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range hood, cfms, muas -- what would you do?

Posted by ElectraToo (My Page) on
Tue, Sep 4, 12 at 2:27

Cross-posted to the Kitchens Forum.

So the HVAC contractor is starting this week on the system for my new build. When we spoke on Friday, he asked me to consider over the weekend what kind of range hood I'm going to want since he'd like to at least stub in the duct work for it while he is doing everything else. I was pretty sure I was going to tell him to go with 8-inch ducting as that would probably provide me with the most options. It would vent directly to the roof and the run is probably around 15 feet or so. I have had my heart set on an Electrolux 36" gas cooktop with six sealed burners and a griddle. The total possible BTU is 60,000, with 18,000 being the hottest burner. I don't think this is terribly hot for things like wok cooking, although it does come with a wok ring. Nevertheless, stir fry is not something I do often. I might sear a steak once in a while but more than likely, I would grill it outside.

So all of that said, I was considering the VAH hood insert with a 630 CFM internal blower which I feel would be more than adequate for my purposes. I do enjoy cooking (esp. with gas), but as we are a household of two and only entertain a few times a year, I can't conceive of a circumstance where the blower would ever be operating at full capacity unless I burned something, and in that case, I would open the windows anyway (which is what I've always done in the past).

However, I then started reading here about the new MUA requirements and went to check my state mechanical code and found that sure enough, I would need an MUA system if I were to go with a hood that has greater than a 400 CFM blower. As I'm already over budget on HVAC, I am pretty set that I will not be wasting any more money to meet this insane code requirement, so that leaves me with only a few options.

1. Do I just go with a smaller cooktop and therefore a less powerful hood? I am really hacked off at having to give up my six burners. Would 400 CFM be sufficient for a typical 30" professional style cooktop? Would it be nuts of me to try to use it with the 36" (knowing that I will probably rarely use it at full capacity and always have the option of opening the window)?

2. Has anyone tried the alternative solution of installing those Broan MUA Dampers? Do they work as they're supposed to and did they satisfy your code inspector? My architect recommended a Broan hood in his schedule of materials and I am thinking the reason may be because he knew we might probably have to use these dampers.

3. Regardless of the above, would you still consider going with the 8-inch ducting on the theory that if they hood you ultimately chose required 6-inch, it is easier to size down than up?

Thanks in advance for your advice and guidance. Don't know what I'd do without this board as a resource.

Follow-Up Postings:

RE: range hood, cfms, muas -- what would you do?

Have you asked your GC or HVAC sub if MUA is actually enforced in your area? I ask because my municipality's code has the same requirement, but my former GC, who had never heard of the code despite installing many high CFM hoods, checked with our city's inspection office and was told that they have never enforced the MUA code. That doesn't mean that one should ignore the possible need for MUA, especially in a new build where homes are much tighter than older ones.

I installed a 42" 600cfm hood over a 6 burner, 36" CC rangetop despite the usual btu-cfm formula showing that I needed a more powerful blower because I found a great deal on a gorgeous custom hood with blower already installed. I've been cooking up a storm with rangetop and hood for more than nine months now and have done fine. We entertain often; I cook daily from scratch; and I often use a few burners or more at once. Now, its not been long enough to notice any possible greasy build up on walls and cabs if that was going to happen. But on a daily basis, I'm doing well with fewer cfms than suggested. The fact that my hood is 6" wider than the cooking surface, as often recommended here, probably helps. If your hood will also be wider, I think you could get by with a 400cfm hood. I'm only a lay person, not a venting guru. :)

RE: range hood, cfms, muas -- what would you do?

If you have your contractor design the house so that combustion appliances, such as furnaces, water heaters, and gas dryers have their own MUA and are isolated from the kitchen (i.e., in a room of their own), then the safety-based MUA requirement is met, and simple passive devices such as the Broan damper scheme should be adequate if conditioned air is not needed.

Duct sizing should follow manufacturers recommendations, but these should be based on minimal pressure loss while keeping the air velocity at a level minimizing grease accumulation in the duct. 500 fpm is the minimum, but if the duct passes through a cold attic higher velocities in the 1000 to 1500 cfm regime are desirable (values at full power accounting for pressure losses).


RE: range hood, cfms, muas -- what would you do?

Don't forget to consider the noise levels in the kitchen for interior fan units. If it were me, I'd always opt for a roof mounted exhaust unit so that sound is never an issue. Good luck1

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