In general a larger canopy will help with capture capacity. Think of the canopy as a reservoir or buffer when there is a burst of high volume smoke. That burst needs to be contained until it can be pulled by the blower. The other thing I would suggest is getting a 27" deep hood rather than the standard 24". That's perhaps my one regret. I like to have the hood mounted above head level so a 27" depth hood would have been better. If you don't mind a lower mount, then a 24" will do. For the blower and noise, I think the situation will depend largely on how your specific house responds to various factors. Air flow noise is significant but internal blowers do contribute a measurable amount to that. I went with a rooftop blower so noise in my case is almost all airflow. There is significant noise at full but not nearly as much as I had with my vent-a-hood at my previous house which was a third the size CFM wise. For reducing noise, I'd install an inline or roof blower and a silencer. That's about as much as you can do to reduce overall noise.

Good luck.

amcook: the hood suction is inversely proportional to how high it is mounted... so if you were a an avg 5'9" that means the hood has to be mount at 5'10" or higher 70" whereas the ideal height is 66" ish... i think per inch you lose some suction capacity of the hood.

I would agree with Amcook with regard to the capture area and noise factor.
I've had a Proline 900 CFM under cab hood since October 2010.
It's a sturdy well built unit at a great price. Also the people at Proline are great to deal with.

This is a good way to say it" ".... reservoir or buffer when there is a burst of high volume smoke. That burst needs to be contained ..."

I'll turn this into a mantra:
"Reservoir buffer burst. Contain the burst."

Many hoods sold these days have the baffle or mesh filter down low, so low it's flush with the bottom of the hood shape. Dumb. They could have put it higher up. Recessing it would leave a lip all around, i.e. a container for bursts.

Suction at the cooktop is not the principle that is in play in capture and containment. There is some weak air flow from the hood at the cooktop that does tend to follow an inverse distance law (perhaps 1/R^2, but I don't have time to pull the ASHRAE handbook right now to estimate the functional).

Hoods capture rising effluent from cooking surfaces because the effluent is rising. This is due to the high temperature of the surface and that hot air rises. The rate of rise is substantial, about a meter per second. Internally in the hood the flow has to be high enough to keep this air from "bouncing" off surfaces such that it curls down into the room and escapes containment.

Hoods mounted higher from the cooktop surface have to be larger to capture the same amount of effluent because the effluent expands as it rises. They will likely need to have proportionately larger flow to avoid the bounce problem, depending on their internal design.

With enough flow rate, the zone over which the hood can capture the effluent can increase slightly larger than the hood aperture, but this increase is small because the velocity of the air flow outside the aperture lip decreases rapidly with distance and the effluent velocity of a meter per second will dominate the air direction.

Higher hoods are more susceptible to cross drafts interfering with capture, but this is a problem, particularly with island hoods, no matter what the distance.

kas

I had post a reply earlier, but for some reason it isn't showing up now. It was about the Best by Broan K260A hood and a great deal I can get on one at a shop that is clearing things out. I had looked at the Broan E60, but I can get the K260A for less. Cost-wise, this looks like a great deal. However, I was thinking about the capture area and what was mentioned above about the baffle filters sometimes being placed too low, and I wonder if this might be the case with the Best by Broan and standard Broan hoods. The Proline hoods appear to have a much greater capture area before getting to the baffle filter. Is this likely to make a noticeable difference? I really appreciate all the replies.

Some pyramid shaped chimney hoods, but not all, are only available with internal blowers. The CFMs in these are lower than what you can get with a Canopy Hood. With Wok cooking, and the amount of heat, odor, grease, steam and smoke generated, a canopy is the best way to go but if you can find a chimney style that accepts an remote or in-line blower with over 600 CFM,s that way work for you.

IMHO, you will want 800 CFMs or more.

Hoods that approximate the shape of commercial hoods should be most effective, but unfortunately are likely to be the least aesthetic. A low baffle configuration can avoid "bounce" if the air flow through the baffles is high enough. Better though, in my opinion, is a capture area below the baffles because it may allow the flow rate to be a bit smaller, all else being equal.

kas

I just wanted to say thank you for all the replies. Everyone has been a great help.

Please be aware that in some cases a powerful hood needs what is called make-up air to prevent the air pressure inside the home dropping into the negative. If you have fuel burning appliances this can suck chimney gasses back into the home raising carbon monoxide in the air to dangerous levels.

