system. Therefore, it is just as watertight as others at this location.
The misunderstood logic is explained in the following quote, stating that 125mm (5 inch) is the smallest reasonable pipe size. The mention of the 50mm (2 inch) pipe was inappropriate.
Bigger = better -> 150mm (6 inch) pipe and no 125mm (5 inch). Okay, thanks, then we’ll go with that.
Summary: Don’t just increase the duct diameter—use smart solutions.
.
Full version:
Ask Lesch about it
(Not only) he can explain physics better than I can. I will try anyway (after having a good exchange with RobsonMKK today, I must finally give him a chance to shake his head as usual over my words at the end of the day).
So, let’s imagine very simply what happens: the fumes need to be extracted. There should be no mess inside the duct, even if the fumes are greasy vapor. So a filter is necessary. Unfortunately, the filter creates resistance to the airflow. This resistance needs to be “overcompensated” by appropriately increasing the air draft. We use a fan to do that.
The fan pulls the fumes through the filter. Right after the filter is a funnel-shaped chamber (due to converting the rectangular inlet cross-section into a round duct).
In this chamber, turbulence occurs (caused by the interaction of an ideally steady airflow and the complex, moderately asymmetric shape of the funnel). This turbulence continues into the duct. Because it hits non-axially (and its wavelength never quite matches the duct’s resonant frequency), it spirals elliptically along it. The air column inside the duct resists this motion with its spring stiffness.
In this process, the airflow compresses the air column in the duct, and its spiral (helical spring) shape supports this “boring” action. Once reaching the outlet, the system begins to vibrate continuously—pulsing only due to the weight of the flaps closing the outlet—until the fan speed is changed.
The perceived noise depends not only on sound pressure but also on frequency; a wider duct tends to produce lower-pitched sounds due to longer wavelengths.
Is a larger duct better? Not necessarily:
While duct diameter increases the flow speed of the spiraling fumes, it also increases the spring stiffness caused by the weight of the air column in the duct.
Which effect dominates depends on the proportion of horizontal and vertical duct sections: if most of the duct runs horizontally, a wider duct will increase speed; the greater the vertical section, the more the air column’s weight matters.
.
My suggestion: do not rely solely on pushing the air column (as is usually done with a single fan just behind the filter at the start of the duct), and also not solely on pulling (a fan only at the outlet), but use a combination of both. Running at the same speed, this (as optimal as theory allows) practically reduces the effective duct length to zero—at least regarding the spring stiffness of the air column.
Clear so far?
https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/
.
Full version:
Knallkörper schrieb:
A larger duct (150 instead of 125 millimeters (5.9 instead of 4.9 inches)) does increase airflow, but it probably has less impact than increasing a setting on the hood. Unless your duct is very long, then the diameter matters more. After all, a 150 mm (5.9 inch) duct has a cross-sectional area 44% larger than a 125 mm (4.9 inch) duct.
Fact is: Larger duct = less noise + more airflow at the same setting
Kaspatoo schrieb:
bigger = better -> 150 mm (5.9 inch) duct and no 125 mm (4.9 inch). Okay, thanks, then it’s going to be like that.
Ask Lesch about it
(Not only) he can explain physics better than I can. I will try anyway (after having a good exchange with RobsonMKK today, I must finally give him a chance to shake his head as usual over my words at the end of the day).
So, let’s imagine very simply what happens: the fumes need to be extracted. There should be no mess inside the duct, even if the fumes are greasy vapor. So a filter is necessary. Unfortunately, the filter creates resistance to the airflow. This resistance needs to be “overcompensated” by appropriately increasing the air draft. We use a fan to do that.
The fan pulls the fumes through the filter. Right after the filter is a funnel-shaped chamber (due to converting the rectangular inlet cross-section into a round duct).
In this chamber, turbulence occurs (caused by the interaction of an ideally steady airflow and the complex, moderately asymmetric shape of the funnel). This turbulence continues into the duct. Because it hits non-axially (and its wavelength never quite matches the duct’s resonant frequency), it spirals elliptically along it. The air column inside the duct resists this motion with its spring stiffness.
In this process, the airflow compresses the air column in the duct, and its spiral (helical spring) shape supports this “boring” action. Once reaching the outlet, the system begins to vibrate continuously—pulsing only due to the weight of the flaps closing the outlet—until the fan speed is changed.
The perceived noise depends not only on sound pressure but also on frequency; a wider duct tends to produce lower-pitched sounds due to longer wavelengths.
Is a larger duct better? Not necessarily:
While duct diameter increases the flow speed of the spiraling fumes, it also increases the spring stiffness caused by the weight of the air column in the duct.
Which effect dominates depends on the proportion of horizontal and vertical duct sections: if most of the duct runs horizontally, a wider duct will increase speed; the greater the vertical section, the more the air column’s weight matters.
.
My suggestion: do not rely solely on pushing the air column (as is usually done with a single fan just behind the filter at the start of the duct), and also not solely on pulling (a fan only at the outlet), but use a combination of both. Running at the same speed, this (as optimal as theory allows) practically reduces the effective duct length to zero—at least regarding the spring stiffness of the air column.
Clear so far?
https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/
K
Knallkörper23 May 2017 23:4611ant schrieb:
Although the airflow speed inside the screwed ventilation duct increases with the pipe diameter, the spring stiffness caused by the weight of the air column inside the pipe also increases.Hello 11ant,
I enjoyed reading your post, but I cannot quite agree with the quoted passage from a technical perspective. Generally, the flow velocity decreases as the cross-sectional area of the duct increases. The “spring constant” also decreases because a lower pressure builds up in a larger pipe.
Similar topics