Heated, passive MUA for 1200 cfm hood
N Johnson
3 years ago
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N Johnson
3 years agoRelated Discussions
1200cfm oversized hood...Overkill for 30" BlueStar range?
Comments (47)Well, "any" is potentially a very small amount, and I'm sure that there is some slight amount. My attic, however, is vented all around the periphery, at the ridge, and also with an attic fan housing. Ice dams at the roof edges tend to occur when the outside air warms up during the day and heats the snow bottom up through the vents instead of top down. In my present half-completed configuration, the ~ 2 x 2 ft heat exchanger sits above a Cooley and Hart 3 x 3 diffuser in a hall ceiling. There is a furnace filter taped down on top of the heat exchanger. This style of filter (Honeywell Filtrete) needs significant pressure drop to move much air through it, and I was pleased to observe that warm air doesn't seem to rise through it in the winter, or hot attic air settle through it in the summer. The sides of the heat exchanger and its plumbing are buried in insulation. When kitchen fans are running and the house is closed up, air is pulled through and the thermostats, the larger-than-typical-size Taco pump, and the furnace controls do their thing to heat the air. (The circuit is capable of well over 100k BTU/hr depending on furnace and air temperature.) The problem, as I have pointed out a few times here, is that pulling air through a filter and heat exchanger requires a pressure drop -- a drop too large for combustion appliance safety. My only combustion appliance is an oil furnace, and it now uses its own MUA system to avoid back-drafting. Nevertheless, if I were to run both kitchen fans at once at full power expecting all flow to be made up through the heat exchanger without fan boost (possibly 1400 actual cfm with restricted MUA), the pressure drop in the house causes some furnace exhaust to be pulled into the house via tiny cracks not normally relevant when there is a positive draft, so I don't do dat without an opened window. My intention is to duct the heat exchanger to a mushroom air intake already installed on my roof via an axial in-line fan and a four-inch pleated filter caddy so that at the maximum possible flow rate (possibly 2000 cfm, say) the house pressure can be balanced and all the MUA comes from outside without mingling with the attic air. A damper may be needed, although as noted, the Filtrete coating will be pretty resistant to air passage without some deliberate pressure drop from the in-line fan. kas...See Morehvac guy suggested 300 cfm vent a hood to avoid mua--thoughts?
Comments (31)It is hard to know where to start here. The flow rate (cfm) is determined by the uprising velocity of the cooking plume effluent and the aperture area of the hood, and not to first order by the BTUs. I would start at 90 times the area in square feet. Baffles will not separate grease from the air at really slow air speeds, so if one intends to go as low as possible, then a mesh that is routinely cleaned is probably better. However, mesh hoods typically have undersized apertures, so capture is degraded at the hood periphery. In other words, the hood is smaller than listed. Baffles will at any speed provide fire blocking, their other purpose. All fans have fan curves, including those made from magic lungs. The fan curve plots flow rate versus pressure drop across the fan, which results from duct friction, duct transition flow disruptions, mesh or baffle restriction, lack of MUA, etc. Typical fan curves are slightly convex, with cfm on the abscissa and pressure on the ordinate. When the pressure drop reaches some maximum, such as an inch or two of water column, the flow reaches zero. At zero pressure drop, the flow is (should be) the rated flow. The pressure drop is never zero in situ. VAH may be counting fan and hood, which can also be the rating used by some others at some times. It depends on whether the rating is for the hood with fan or for the fan only. The VAH rated flow certainly does not include the losses from the ducting and duct transition to the cap at the outside. Unfortunately, unless susceptible to a calibrated measurement, code enforcers will look at the fan rating and not actual flow for enforcing MUA rules. Ideally, they would test for negative house pressure vs. what combustion appliances present are not allowed to exceed without risking back-drafting. The relative loudness and ugliness of outside fans has to be compared to the relative social ugliness of loud inside fans. YMMV. I would not, however, duct to my neighbor's door. Some other path should be adopted. kas...See Morerange hood, cfms, muas -- what would you do?
Comments (3)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. :)...See MoreFinding an affordable range hood that is 27" deep and 1200cfm
Comments (19)Think of it as a shiny tile backsplash with poor vertical alignment. :) There are some pre-patterned stainless steels that can look fairly pretty. Just keep the angle of tilt-out (from vertical) shallow enough that a rising expanding plume that hits it is still reflected upward. Three inches of tilt over 32 inches (less if the range has its own backspash) is easily sufficiently shallow in angle. Even a less observable short metal piece close to the hood could be built and be 30 degrees or less from vertical. For example, a six-inch piece of metal tilted out 3 inches at the top would be at 30 degrees. Tour http://finishedkitchens.blogspot.com/p/kitchens-alphabetically.html for ideas. In any case, a serious gas range needs some consideration of what the wall behind it is made of, and I'd start with 5/8 firecode sheetrock (or stone or brick) covered by something fire resistant, such as metal or tile. Assume a raging grease fire in your planning, because that is what the range manufacturer did to comply with UL and to specify offset distances to combustibles, and what the hood manufacturer did to comply with the hood filters performing their fire blocking function. kas...See Moremtvhike
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