New Hood meets frustrating new city code for MUA/CFM
Macswim
6 years ago
Featured Answer
Sort by:Oldest
Comments (16)
hvtech42
6 years agolast modified: 6 years agoMacswim
6 years agoRelated Discussions
hvac 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 Moremyths about cfm's and mua
Comments (17)A make-up air system for the range hood isn't going to run constantly like a normal HVAC system that heats/cools/filters your internal air throughout the day. It also cannot be compared to a commercial MUA unit for a vent hood over a range in a restaurant because they cook from the time they open to the time they close. So a residential MUA system will have much less of an impact on internal comfort levels than what most people are envisioning. Most HVAC systems are designed to run with slightly negative air pressure to begin with. That's why things tend to blow into your house rather than out. Ask yourself, "how often do I turn on my range hood compared to opening the back door/garage door/front door of my house?". Every time you open a door to your home you are letting in a large amount of unconditioned freezing cold or hot and humid air. And when enough outside air is mixed with the internal air to affect the temperature, your HVAC system will activate and begin exchanging it for conditioned air. The same process will occur with a passive make-up air system. Everyone seems to be afraid of the air outside their homes, but who ever says, "I'm gonna step out for some dirty hot and humid air"...no one. Everyone always says, "I'm going outside to get some fresh air." It really is as simple as allowing your house to "breathe". And like foodonastump said, no house is sealed 100% airtight so your home is going to find a way to bring in outside air. You are just giving it an extra set of lungs by adding a make-up air system during operation of a high CFM range hood to avoid creating enough negative air pressure to stall or backdraft combustion vents. foodonastump, the 2009/2012 IRC M1503.4 code provision reads "Exhaust hood systems capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be provided with makeup air at a rate approximately equal to the exhaust air rate". So the relationship between the exhaust opening and fresh air intake need to be symmetrically proportional at minimum. And the building inspectors will default to the commercial code which reads that make-up air needs to be supplied at 80% of the exhaust rate. And because there are residential range hoods that can exhaust 600 CFM through a 6" or an 8" round duct (several manufacturers exhaust up to 1000 CFM through an 8" duct while most use a 10"), the question becomes even more confusing. And based on the fact that any exhaust vent in your home, combustion or not, is fitted with some type of backdraft damper/flapper, the reasonable path of least resistance for fresh air intake would still be an open and clear 6" duct, even at a 600 CFM exhaust rate. davidrol1, An air curtain is a great application for commercial settings but is just too uncomfortable for a residential homeowner. Hope this helped a little. Nate...See More300 CFM vent hood and BACON
Comments (30)You'll be fine. People have fried bacon for years without venting at all. I use an electric skillet for larger amounts of bacon instead of using my range. No problems no horrid smells. No grease on the cabinets or counter. Properly cooked bacon is a slow process over low to medium heat. I only turn on my vent for heavier cooking. All this angst on this forum about venting is downright silly. Unless you are high heat cooking for hours on end on all burners the recommendations are way over rated. And some people just love to spend other people's money. It's refreshing to hear the appliance dealer didn't try to upsell you. Cooking smells in houses is a good thing. It's what makes a house a home. Range hoods are for removing combustion products,smoke, heat, airborn grease, and steam from the area above the cook top. They were never designed to be whole house ventilation systems. Windows let in fresh air. Range hoods remove cooking effluent....See More300cfm Hood vs. 600cfm Hood Requiring Make Up Air?
Comments (11)It would be best to start any quest involving kitchen ventilation to study the literature, or at least study the many many threads here on the topic. I would prefer to write answers to questions that are based on my possibly inadequate previous writing than generate yet another long message covering the entire topic. You may wish to start with the "Hood FAQ" which can be found by searching. I believe it is listed under @opaone. Hoods are about capture and containment, and success in this depends on many factors, the most critical of which is sufficient air flow velocity at the hood entry to ensure that which is captured is contained and expelled from the residence. This velocity is independent of how many burners are running at once, but rather what the cooking surface temperature is plus plume heat augmentation by gas burners. In any case, the 100 CFM per 1000 BTUh is derived from certain standardized commercial hoods, not residential hoods. See the Greenheck Guide available at https://www.tagengineering.ca/wp-content/uploads/2015/02/KVSApplDesign_catalog.pdf for comparison between the 100 CFM per rule of thumb and the hood aperture velocity rule. I suggest 90 CFM per square foot of hood entry aperture, and this aperture needs to be large enough to overlap the rising and expanding cooking plumes. Let me adjust the salesman's statement: "...because the design pulls more air than a filtered/baffled hood of the same [zero static pressure rated] cfm [when hanging unattached to anything in free air]." Ventahood use squirrel-cage blowers that also perform the function of baffle filters, thereby eliminating that cause of pressure loss. However, as blowers go, they are more susceptible to other pressure losses than conventional blowers, and hence if there is extensive ducting, or inadequate MUA, their flow rate will be less than the actual rate asserted. My guess is that you should have a 24 x 42 inch hood aperture, requiring an actual 630 CFM obtained from a 900 - 1000 CFM rated blower. Read MUA related threads to determine how closely the interior air pressure has to match the exterior air pressure for safety. Your state may require heated MUA. There are a lot of ways to accomplish MUA under these conditions, and the threads here should help you understand what you have to tell us to allow better tailored help. kas...See Moreweedmeister
6 years agoMacswim
6 years agoJoseph Futral
6 years agohvtech42
6 years agolast modified: 6 years agoMacswim
6 years agoMacswim
6 years agoMacswim
6 years agoMacswim
6 years ago
Related Stories
MOST POPULARHow Much Room Do You Need for a Kitchen Island?
Installing an island can enhance your kitchen in many ways, and with good planning, even smaller kitchens can benefit
Full StoryBATHROOM DESIGNHow to Choose Tile for a Steam Shower
In steamy quarters, tile needs to stand up to all that water and vapor in style. Here's how to get it right the first time
Full StoryKITCHEN DESIGNThe Most Common Kitchen Design Problems and How to Tackle Them
Check out these frequent dilemmas and expert tips for getting your kitchen design right
Full StoryHOUZZ TOURSHouzz Tour: Ready for Hurricane Season on the Jersey Shore
This family home was designed to stand up to storms. See how the architects built resilience into the design
Full StoryKITCHEN DESIGNKitchen Remodel Costs: 3 Budgets, 3 Kitchens
What you can expect from a kitchen remodel with a budget from $20,000 to $100,000
Full StoryCONTRACTOR TIPSThe 4 Potentially Most Expensive Words in Remodeling
‘While you’re at it’ often results in change orders that quickly add up
Full StoryKITCHEN DESIGN8 Kitchen Design Tips for Foodies
If you own at least one pricey knife and have a slew of kitchen tools, you’ll want to read this
Full StoryKITCHEN DESIGNKitchen of the Week: Function and Flow Come First
A designer helps a passionate cook and her family plan out every detail for cooking, storage and gathering
Full StoryKITCHEN WORKBOOKHow to Remodel Your Kitchen
Follow these start-to-finish steps to achieve a successful kitchen remodel
Full StoryKITCHEN DESIGN7 Tricky Questions to Ask When Planning Your New Kitchen
Addressing these details will ensure a smoother project with personalized style
Full Story
hvtech42