You might get more technical advice from the vent experts at the Applaince Forum. Many old threads there, too. FWIW, I do not think it sounds remotely like too much air flow for gas.

Frankly even for our 48" range, our AC guy told us we didn't need more than 600CFM. He told me that the point of the blower vent was to direct heat towards the vent.

For the smells, I got an air filter for the AC instead which is what the restaurants use to get rid of cooking smells. Apparently CFM has nothing to do with the smell.

You don't *need* 1000 CFM. If you have a tight house and no makeup air, a powerful hood will cause issues but that would be true of 600 CFM as well. Jumping up to 1000 could drive of the cost of makeup air. Other than that, more CFM gives better performance on high speeds, and when you use it on lower speeds (which you will be doing for most cooking), it will give equivalent performance to less powerful hoods while making less noise.

Frankly even for our 48" range, our AC guy told us we didn't need more than 600CFM. He told me that the point of the blower vent was to direct heat towards the vent.

He gave you bad advice. I can see where it came from... when you exhaust more air, it has to be replaced, and that can cause issues elsewhere in the home if not dealt with properly. That is his main concern as an HVAC contractor, not how well the hood will perform with heavy cooking.

For the smells, I got an air filter for the AC instead which is what the restaurants use to get rid of cooking smells. Apparently CFM has nothing to do with the smell.

I would hope you were already using a filter on your A/C anyway! If you mean you upgraded to a more effective filter - makes sense, but those filters can cause issues of their own with air restriction. Gotta have good duct design on your HVAC system.

For just a hundred bucks, go for the 1000CFM. You may never need it but it'll be nice when you do.

We have a 30 inch Bluestar with a 36 inch 600cfm Broan with variable speed blower. We have makeup air. Our duct makes one 90 degree turn. I don't do any high temp searing or wok cooking. I do some pan frying and sautéing.

Sooooo...even at the lowest setting, the hood is carrying most of the cooking smells out of the house. If you stand on the deck outside the kitchen, you can smell what's cooking while inside the kitchen there is almost no odor at all. I'm very happy with its performance. We have the blower right in the kitchen (is that "in-line" not "external"?) and the hood is kind of loud. Luckily the lowest setting does what we need.

1000cfm is probably overkill. If you cook like me, you will never use that many cfms and the noise would be too much.

We have a 1200 Prestige range hood insert with the remote blower. I forget the model but we have a 48" gas cooktop and I really can't hear much from when it's going at any speed but it works beautifully, which is the purpose. We have a 45 and a 90 degree turn in the attic too. Would I buy it again? Yes. No smells, ever. If you're still designing, try to make your sump (capture area) as large as possible.

Those 90s slow things down. A thousand CFM is like a pistol on your nightstand. Better to have it and not need it than to need it and not have it.

With a run that has multiple 90's I would not want less than 600 CFM.

1000 may be slightly high but if you are happy with an in line blower and this brand, it really is a no-brainer for an extra $100. Multiple 90's can eat up a lot of air flow in a hurry. You may also want to consider the diameter of the exhaust pipe you can use. At 1000 CFM I'd hope for 10" pipe. High CFM through a smaller diameter pipe can create turbulence and noise.

Some hood companies will give you a deduction chart with a breakdown for loss of CFM on turns, length of run etc. You can find some of these on line. I know Vent-a-hood offers charts for their products.

I'm not familiar with this brand however in line blowers should be quieter than a blower in your hood.

I'd also consider having an HVAC guy install your exhaust. I've found I get a much better install (and advice) this way. A HVAC installer should seal your seam joints properly and with a flat run in between two 90's you don't want seams leaking grease in a few years.

We recently had our kitchen renovated and had a 1,500 cfm roof-mounted fan installed for the insert over our 36" rangetop (our contractor recommended 1,200). It's a one story house with a straight run to the roof. My logic was that you can always turn the speed down (and a higher cfm fan at low speed is quieter than a lower cfm fan at medium-high speed). At high speed, the fan sounds like a jet engine and it can atually pull up the baffles in the insert (which are pretty heavy), but at low speed you can barely hear it.

I think the make-up air issue is a red herring. As long as you're aware of it, you can open a window when using the fan at higher speeds. I really don't see why every system in the house needs ro be idiot-proof.

in any event, it was raining the other day, and I decided to cook our sausages on the indoor grill, rather than get soaked cooking them outside. As expected, the fan was noisy as hell, but not a bit of smoke entered the house. When I was done, the baffles were a greasy mess, but the air in the house was perfectly normal.

There are many discussions about this on the Appliance forum, which will give you formulas about the proper fan size. It all boils down to the type of cooking you'll do, how much heat you'll generate and the specifics of your venting. In this case you're getting a very high heat range (which implies you're planning to make some smoke) and have a 90 degree bend in your vent. If it were me, I would get the largest fan possible. As Joe said, you may never need it, but you'll be thankful for it if you ever do.

Sorry if I cam off as flippant in my post. I understand that make-up air is an important issue, and one that probably 99% of homeowners no nothing about. I researched this issue quite extensively before starting our kitchen renovation (if you do a search on the Appliance forum, you'll see that I posted a number of MUA questions there), and I eventually decided against installing one.

