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1400 cfm remote with 8" duct

Posted by ctycdm (My Page) on
Tue, Feb 26, 13 at 12:21

Just got word on shipping of my long awaited custom Prizer Hood. (finally! after 11 weeks) I had ordered it with a 1000 cfm remote blower, and an 8" collar, due to very tight restrictions in my roof framing area. Because of the long delay, Prizer says they are shipping the blower as a 1400 cfm "free upgrade" I know there are some cfm/hood gurus on here. Will this work ok? As 1400 calls for 10" Duct, while 1000 can go either 8" or 10"


Follow-Up Postings:

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RE: 1400 cfm remote with 8" duct

Well, it might be noisier (wind noise) on the highest setting, but chances are you won't be using the highest setting that much if at all. The lower settings might end up being quieter.


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RE: 1400 cfm remote with 8" duct

I was thinking the same thing weed. So should I consider this to be a "free upgrade"? , or more of a compromise. Blower will be less than 10 feet away from the hood, which I know is not optimum for noise reduction, but it's all I can do. Hoping the low or medium speeds will be sufficient most of the time.


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RE: 1400 cfm remote with 8" duct

It won't push out the full 1400 CFM, but it will work with 8".

Better to get the 1000cfm if you can in a timely manner.


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RE: 1400 cfm remote with 8" duct

xedos, curious why you say it would be better with the 1000? If I run the 1400 at a lower speed, isn't that the same thing? I was thinking cfm is like horsepower, it's there when you need it and use it, but not if you don't...


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RE: 1400 cfm remote with 8" duct

Because the 8" duct is better sized for 1000 !

Would you want Taurus tires on a 500+ H.P. Shelby Mustang ?

They'll work - just not well, and the xtra H.P. would be worthless.

If you're the only one using the set up it's fine, but what if someone else uses your kitchen or car and decides to give it the beans?


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RE: 1400 cfm remote with 8" duct

Hmmm. The question cannot be answered with certainty without fan curves for both fans, but if they are just scaled versions of each other, or otherwise have the same blade form, then at all points on the fan curve the higher cfm rated fan should have a higher cfm at the same pressure loss. This means that at worst, the restrictive duct will degrade the 1400 cfm fan to a flow rate only slightly higher than the 1000 cfm fan is restricted to at the same pressure loss. Only if the fan curves crossed one another would the 1400 cfm fan potentially yield lower cfm than the 1000 cfm fan. This is possible for two different fans, but certainly not a given.

As it happens, except for some motor losses, fans draw power in proportion to the work that they actually do moving air. So, unlike the Shelby Mustang, little inefficiency is caused by using the "more powerful" fan. It won't burn rubber.

This raises the question of whether they provided a fan that requires a transition from 8 to 10-inch duct. If so, the closer to the hood that the transition can be put, the lower the duct loss and higher the potential flow rate. If not, then if this fan is actually rated for use with an 8-inch duct, there is even less cause to whine.

The only negative factor that I can think of is if the fans are identical but the motor of the 1400 cfm version is a higher speed. Then behavior will be the same up to the speed that the 1000 cfm unit can get to, and then improve at higher speeds at the cost of higher noise. If the control on the hood only has a few speeds, then it is possible that the point where the 1400 unit's flow and noise is equal to the full speed flow and noise of the 1000 unit is not available. This may not matter much.

kas


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RE: 1400 cfm remote with 8" duct

Thanks kas, I think you answered my question?;) I'll let them provide me with the 1400 "upgrade" for the same cost, and take my chances on more noise at some given speed. I guess it would be safe to assume it will be no less efficient at removing hogs than the 1000 cfm utilizing the same duct?


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RE: 1400 cfm remote with 8" duct

ctycdm:

I would expect the 1400 cfm unit to be no less efficient up to the level that the 1000 cfm unit can achieve. After that, given a definition of efficiency as a the ratio of the marginal increase in air flow to the marginal increase in power input, then to answer the question using data one would need fan curves as a function of power input.

However, given that drag and other losses are not linear functions of speed, I would expect efficiency to drop as the fan "pulled harder." Of course, in this example, the 1000 cfm fan can't pull harder so the 1400 cfm fan efficiency drop is only being compared to its efficiency at lower speeds.

In terms of full power efficiency for fans operated at their design points, efficiencies are probably similar for fans of a given design class even though rated for different flow rates. Changes in class, such as axial to radial, stamped flat blade to molded blade, sleeve bearing to ball bearing, single phase to multi-phase, alnico magnet to neodymium magnet, etc., can change the efficiency.

Efficiency data are not usually available from websites of residential fan manufacturers, although Fantech publishes a fair amount of data for their in-line axial fans. Manufacturers of fans for industrial and military use, such as Rotron, would typically publish more data as their customers are engineers who would ignore them if the data weren't readily available.

