How much is make up air?? It’s pretty but agree i would buy the range you can afford with appropriate venting. 400 cfm seems very low for a 48” hood. we have 600 cfm 36” hood over induction that works perfectly well - we just didn’t do make up air. 😬

Did you read the installation instructions for the two Zephyr hoods in the comment on your previous insert-hood question?

The larger the hood entry area, the more flow rate (CFM) is needed to keep the air velocity (ft/min) at the baffles high enough to assure full containment of the cooking plume(s) that the hood overlaps. (Plumes expand as they rise.) I recommend an actual 90 CFM per square foot of hood entry area, which generally requires a blower that is rated at 1.5 times that value.

There is always make-up air (MUA). No air leaves the kitchen that isn't replaced. The hood is just gagged by the MUA restrictions to some low flow rate if one doesn't provide a low enough pressure loss path for the MUA. What is provided by house leakage may pull moisture into the walls and may cause back-drafting of combustion appliances and thence a carbon monoxide hazard.

Deliberate make-up air will be needed in most municipalities above a rated 400 CFM. How complex the MUA path/system is depends on a number of factors. Do you have other combustion appliances? If so, do they take air from a room connected (air wise) to the kitchen, or do they have their own MUA? If no combustion appliance back-drafting is at risk, a passive (no blower) MUA path (with filter and damper) may be sufficient. Otherwise, see Fantech or others for potential MUA systems.

If you absolutely can't fit or afford MUA, then you might be better off with a recirculating hood if the needed flow rate is available, but one capable of the flow rate and cooking load may be expensive also. Depends on your cooking. See the thread:

https://www.houzz.com/discussions/2347276/vent-a-hood-ductless-ars-range-hood-update#n=181

Sell the range if you cannot afford the required accessory parts. It's like buying it, and then not having the money for the gas connector, or the electrical cord. What you want cannot happen without having all the parts you need in place.

@kaseki thanks so much for the helpful information. The range is induction, and there are no combustion appliances in the kitchen. It’s an older home, so we could only fit a 6” duct in the ceiling. Hood and stove have not been installed yet, so there may be time to revisit the fan cfm and the MUA passive system

I forgot- the kitchen is somewhat open concept, and there is a wood burning fireplace insert about 20’ away

And that wood burning fireplace will backdraft CO1 and live coals into your home if you don't have the appropriate ventilation. That's why MUA is a code requirement.

Besides the fireplace, which will likely be particularly susceptible to hood induced back pressure, the backdraft requirement extends to any combustion appliances in the residence that take air from rooms connected to the kitchen air. Separate MUA systems for furnaces, hot water heaters, etc., are recommended so that imperfect MUA can be provided.

Don't use the fireplace when cooking. Balancing the internal air pressure vs. the outside air pressure requires an MUA system with closed-loop controls. If this is the only combustion device in the house, and if you forgo the fireplace when cooking, then a passive "hole in the wall" type of MUA should suffice.

A six inch duct is too small for the size of hood needed to overlap the cooking surface. Try to find a duct route that can allow at least 8 inches of duct.

Back-Drafting Considerations for Mechanical-Draft Appliances

Heating systems with fan-powered exhaust systems can withstand higher negative pressures than natural-draft appliances. Some types of fan-powered systems are much better than others, however. In order of effectiveness, the choices are:

* Sealed-combustion. Also called “direct vent,” these appliances draw all combustion and dilution air from outside. These can typically tolerate negative pressures in the range from 25 to 50 Pa.

* Power-vented. These draw their makeup air from indoors and are also called fan-assisted, forced-draft, or mechanical-draft. These can typically tolerate up to 15 to 20 Pa of negative pressure.

* Induced-draft. These have a small fan added for energy performance, not to overcome house depressurization. These can typically tolerate 5 to 15 Pa of negative pressure.

By comparison, an atmospherically vented furnace can back-draft with as little as 5 Pa of negative pressure, and a gas water heater will have spillage at 2 or 3 Pa. Fireplaces can start having problems at about 3 Pa. Canadian codes limit negative pressures in homes with atmospherically vented equipment to 5 Pa. U.S. codes do not currently address the issue (in a plainly spelled out "prescriptive" number).

1 pascal = 0.00402 inches, water column

1 atmosphere = 407 inches, w.c.

An Ilve 40” Majestic 2 Induction range is an $11,000 appliance.

It is meant to be purchased by customers who can and are willing to pay for the ducting, hood, blower, MUA, etc that it requires.

The old house is not the obstacle. If you can fit a 6” round duct, then you can fit a 6” x 14” rectangular duct. Heck you can split to twin 6” x 14”.

