Clarification: the note regarding the fantech 8" was that it worked well for their mondo-range.

Question: Do the name brands' external blowers (e.g., Dacor, Broan, KA) need to be used with their hoods alone or are the external blowers free agents?

Use any blower. Build a duct route that produces the right range of resistance to flow as the blower is built to handle. Resistance to flow includes the filters. Story over.

They will never tell you that.

Just like with RPM and torque for a car motor, there is a large range, in terms of what the blower can handle. Of course it's always good to get the sweet spot and all that, but don't sweat it. And it is true that less noise is produced when your air flow resistance is in the range that the blower handles well. What's acceptable to your blower is something you have to guesstimate. But they'll never tell you that. They don't have to. And the testing labs that certify things for them don't even have to insist on installing a real life length of typical duct with bends. But they'll never tell you that.

hth

Blowers have the property of pulling or pushing some volumetric flow rate (cfm) as a function of the pressure drop across them. Blower flow rates are normally rated at zero static pressure, and with more pressure loss, move a lower cfm. At some pressure loss, the flow goes to zero. The relationship is given by the fan curve for the fan. Such curves should be available from reputable manufacturers, and I have seen curves for Broan/Nutone fans on their website.

So, in general as davidro1 suggested, these blowers will pull or push air whether sitting in the open, or attached to a length of duct, pulling air through baffles or mesh filters, even with marginal makeup air reducing the house pressure. It just moves less air than when in the open.

Ideally, using data or estimates to determine what the pressure loss will be, one chooses a blower that pulls the desired air flow at the pressure loss one estimates will result from pulling that air flow from the house interior. This takes some study to approach a valid estimate. You may find it more cost-effective to just over-specify the blower because the added cost may be less than the value of the time it would take to arrive at a better justified selection. Perhaps a static pressure capability of 1.5x the desired flow rate would be adequate.

kas

One of the differences is the noise produced by these blowers, both outside and inside. The inside noise depends on many factors (and you can get silencers) but the outside noise may be important if you have neighbors, depending on the location of the blower. Abbaka has some of the quietest blowers. Broan won't give you a noise level, and I tried through their technical support and engineering department. So I went with Abbaka.

Billy

We put in Fantech 8 inch 409cfm external mounted fan on our outside wall. It works well enough for getting the grease and steam out of our 4 burner range. We will not be keeping it because we cannot run it beyond 1/3 max of the capacity.

The problem is the noise. The fan is mounted outside wall of our kitchen, and it is too noisy for us and our neighbors. We flunked the noise ordinance of the city at max level of the fan. When turned on at max, it sounds like an airplane taking off.

The noise issue was really important to us. We called the company to see if we can get some noise data. They simply reassured us that it was "quiet" but would not give us a specific noise level. We "trusted" the company thinking how noisy could a fan really be... (shame on us!) Guess what, we were really wrong on that trust and assumption. It produces about 80dB of noise at max at the property line which is about 5 ft from the fan, which is WAY too noisy in a neighborhood with close neighbors. The noise ordinance states 55dB at the property line. Not only that it is so noisy, we really cannot use it at the max level inside the house either. I suppose if the fan was 20 ft or more away from the kitchen AND it was far far from the neighbor, then the noise would not have mattered. That is NOT the situation for us.

Don't bother with this fan. You get what you pay for. The reason that this is cheap is probably because they did not put "quietness" into the cost of engineering and manufacturing.

As soon as we can make it happen, we are replacing it with an Abbaka fan.

Wow, Kas -- so the length of the duct doesn't matter? If I could ensure a closed path from here to Mars, the suck/cfm would be the same as from here to the next room? There has to be something wrong with that.... But I guess the velocity of the fan blades just does not care what volume of air is behind it, only the pressure change on either side ... but why wouldn't the volume of air in the vent affect that all-important pressure change?? After all ambiant air pressure matters.... (is that what "static pressure" means? If that a way of normalizing atmospheric pressure?)

OK, I think I get it. These rules of thumb about needed cfm being 10 per 1000 btus (or whatever they were) are just guesses incorporating random, wild estimates of pressure loss in some random generalized average ventilation system. You're talking about actually calculating needed cfm but the missing parameter is not cfm loss in the system or even total btus but amount of (true) cfm clearance needed per btu. There is just so much variability at so many steps of the way, that attempting a calculation just seems to pretend more precision than is in any way possible.... I think that's davidrol's point. So: what is the desired flow rate anyway???

Kaismom -- that's a sobering story. I take it you don't have room for an inline silencer, not that this would affect the dB level at the lot-line. I think that if you click on the pdf for fantech specs it might be saying that your fan's sone level is 9-11 and 69-72 dB. Wow.

...and interestingly, the specs for the inline fkd, their 'quieter' model are at least as bad: 14-16 sones, 69-72 dB. ouch! The silencer will mitigate sound down at the range I think, but will it matter near the output, upstream from the silencer?

� Does the silencer function to quiet output as well as noise in the kitchen? Both up and downstream as it were?

Here is a link that might be useful: Fantech specs

fwiw, I have an inline blower, incredibly quiet, less than 4 feet from the duct opening above the cooktop. It's in a concrete building (these usually make noises sound louder when you are in the same room as the noise source). The building gives me a 6" diameter duct in my condo. This 6" duct goes to a larger common duct where a roof blower pulls air. My job is to get my exhaust over to the common duct. My fan motor is in the kitchen, next to the ceiling, a concrete ceiling. You could have an inline fan INSIDE the kitchen too.

I widened my duct for three feet before my fan. It's an FG-6 from fantech.net

It is true that to say that I found it hard to estimate with clarity, since there are so many factors that combine. I just kept it "reasonable" in my mind and it all turned out great. The fan is small in terms of CFM. My duct is small too.