I believe that according to some local codes any ducted vent more powerful than 300 CFM will not pass inspection without provisions for make-up air. This would be a fan that brings outside air in to keep air pressure neutral and can be made to heat the incoming air, however inefficient and expensive these may be.

Good point about the make-up air. I have seen on commercial hoods what appears to be a make-up air inlet built into the hood. Residential hoods don't have this, but it doesn't look like it would be difficult to wire in a damper that opened whenever the blower kicked on. From what I have read, some put this damper into a line that attached to their return air system. I have also seen dampers placed close to the range hood itself.

I wonder how effective a damper feeding the return air system would be, compared to a damper that feeds a vent just below the hood? Any thoughts?

MUA supplied into the area around the hood has been shown to disrupt the capture process. MUA introduced below the cooking unit works well, but can freeze the toes, as does air introduced well away from the cooking unit and given distance to smooth the flow.

Unfortunately for our wallets, the amount of negative house pressure that can cause backflow from a combustion appliance is very slight (less than -0.05 inches w.c.). The pressure drop through an air filter and a passive damper is generally higher. So the passive system achieves its safety purpose only if one has no combustion appliances (gas furnace, water heater, or clothes dryer) or the appliances have their own source of MUA.

(My only combustion appliance is an oil furnace, and it now has its own MUA, but if I want to operate my fireplace I will still need pressurized MUA. This is still a work in progress. Also see below.)

Even without the safety considerations, such as they may apply in a given case, achieving a large flow rate from the hood depends on its fan curve, and if the house pressure becomes significantly negative, the actual flow will drop (a lot) until the house pressure plus losses in the hood and ducting matches the fan curve flow rate equal to that which the house leakage can supply at that pressure.

For high flow rates, room heaters cannot heat the air at the rate it can be pulled out of the house, so in many climates a supplementary means of heating the MUA is desirable. (I use a car radiator sized heat exchanger fed by a hydronic loop to my oil furnace.)

For scaling purposes, but your leakage may vary (YLMV), with my house mostly closed up, a temporary 1-inch filter in series with the heat exchanger, and no MUA blower support, my two kitchen exhaust systems on high could bring the house pressure to -0.07 inches of w.c. and backflow my oil furnace (pre MUA) enough to set off the nearby smoke alarm.

At that pressure, total flow was possibly 1000 cfm, whereas at a setpoint of -0.02 inches the flow is anticipated to reach 1500 cfm. (Pressure losses in ducting are also contributing to the loss, as the zero static pressure rating of the two blowers sums to 2500 cfm.)

kas

Thanks for the information. We currently have a heat pump and electric water heater, so no concerns there. If we were to switch to gas for the water heater, wouldn't a direct vent alleviate any concern?

We do have a wood stove, but it has an outdoor air supply kit, so I don't suspect any issues there either.

If problems do arise, it sounds like installing an ERV or HRV is the way to go, since the benefit of a passive damper is minimal.

What would be the best, make up air introduced at the base of the stove, or make-up supplied through the air handler?

Reading your post again...can you explain what is meant by, "...a setpoint of -.02 inches the flow is anticipated to reach 1500 cfm."

This is all unfamiliar to me, so I am having difficulty conceptualizing what is happening here...

a fireplace has make up air introduced at the base of the firebox. It is used for combustion and it goes up the chimney. It is unconditioned air. Your question is a good one. "....make up air introduced at the base of the stove, or make-up supplied through the air handler?"

I'm not sure that there is a particular advantage of under the stove vs. using an air handler for MUA. In hot commercial kitchens, under the stove avoids having to heat the replacement air. In a residence, it is likely easier to intercept the furnace air supply than duct to under the stove.

To answer Transy416's question, a little pedantry on fan curves seems necessary. This requires an appreciation for graphical plots.

Imagine a particular fan or blower, however you like to call them; I'll use fan for brevity. It would normally be specified for flow rate (cfm) by the flow rate achieved when it is hanging in the open. This is the zero static pressure case because the pressure on both sides of the fan is the same, so the differential is zero. Zero static pressure is not the case when the fan is in series with ducts and hood components.

Now imagine that this particular fan hanging in the open has a restricting device placed on one side or the other such that a pressure difference occurs between one side and the other. At some point of increasing restriction, the fan will move zero cfm. Now we have two data points, the flow rate when the pressure is zero, and the pressure when the flow rate is zero. Assume that the maximum flow is 1500 cfm and the maximum pressure differential is 2 inches.