The renovations on our house ran well into six figures, and installing a simple MUA system would barely have made a dent in our budget. Installing a heated MUA system is a different story though. I found only one system that could heat incoming air effectively: it was very expensive and required an 80 amp circuit breaker (which would have required an electrical service upgrade as well). That probably would have caused a significant dent in our budget and greatly increased our winter electrical costs, all for a marginal increase in the functionality of our kitchen.

Some might call this cutting corners, but I think of it as allocating a fixed resource in the most effective way possible. In our case, I could choose 2 out of the following 3 options: 1.) replace all of the 1960's windows in our house with new Marvin windows, 2.) replace all of the soffits in our house with vented ones and put in a new roof and gutters (we've had significant problems with ice dams most winters), and 3.) install a heated MUA system. I chose to do 1 & 2 on this list, as I thought these upgrades would give me the most bang for my buck.

For me the issue boiled down to the fact that I wanted control over where and how the MUA comes into our house. In summer I might want to let it in near the AC return to help circulate some fresh air throughout the house. When the temperatures are in the single digits though, I'll probably open a window close to the hood so most of the frigid air will exit right away. My wife and I are used to opening windows when we cook anyway (as our old 400 CFM hood never vented very well), so continuing to open a window seemed like the easiest solution.

All of the residential MUA systems I've seen are passive systems with a damper that opens to let in air. I'm certainly not an expert, but I've never seen a forced air system for residential installations. There's a 10" vent pipe on my hood and the largest Broan MUA system that I've seen has a 10" duct as well. My old high school geometry tells me that this equates to 78.6 sq. in. of space, so I don't understand why opening a window about 4" wouldn't let in enough air to avoid any problems.

The town I live in is very strict in code enforcement, but the building inspector told me that he only requires MUA in commercial establishments. Likewise, my appliance dealer said that he sells lots of high CFM hoods, but rarely sells MUA systems with them. My HVAC guy said we could talk about MUA when I replace my AC system, but also didn't think there would be a problem with just opening a window. I know that the State of Massachusetts recently started requiring MUA systems in new kitchen installs but, outside of that, I'm not aware of any localities that require MUA for residential kitchens. So I really don't see this as a code issue at all.

The only person who might conceivably cook in our house (other than me and my wife) is my mother-in-law, and we're certainly planning to explain the MUA issue to her the next time she visits. I'm also not planning on selling the house anytime soon. If I did though, I'd be happy to explain the MUA issue to a prospective buyer who wanted to talk about maintenance issues, even if that meant that I'd have to knock a thousand or so off of the purchase price. If the buyer were an arrogant snot who thinks he knows everything though (and there's more and more of those types in our town), then I don't think I'd be under any particular obligation to bring it up. At any one time, there's hundreds of houses on sale in our area with high-end kitchens equipped with high CFM hoods, and no MUA system installed. I know this because we considered buying a new house before getting ours renovated, and looked at a lot of houses. No one we spoke with even knew what MUA was (neither did I before joining GW for that matter). So I don't see why I should be held to a higher disclosure standard than anyone else in this area.

By the way, I will be getting a dedicated MUA system for the new boiler we had installed in our basement last year. I started to notice a smoky smell by the fireplace in our living room every time the heat turned on last winter, and the problem was alleviated by cracking a window in the basement.

In any event, my original intent was to say that the OP should think about getting the right size fan and then think about MUA, and not let the MUA issue scare them off from buying the right size hood.

The issue with the length of the duct and the 90 degree bends has to do with what's called the static pressure of the ducts. The exhaust fan's CFM rating represents the CFM at a standard static pressure. If the static pressure of your ducts is higher than that standard due to length and the number of bends, then a lot of the work of the fan goes to just overcoming the static pressure, and not into actually moving air. Which is to say, a 600 CFM won't deliver 600 CFM with your duct setup.

I'm not an engineer, so I can't tell you what size you do need. Just that the reported air volume is not going to be the same as the air volume you get with your duct setup.

Wow...so much priceless information.

Ever so slightly OT: the MUA discussion and talk of drawing combustion gasses back into the house is making me wonder a couple of things. First, is there an art to where the MUA inlet is placed and the pressure needed to open it? I.e., might it be "easier" for air to come back down the chimney, bringing the smoke from a cheery winter fire along with it, than for the MUA inlet to open? Does where the MUA is located make a difference in that?

Second, do I need to worry about a high-CFM fan (current spec is for a 42" 1200 cfm Zephyr with a 10" duct going straight up through the roof) blowing out the pilot lights on our old O'Keefe & Merritt?

Regarding MUA, interesting, our town just started this code. What happened to all the homes prior to this code? The high CFM pro style hood craze has been around awhile now.

I was browsing the 2009 IBC to try to see where it discusses MUA so I can show our builder but it is a rather long document :P

R

Pay attention to what hvtech42 is saying. He is right. Too much blower is actually more dangerous that too little. As a rule of thumb, any vent with a capacity of 350 cfm or greater needs makeup air, which means air coming in from outside the building envelope. In most jurisdiction the mechanical code requires make up air for a vent with a capacity of over 350 cfm. In winter that is cold air. So you are exchanging 1000 cfm air that you already paid to heat for cold outside air that you will now have to pay even more heat. To avoid the problem there are heat-recovery make-up air systems. But, for a guy who is worried about saving $100, the $1000 price tag of such systems, installed, may be a little daunting.

What does the range manufacturer recommend for ventilation?