Perfect is the enemy of hood

mabeldingeldine_gwMarch 8, 2012

I have been researching the kitchen ventilation issue with great interest. I have an 1880 Cape in Maine. We recently re-sided and added an exterior vent. The vent passes through the outer wall of the house into the house at floor level behind the kitchen range wall in an understair closet at floor level. The duct will run for 5-6 feet, bend 90 degrees travel vertically for another 6 feet, another 90 degree bend into the kitchen for hood access. The ducting will not be enclosed in the wall for 99% of the run. My primary goal is grease capture. I will have a 30" Capital Precision range.

I know this is NOT the perfect scenario, but it is by far the best option at this time. Future plans call for moving the range wall when we remodel, but that is a long way off. Given this scenario, I have some questions.

1) Can I use an inline blower or combination under the stairs to make this system more effective?

2) What kind of CFM do I need to capture most of the grease.

3) Will I need MUA in this scenario given my leaky old house and no local code requiring it, and if so, can it be passive air entering near the vent exit?

4) Should I just add a good quality recirculating hood and wait 10+ years for the kitchen reno?

I have read many of these threads and out links and have learned a lot, but I am still a bit bewildered given my situation. We had an energy audit a few years ago and believe me there is PLENTY of air exchange, I am more concerned with where the air is coming from especially if I need a high CFM unit.

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There is no such thing as a good recirculating hood for your described purposes, I'm afraid.

BTW, love your post title. :)

    Bookmark   March 8, 2012 at 9:27AM
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I was intrigued because I thought it read "perfect is the enemy of food". But I'm not too much clearer on the correction. No worries about explaining better to sleep-addled me. I love the glimpses of your house from another thread. Good luck with this! [I've no substantive input as I'm a certified layman here].

    Bookmark   March 8, 2012 at 12:05PM
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Writersblock and Aliris, the title came from on of my favorite English profs who used to frequently observe that perfect is the enemy of good. I use the expression regularly, and remember him fondly every time.

And Aliris, I think perfect is the enemy of food, too. All those pesticides in hopes of unblemished fruit, sigh.

    Bookmark   March 8, 2012 at 1:02PM
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yes, I'm beginning to get it! I like it... (the saying). Thx... [this is why I am not an English major!]

    Bookmark   March 8, 2012 at 1:36PM
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I, too, have an old house (1858) & MUA was never a consideration; after 10 years, there haven't been any problems. My range has (6) 15k BTU burners with a 600 CFM vent (internal with 1 turn) which is what was recommended in 2002 although most on this forum will consider it inadequate in 2012. I have no complaints - if I grilled inside, I suspect I would need a larger system. Sorry, but I have no experience with inline blowers.

    Bookmark   March 8, 2012 at 2:51PM
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I think an inline blower or an externally mounted blower will work fine. If you go with inline, make sure there is a bend between it and the hood. Ideally you should have some straight duct run between at both the intake and exhaust of the inline blower.

I think you can get by without makeup air in your older house.

600CFM is probably fine but I would check with Capital. What size duct and vent did you install?


    Bookmark   March 8, 2012 at 3:20PM
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Billy, we have yet to install the duct. We currently have a microhood, which I am eager to send to a "farm in the country" and replace with a vent. The exit through the house is rectangular, I'm not sure of the size right now, I'll have to check it in the daylight.

Capital recommends 600 cfm, I just thought with two 90 bends we'd need more?

    Bookmark   March 8, 2012 at 7:41PM
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Fortunately, perfect is almost unobtainable in kitchen ventilation, and good enough is, to a great degree, a personal preference.

Do you have any combustion appliances (other than the CP) including a fireplace? If not, house leakage may be adequate, so long as having the actual cfm significantly lower than the rated cfm is OK.

If you can introduce air into the kitchen such that it doesn't cause cross drafts at the range, that would be better just using leakage. Winter operation may bring the expression "chill out" into focus, but this could also be true if the leakage is adequate. Some of the decision space here is dependent on your tolerance for odor and grease.


    Bookmark   March 9, 2012 at 1:59PM
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I have an oil-fired HWBB heating system and a wood burning stove on separate flues.

We are working to tighten the place up, but there are years of work ahead of us. I can always crack a window when I really crank the hood, but installing some kind of passive MUA unit is not impossible. Once you've cut a hole in the side of your house a few times it becomes far less scary.

I have never lived in a place that had a range hood. I love to cook and now that I have a better range I suspect I'll be cooking more. My priority is grease and smoke form searing meat and the occasional pizza spill on the pizza stone. Really anything will be an improvement, but wading through all the details is confusing, especially considering it is mostly written with new tight construction in mind. I appreciate any help you I can get. There is a very real shortage of knowledgeable contractors on this topic in my rural location.

    Bookmark   March 9, 2012 at 5:26PM
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A wood burning stove would be a great test subject. If you do the installation in stages, you can fire up the hood (so to speak) and see if it back-drafts the wood stove. If it doesn't then you are MUA OK; if it does it will be obvious.

If it does, then when your lungs have recovered you can look into putting a damper controlled passive hole in a wall that doesn't put the cold air where it is most annoying. If that doesn't solve the wood stove backdraft then a blower will be needed in series with the MUA port.

I suspect that passive will be OK in your case, but have no experience with this combination of factors. A damped down wood stove has a very limited heat aided draft, and not a few potential locations for leaks, so it might not take much hood-induced negative pressure to make the stove smoke into the kitchen.


    Bookmark   March 9, 2012 at 5:56PM
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You seam to have a good grasp of things. With the two 90 degree turns you will decrease the CFM about 18 % and for each 10 feet you are losing about 10 % more. Therefore actual CFMs at maximum setting is closer to 450. An 800 CFM in-line blower under these conditions would provide about 600 CFM and would be my recommendation. Also, the duct size would be larger, therefore quieter.

You also want a hood to remove moisture effectively, otherwise you may end up doing a remodel sooner than you want.

MUA is probably irrelevant in your situation but Kaseki has some good inout regarding air flow

    Bookmark   March 9, 2012 at 8:28PM
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Hmmm... what does this mean exactly "a blower will be needed in series with the MUA port?"

Does it mean a fan powered MUA somewhere?

Aprince, can an inline blower be used inside the house under the stairs as I described? And how loud are they? Will it have to be boxed in with drywall?

    Bookmark   March 9, 2012 at 10:47PM
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When evaluating the pressure drop seen by an in-line blower due to duct length and bends, one must also include the transition losses at the hood and the hood baffle losses. These, of course, change with the actual air flow, and in any case are almost never reported for residential hoods.

These losses will move one farther up and to the left on the fan curve, reducing actual flow still further. The exception would be if the fan were built into the hood and the manufacturer claimed only the cfm that were achieved in that circumstance. (This might be difficult to ensure for most manufacturers.)

I did mean that if a passive port were not sufficient, then adding a fan to the MUA path would be needed. Without making the system really complicated, you could turn it on when the hood fan was on, or build a pressure balanced damper scheme, or perhaps use some other approach that a commercial building HVAC experienced company might suggest.

At this point I would recommend that you ensure that an MUA path is possible, and then wait for leakage MUA results with your new hood.


    Bookmark   March 10, 2012 at 1:57PM
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