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Discussion Starter #1
My new home is being built very 'tight'; icynene and everything sealed up. The HVAC has been engineered with this in mind, and a 3rd party inspector is checking it all out. With that in mind, I need to ask for help in choosing a range hood. We'll have LP and a 30" range. We are average cooks - just feed the family. The challenge is this: the vent duct goes straight up about 25 feet through the roof. The ceiling is 18 feet. The kitchen/breakfast room is about 15X26 and is pretty open (but not wide open) to the living which has the same high ceiling.
I just don't know how powerful a hood I am going to need. I'd appreciate any help!
 

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200CFM give or take.
Don't forget the make up air you'll need for it, specially if your house is tight. ERVs/HRVs are not make air units.
 

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Yep. When I built a house in Calgary the code required that any range hood above 150-200 cfm I believe, needed to be interlocked with the furnace fan and a combustion air pipe be installed in the furnace room. The exhaust fan can create a serious negative pressure in your house and cause problems. Especially if someone is using a bathroom exhaust fan and the central vacuum is running at the same time. Check with your local building and mechanical inspector for the code requirements. Buy the best quality/expensive one you can afford. They get a LOT quieter as the price goes up and you will thank yourself later not having to listen to a rumbling fan while cooking. You get what you pay for with them. Mine is directly vented horizontally thru the side wall of my house. They made a rectangular box in the wall for a 'S' air trap (slows down air and heat migration and drafts in the off cycle) then connected the fan to the lower end and the sidewall vent from the top of it. Similar to a sidewall vented gas fireplace.
 

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I'm surprised that I don't need more. I am in Houston and we have lots of heat and humidity (though this is the coldest winter in 20 years:thumbdown:) I thought that the duct being so long and straight up would require more pull. I definitely want quiet, and it seems that the more expensive and quieter ones are also much more powerful.
 

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They are. I would go to a higher end appliance store that sells Jenn Air, Bosch, Kitchen Aide, Miele type appliances and get some professional advice on the size and type you need. The larger the stove the bigger the fan in CFMs you need. Gas stoves and grilling type need more CFMs. If you cook large pots of pasta and give off lots of steam you need a larger one. A high end appliance dealer (non big box) should give you good advice. They usually have reputable installers and sub contractors to properly install it also.
 

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1 CFM per 1,000 BTUs input.

CFM is not pull. Pull as you refer to is actually velocity. 200CFM through a 6" pipe has more pull then 300CFM through a 10" pipe.

Most of the ones rated at 400CFM. When piped right, are only doing 175 to 250CFM.

You need to know the size(sq in of area of the exhaust pipe). And the length to determine what size rating the fan needs to be. The actual air flow won't be the 400 or 600 CFM rating. And you don't want it to be.
They might be rated at 400 to 600 CFM at 0" static, but drop off quick. You'll need one that does roughly 200 CFM at your exhaust length.

You bring in the hot humid Texas air at that rate, and you'll add 2 to 3 tons of cooling load to your home while the exhaust is running.
 

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Discussion Starter #7
Thanks so much for your expertise!

My pipe is 10" diameter and goes up 24 feet. The maximum btu's on one burner is 16,200 (natural gas, so I don't know how this translates to LP) and the bake burner is 18,000, also natural gas.

The price range on these hoods is amazing.
 

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When converted to propane, the BTUs will be the same.

The 10" pipe is for a large volume. In excess of 400CFM. You'll pay a lot to recondition the air it exhaust.
What ever amount of air you exhaust, you have to bring in make up air(or 90% at least).

Does your hood have make up air ability?

On a mild Texas day. 90OD temp at 50% RH, and maintaining 80 Indoor temp at 55%RH. Bringing in 400 CFM of fresh air is a A/C load increase of 10,993BTUs an hour. Or 183 BTUs a minute of exhaust fan run time.
 

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Discussion Starter #9
So I wonder why the builder put in such a large pipe. It definitely sounds like I don't need it and will just be wasting my electricity.
The large pipe has already been installed through the roof to the top of the ceiling. It needs to be repiped (drywallers dented it) down the wall to the hood. Can a smaller pipe be fitted to the larger one?
 

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Kind of too late for that now.

Builders have bent to the misguided wants of people. Everyone one jumped on the big exhaust band wagon. And it has become an energy hog standard procedure.

Along with builders often not knowing for sure what size range will end up being installed for sure.
Often people opt for a larger range then originally speced. So a large hood gives then safety.

Have you spoken to your builder about make up air yet?
 

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I would imagine you can hook a smaller pipe to the larger one or even use the larger one as a liner (small one run inside). You would need to find out from the hood manufacturer what size of pipe they require/recommend so the fan works properly according to their specs so you get the proper flow AND velocity otherwise it may overload/burn out/void warranty. A professional sheet metal company should be able to work around your situation. Everything has to fit and meet the code so I would get it approved BEFORE you do it. Some of our high end appliance dealers here work with professional installers/subcontractors and that is who I would recommend. Generic builders are just that. That gas code and gas inspector may want to have a opinion on it also, they are VERY picky here.
 

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Using the 10" for 200CFM drops the pipes velocity a bit low. Grease can build up quickly. Using it to run a liner through is a good idea.
 

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10" pipe is more like a chimney than a vent. Belongs in a restaurant or laundromat. Get the proper size pipe which MUST match the manufacturers specs or it won't work properly.:thumbsup:
 

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Discussion Starter #14
I'm sorry. I was wrong. I went out to the site and measured again and it turns out it is 8". So that's better at least.
 

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Thats a bit better.
Should be able to keep your CFM to 300 or so.
 
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