Commercial ventilation sources such as Greenheck would have efficiency data available as efficiency is a big deal to the operating costs of restaurants and commercial kitchens, but I can't recall if the data are published on their website.

kas


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RE: 1400 cfm remote with 8" duct

It's very common to run a hood at it's lowest speed when not cooking something that requires a lot of venting. This minimizes the amount of outside air you are pulling into your house (important in cold winters and hot summers). A 1400 CFM blower may have a higher minimum speed than a 1000 CFM blower, so you may not have a setting that is as low as you would like. This is a question to take up with the maker of the blower.


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RE: 1400 cfm remote with 8" duct

I echo Caliente63's post. While your situation may be completely different from mine in that you are planning remote blower, I can relate that all this talk of "quieter on lower speeds" talk that advocates bigger blowers can often seem to miss question of HOW LOW CAN IT ACTUALLY GO? I have a Bluestar/Prestige hood, purchased 3 years ago. Very happy with hood. 36" hood over 30" Bluestar range. I elected to go with 1200 rather than 600 CFM motor, nominal upgchage, figured more is better...anywho...while I can attest to the usefullness of the full 1200 CFM at times of crisis during wok emergencies etc, 90% of time I seem to run hood on LOW...boiling tea kettle, just the oven on while we are at dinner table, one pan etc..... Of course, these low intensity stove use times are also the times when I would like to either hear myself talk at the dinner table or hear my background music or whatever. I certainly don't mind the fan nosie (I am on an 8" duct, somewhat undersized for full 1200 CFM but mostly I am never above half speed) when I am fully in action at the stove. But during the long cooking sessions when I want to listen to music or whatever as I go, and all I need is just the oven baking cakes, or one pot simmering or whatever, I found that even the LOWEST setting is just too much. So, I would seriously consider that issue.

After 3 years of being upset with my hood on low, I had the idea that I could install some kind of switch to control the two motors separately. The Prestige/Bluestar hood achieves 1200 CFM by way of two identical motors. After discussion with Prestige and some research I just had my hood rewired to include a separate remote X10 controlled wall switch to control the second motor on or off. It is a night and day difference. Now for everyday tasks I use the hood on ONE motor on low. As soon as I want anything remotely intensive I flip the switch to get TWO motors on low. Then of course if any more is needed I can dial up to the full 1200 CFM....This works very well for the Prestige because the two motors are controlled by a single continuously variable switch. My estimate is that the LOWEST setting of this switch is ca. 1/4 to 1/3 of full throttle. So by installing the secondary control switch essentially I have dialed down my lowest setting from ca. 300 - 400 CFM to about half that.

If I had the choice again I would go remote blower. That not being possible I am now VERY happy with current setup However, I was at the point of ripping out my second fan completely due to noise on the low setting which was spoiling my time in the kitchen, so if I were stuck with small duct and no remote option I would seriously look into the controllability of a large CFM motor before taking it over a smaller unit that would be adequate for the job.

Very best of luck and keep us posted on your progress.


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RE: 1400 cfm remote with 8" duct

buffalotina-thanks for the head up on the X10 switch. I read up on it, and believe its the on/off switch modifier some of the general contractors refer to when discussing our current 5 amp infinite wall switch (off, high, low, medium) integrating with a new Wolf hood. The more I understand the better.


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RE: 1400 cfm remote with 8" duct

Thanks for all your responses and the helpful info on your first hand experience buffalotina! I talked with an HVAC company owner who I've known for many years, and he really thinks I should stand firm with my original choice of 1000cfm for several of the reasons you mentioned. I let Prizer know today that I am not interested in the "free upgrade"


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RE: 1400 cfm remote with 8" duct

OP: I emphasize that a lot of the time in my particular case the hood gets used for light duty cooking, tea kettle etc. I always open a window for MUA when I have the hood on so another benefit of a lower LOW is less cold air drawn in in the winter when it is just not really needed. The reduction in noise is the big plus. Now this is not to say that I don't really appreciate the second fan when needed. If I am doing a few dishes and the oven on then I definitely need at least two fans on low and sometimes more.

Now my fans are in hood, so perhaps with a remote blower the noise would be no factor at all on the low setting. But most of the additional noise when I have the second fan on I am pretty sure is from the air flow through the baffles. I have tried removing one of them and the fans/duct themselves are not making the extra offensive noise I hear when I have two fans on.

You don't say what size hood/stove you are dealing with and your cooking type.

SparklingWater: I am attaching the link to the device I installed in my hood to control the second fan on or off. It is a remote controlled relay. It is powered by 120V but can control a load at a VARIABLE voltage from essentially nothing up to 308V or something crazy. It is controlled remotely by a matching wall switch purchased from same company. Took a bit of work to figure out the wiring scheme needed, but so far it is working out great.

Here is a link that might be useful: X10 Remote Relay


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