Custom Range Hood Help, Please.

pretzels

I would like some advice/ideas about, well, my idea.

Our house is just a couple of years old. I did all the design and some of the work myself. It is a modern/industrial design. Our A/C ductwork is exposed spiral duct.

With that said, we planned initially for a recirculating range hood. However, we have run into some really bad condensation issues in our house, (its sealed really tight with no venting besides bath fans), and we are thinking that we need, or rather have decided we are going to install an outside venting range hood. We have a gas range installed on an island. Our house is 2 story, and there is a bedroom above the kitchen area. We can't vent up through the roof/bedroom, and the range is on an island so it cant vent directly out a wall. We have a 36" whirlpool propane cooktop with 5 burners.

My idea is this. Get this Broan externally mounted blower: http://www.broan.com/display/router.asp?ProductID=1255

Use 10 inch spiral duct work, exposed. Which i think will look nice, and match the rest of the design. The duct work will have to go straight up about 5 feet, then have a 90 degree elbow, then about 10 feet of more duct work, then a 45 degree elbow, and lastly about 1-2 more feet to the outside. I would then build a box/hood the same size as the cooktop to attach to the bottom of that spiral duct, and some how get a grease filter and lights up in it.

Any thoughts?

My questions/concerns:

What to do about replacement air?

Is the fan big enough, too big, or too small?

Below is a link to a picture of our kitchen. The red x is where the ductwork would exit the walls to the outside.

http://www.flickr.com/photos/61280032@N06/6126184226/

Thank you.

pretzels

anybody have anything to say?

fabrk8r

Sounds like you have a good plan. You will definitely need make-up air when exhausting 1,200 cfm with your home is as tight as you say it is.

If it were me I would add another spiral duct to bring in make-up air directly into the hood and use a motorized damper at the intake and wired to the exhaust fan switch with a delay so the intake damper would open at least 1/2 way before the exhaust fan turned on. This would eliminate pulling conditioned air out through the hood.

I would simply use a spring loaded or gravity backdraft damper on the exhaust.

__________________
"The physician can bury his mistakes, but the architect can only advise his clients to plant vines." -- Frank Lloyd Wright

pretzels

do you think 1200 cfm is over kill? they have a 600 and 900. I just figured since the length of the duct was pretty long, with a 90 and a 45 bend in it, it would need the 1200.

would it be bad to reduce it down to a 9inch spiral duct? that is the size of the a/c ducts, so it would match, but i imagine the difference isnt that noticeable.

any other help would be much appreciated.

i dont know how that would look with another spiral duct going to it, that may look too funny.

hvac122

What type of heating system? Gas or electric? Water heater? With a 1200 cfm Hood in a very tight house it could cause many problems. Even bringing in makeup air you would need to condition it. The air will get in some way.

pretzels

I have a heat pump. Everything is electric except cooktop.

by saying "the air will get in some way" you mean it will find its way through every crack it can basically?

What do you mean by condition it? Have it run through the a/c return vent before its released into the house or something?

thanks

fabrk8r

If the exhaust hood is carefully and correctly designed you could make both the exhaust and make-up air appear to be one spiral pipe by running the exhaust inside of a larger pipe used for make-up. Think of it in terms of the concentric vents that are used for high efficiency furnaces.

This design is somewhat similar to a commercial kitchen hood that has the make-up air form a sort of air curtain to keep from pulling too much room air out of the exhaust and may require a fan on the make-up air supply.

Putting the custom hood design considerations aside for now, moving 1,200 cfm through a 9" pipe the air will be moving at about 2,500 fpm. That will be pretty loud. Reducing the exhaust fan capacity to 900 cfm and maintaining the 9" dia. pipe will still move the air at 2,000 fpm which is a little better, but probably still objectionable for a residential kitchen.

__________________
"The physician can bury his mistakes, but the architect can only advise his clients to plant vines." -- Frank Lloyd Wright

pretzels

wow, i just wrote a lot, then it got deleted, oh well, ill just ask some more questions.

What size ducts, fan, and how exactly do you recommend setting it up then?

Even if this fan is externally mounted and has a large duct like 9-10" inches you really think it will be louder than the majority of the non custom range hoods?

Most of the range hoods i see all have 4-6" ducts, wouldnt that be way to small for my application/needs? I looked at tons of ranges for a while and they were all 4-6" ducts, then i saw this external broan with a massive 10" duct and i was like....holly you know what.

obviously im not expert, and i dont intend to imply that i think i know better. however, it just seems that the range hood would be way less efficient if it had the make up air directly in the hood like that. as in, it would be sucking more of the outside air back outside, and causing less of a suction on the contaminated air. on the other hand i do under stand the whole 'air curtain' theory. i believe its the same as like in Walmarts and other large stores where they have these massive fan/vents blowing down on the entrance doors, or down on where the buggies come into the Walmarts and they leave the door open all day long, but it has a huge fan/vent blowing downward, which keeps the outside air out and the inside air in, somewhat. That does make since to me, but the idea applied into the exhaust hood is confusing, since my actual intention is to suck the contaminated air out, rather than keep outside air out and inside air in. Does that make sense. Now i do understand the need for make up air, not fully, but it makes sense to me. Which i came across all of that when i was researching how to handle my condensation issues with a house thats sealed to tight.

so yeah, any recommendations on exactly what sizes of fans and ducts to use and how to set it up in a correct manner would be greatly appreciated.

thankyou for the help.

fabrk8r

If I have a lot to write in a post I like to write it in wordpad first, then copy and paste it in the reply. I've been frustrated too many times by losing my reply.

I like your plan. It makes it really easy when you have exposed ducts and are already going for the industrial look.


With an externally mounted fan it isn't the fan noise to be concerned about so much as the noise of the air moving at a high velocity.

4"-6" diameter exhaust duct will move 250 cfm @ 1,200 fpm, which should be more than enough to exhaust the heat and cooking fumes from a residential stove using the rule of 5 cfm per inch of range top width. In your case the minimum fan would be 180 cfm. The extra cfm of a larger fan will help remove more heat and grease.

This hood will need to be designed specifically for your situation if you want to have make-up air supplied directly to the hood. This isn't an off the shelf item unless you have a local restaurant equipment supplier who can order one built to your size requirements. You can make one yourself, but it will require some expertise in hood design and air-flow. Most of the hoods I make are either huge restaurant hoods or small hoods for custom residential kitchens. I've never made a small hood that required make-up air into the hood like commercial hoods do, but it would be based on the same principle.

__________________
"The physician can bury his mistakes, but the architect can only advise his clients to plant vines." -- Frank Lloyd Wright

maxfischersweet

so i guess my main question now is, is the 1200cfm fan overkill and should i get one thats more like 600-900 cfm, or is the 1200 a better fit for my set up.

hvac122

Most houses are 300 or less. Depends on your needs.

hvac benny

What does the literature that came with your cook-top specify for an exhaust? I'm surprised that you were allowed to install a gas cook-top without proper ventilation; around here you must have it to pass inspection.