For historical consistency, the pressure is measured in inches of water column. A metric alternative is pascals.

On grid paper imagine a pair of axes drawn in the usual manner of an abscissa and ordinate. On the abscissa mark off a cfm scale, and on the ordinate mark off a pressure scale. Plot the two data points.

The fan curve is the function that connects these two data points by a line, generally convex in curvature relative to the origin of the graph. (Representative fan curves are available from Broan-Nutone's web site.)

Now, given a 1500 cfm at zero static pressure fan, if I attach it to some ducting and a hood with baffles, there will be some pressure drop as a function of fan flow rate through the hood and its ducting. This can be represented by a straight line from the origin of the plot with values of pressure drop increasing as cfm increases. The actual flow of the combination ducting, hood and fan will be the point where the fan curve and this pressure loss line intersect. For this imaginary system design, assume that the line intersects the fan curve where the flow rate is 1000 cfm at a pressure loss of 1 inch.

Now start sealing up the house so that the overall pressure loss would be greater than one inch at 1000 cfm. This point is not on the fan curve so it isn't a valid operating point. Instead, there has to be a new line and new intersection point where the flow is less than 1000 when the pressure is higher than one inch. We want to keep the added pressure loss small with MUA.

What I was hinting at was that if I tailor my MUA control system to try to keep the house pressure no more negative than -0.02 inches relative to the outside air, I will avoid the combustion backflow problem and minimize the degradation of fan flow beyond its baseline due to pressure loss from the ducting and hood.

So, under some condition of MUA supply for this imaginary system, the ventilation is pulling 1000 cfm, and the MUA is supplying almost 1000 cfm (house leakage at -0.02 might supply a bit of flow). The MUA will have to supply this air through its intake (some pressure loss from screening and transitions), ducting, air filter, heat exchanger, and diffuser. These losses may well be higher than for the hood ventilation. One can strive for zero house pressure, rather than -0.02, but this may be difficult even for a PID controller to manage stably (TBD).

In my case, the house has several exhaust fans of 1500, 1000, 80, 80, and 110 cfm (static pressure), which can be on in any combination, and the two high flow kitchen units have variable control. Actual maximum flow with the house in the zero to -0.02 range is estimated to be around 1500 cfm. To stabilize the internal pressure, a servo control loop through the MUA fan motor is required. Most here may be able to avoid such complexity in their own MUA systems.

kas

Thanks for the detailed explanation. This helps me a lot, not just with understanding more about range hood ventilation, but also with excess static pressure in my zoned Carrier Infinity system. I wish the HVAC folks had explained things that way. That's another story though...

When you were talking about a servo control loop through the MUA fan, does that mean the speed of the MUA fan would increase/decrease along with the vent hood fan? That is, for those cases when the vent fan is run on low speed and perhaps increased to a higher CFM later during cooking.

What you have said has made me realize the importance of a large capture area so the vent hood fan can be run at potentially lower speeds, thereby possibly reducing the amount of MUA required. We will be doing a lot of wok cooking and I was considering the Best by Broan K260A. However, it appears the capture area below the K260A filter may be less than other hoods, thereby requiring the vent fan be run at higher speeds compared to a hood with a larger capture area below the filters. And consequently, the need for MUA may be greater for a hood such as the K260, at least in the case of wok cooking and running the fan at high speed, compared to a hood with a larger capture area? Am I understanding this correctly? If so, I am talking myself out of the K260, which is too bad since I can get such a great deal on it....

Servo loop: The power to the MUA fan would ultimately derive from measurement of the differential pressure between the kitchen and my well ventilated attic. (The attic was chosen to mount a BAPI wind resistant sensor to minimize fluctuation from wind. Devices that are very good at ignoring wind effects on pressure measurement are quite expensive. The affordable BAPI sensors will still be influenced if mounted outside.) So the speed will increase or decrease with ventilation fan speeds, but this will depend on how closed up the house is.

The capture area is the aperture of the hood, plus maybe a tad larger. The capture volume is the volume under the baffles. The area has to be large enough for the rising effluent to be captured. The volume has to be large enough for dynamic changes in effluent that might otherwise overwhelm the hood. For steady state cooking, I don't think the volume is as important for volume sake, but it does form the shape that controls where the first "bounce" is directed.