If you absolutely don’t want to have the appropriate venting/MUA, and you have high enough ceilings, it might be possible to have someone design, custom fabricate, and install a hood with huge capture volume. For example 48” W x 36” D x 36” H. Using high capture volume to make up for low cfm.

But you’d need to consult a specialist, not your contractor, and the cost of fabricating such a large hood from stainless steel - well, it will again come back to money.

High end kitchens are a ”pay to play” game, unless you DIY.

Thanks again @kaseki… you are so helpful!
the fireplace is a direct vent, with an enclosed burner box and blower.
As a New Hampshire transplant from Southern California, this is all new to me. My CA kitchen had a 48” GE Monogram gas range with a 54” 1400cfm hood, and MUA was never brought up by anyone- the architect, kitchen designer nor the inspector.

I believe the range hood make-up air requirement was introduced in the 2009 International Residential Code (which forms the basis of most statewide building codes.) Some states, like my home state of Virginia, adopt the IRC with a three-year lag so it wouldn't have applied until 2012, and possibly 2013 depending on when permits were applied for.

Houzz just ate my long comment. I'll try to reconstruct it later.

About how much does it take to install MUA? Just a range, please.

$200-$20,000. It depends on location, CFM, heating and cooling needs, route, and a dozen other site conditions, as determined by your GC.

Dumb contractor with a dumb idea.

I'd say dump the 40" range. I just pulled out a yardstick to see just what a large range that is -- and all this trouble. Go with something in a more reasonable size.

"the fireplace is a direct vent, with an enclosed burner box and blower."

There are a lot of ways that a blower can be used with a firebox. Please explain what is being blown. E.g., Air is pushed into the firebox, air is pulled from the firebox, air is circulated around the firebox to warm the room.

"As a New Hampshire transplant from Southern California, this is all new to me. My CA kitchen had a 48” GE Monogram gas range with a 54” 1400cfm hood, and MUA was never brought up by anyone- the architect, kitchen designer nor the inspector."

Welcome to NH. When I wrote my disappearing message yesterday, the outdoor temperature was 4.7F here in S. NH. The amount of tolerable house leakage here in NH vs. in S. CA may be expected to be different, as will the probability that a kitchen window will be open. This is why heated MUA will be needed unless you like cooking in an ever cooling kitchen. Household furnaces are intended to maintain temperature against heat losses, not against a multi-hundred CFM fan pulling in cold air.

Chef Liu raises the potential of using high capture volume (reservoir volume) to compensate for insufficient flow rate. Some salient points:

• If only informal MUA is available from house leakage, my 2/3 fan curve rule won't apply; you may get only half the rated CFM depending on the house construction.
• Pulling air through wall cracks is not good for you or the house.
• Baffle averaging through use of large reservoir volume only works so long as the plume volumetric rate is less than the actual blower volumetric rate. 200 CFM may be marginal for some cooking with continuous high velocity plume generation, such as wok cooking.

Some bottom lines:

• Performance, aesthetics, cost -- pick two or trade among them
• MUA can be more complicated, and a greater pain in the neck, then the hood system itself; you need to consider the options in the context of your residence's architecture (framing mainly).
• Do whatever is possible to avoid combustion appliance back-drafting. Iff successful, passive (no blower) MUA for your hood system will likely do, and should pass inspection.
• In this case, you will need a "hole in the wall," wall cap, damper, air filter, and heater (electric, hot water, whatever).
• Attic, basement, mud room are all possible locations for MUA ingress. However the release into the house (and particularly if into the kitchen itself, should be as non turbulent as possible when the MUA gets to the hood. Release out of a toe-kick panel, for example can be problematic if at the cooktop's cabinet.

@kaseki again, thank you for all of the info. The HVAC contractor that worked on the house now seems overwhelmed with some personal issues so I think we may hire a specialist to consult on site and give suggestions and costs. I couldn’t seem to get any answers out of the last guy. The hood is not yet installed, and even though the ceiling is closed update we can still revisit enlarging the ducting. Where in So. NH are you? We are in Peterborough

S.W. Rockingham Co.

I was told by an appliance rep for Zephyr, that a MUA kit costs about $2000-$4500 installed.

I have an older home and did my kitchen many years ago....I have a 48" gas range, 54" SS hood with 1200CFM blower. My house was built in 1925 and an appliance rep told me at a product knowledge luncheon....that with my older drafty home, I wouldn't likely need an MUA system. I have two pilot lights in my home....gas fireplace insert and water heater. I only use the LOW setting on my hood except for the holidays when multiple burners are in use.

I've never had a problem in my home (that I know of)...and my kitchen was installed about 13 yrs ago. However, the last few years I started cracking the kitchen window when I use my hood....just in case!