So, when you throw that mixture of flank steak, soy sauce, corn starch, and whatever into the wok, there will be a sudden blast of steam that you want to capture and keep captured.

kas

Transy, if you can wait a month before ruling out the K260, I might be able to help you. I'm installing a 48" Culinarian with a grill. The hood is a 54" K260 with an external fan of 1500 CFM. The hood is being bumped out 3" extend the capture area and to line up with the flanking 15" cabinets. As it stands, it's being mounted about 30" above the range. I'm trying to find a way to mount it at least another couple of inches higher, but I might be SOL on that.

Right now I have passive MUA from underneath a utility cabinet on the other side of the kitchen, cold toes be damned. If I need to, I'll bite the bullet and install an electric toekick heater. I had originally planned on running the duct for the passive MUA to underneath the range, but I had to change plans. The passive MUA is undersized compared to the 10" hood duct. Hopefully, the inspector won't have an issue with my set-up.

I will also be doing a bunch of wok cooking. But I will also be doing a lot of indoor grilling too. Unfortunately, I'm a few weeks away from that, assuming all goes smoothly. The cabinets are being installed this week. The counter top probably a couple of weeks after that. Then another week or two of finish work, unpacking and test driving.

Thanks again, Kas, for the detailed explanations. You've been a huge help!

Jscout - I'd appreciate hearing about your K260. If you can PM me that would be great. What made you decide on the K260 anyway?

I decided on the K260 because it was relatively cheap. Just a year ago, it would have been a Broan for about $100-$200 less. I snoozed and I lost. Broan reconfigured their product line and moved the 54" and larger hoods up to their pricier Best line. Along with the move came a higher price tag for the same product. Frankly, unless you're getting an internal blower, I think all hoods are a huge rip-off. It's just another way to take your money. In an inline and external configuration, the hood is just a large funnel. Other than lights and a switch, there isn't much to it. An insert would also get the job done.

There is one upshot to the K260. When I placed the order in April, baffle filters were optional. I had to wait until July to take delivery because I wasn't ready for it. In that time, Best changed the product and now includes the baffle filters as standard. So in the end, Broan or Best, I broke even, because I think the Broan side of the business still doesn't offer baffles as standard.

For the Bluestar 30" range that sees a lot of wok cooking, I was leaning toward a 36" and 27" hood (or a 24" hood bumped out 3"). I'd like the option of having the hood a little higher from the cooktop, perhaps approaching 72" off the floor, or 36" from the cooktop.

Giving this some more thought, I was wondering if a 42" and 27" hood would be absolute overkill for a 30" range. The price isn't much more than a 36" hood. That, and would a 42" hood over a 30" range look strange, with the 6" of overhang on the sides?

In some publications' photo spreads of poser kitchens, the presence of _any_ hood is strange. But the editors don't have to live with grease deposits and odor. (And maybe the publishers always eat catered meals.)

As always, the kitchen owner has to decide the weight to be given to functionality versus appearance. In my view, a size large enough to capture all of the rising effluent gets highest weighting.

kas

Agreed. Given a 36" x 27" hood and 42" x 27" hood are both wider than the 30" range, would there be diminishing returns with the 42" width hood?

I think 36" x 27" would be more than enough, IMO 42" x 27 over a 30" Range would be overkill from a size perspective.

Sure bigger is better when it comes to capture area, but I think a time comes when bigger is too big.

You must always remember that TRUE capture area is more important then the actual size. By that I mean the actual size of the capture and not the size of the hood outer dimensions.

Personally I know nothing about design, but I would think that a 42" hood over a 30" range would be out of balance from a visual perspective.

Trevor reminds us of a good point, which I'll restate as the correspondence of the expanding rising effluent size versus the capture area size. To address this for wok cooking, it is necessary to decide what diameter of the wok is effectively generating rising moisture and grease, expand this at a conical half angle of, say, 22.5 degrees upward, and see where this cone intersects the hood aperture. This is relatively easy to do with a constant scale sketch from a front and side view with the aid of a ruler and protractor.

Using a larger hood than this sketch effort suggests is needed may be overkill depending on how much the effluent is disrupted by cross drafts.

kas

Good points about the 42" over a 30" range. Most of time a 14" wok will be used, so nothing enormous.

If the 42" wouldn't have much benefit, then there may be no point in it, especially if it is going to look out of place. Anyone have pics of a range hood that is 1 foot wide than their range?