Most hoods come with multiple speeds, the top speed is the listed CFM and the lower speeds are obviously lower.

Why do you want to reduce the CFM, most people want their hoods to suck more...

Just get the MUA that you actually need instead of bending o we backwards to do it wrong with a kludge.

To make sure we don’t have to make up air is more expensive and just not in the budget. We had an appliance guy tell us to get the cfm reducer and I wasn’t sure about it, I even asked an electrician and he wasn’t sure about it either. I understand the higher the fan mode, the higher the cfm, but if code doesn’t allow over 400 cfm without extra make up air, can a person really get a range hood that goes up to 900 cfm plus?

If you can’t afford to do the ventilation right, then you can’t afford the fancy range that is causing you to not afford to do the ventilation correctly. You don’t separate one from the other. They are inseparable components. They must match up correctly.

Switch back to a consumer grade range and solve multiple problems with one decision.

Install a smaller diameter duct, and your hood won't suck all that much air. That'll achieve what you want. But it's a bit of a waste of money.

I have a great hood that during a remodel moved from our old kitchen into the in-law unit. The in-law kitchen has a small-diameter duct (maybe 6"?) and there really isn't a cost-effective way to fix that. The formerly nice hood now isn't even powerful enough to prevent the smoke alarm from going off when we cook.

So, yes, what you are proposing is technically possible. But then I question why you even bother with getting a hood. 400 cfm is not going to do a great job. It's better than a recirculating hood, but then that's not really saying much.

Can you get an unheated MUA system? Would that be acceptable in your climate? That's a relatively inexpensive solution to meeting the code requirements.

I guess this is well more complicated than our builder made it out to be. All I’m doing is looking at range hoods and seeing what I like, most of what I like are over 400 cfm. I was just trying to see if the reducer would work or not, apparently not. Thank you for your input.

I agree that you need to get the hood that is reccommneded by the range and 400 CFM is pretty low so I certainly would be getting MUA and forget the “reducer”

Start at the beginning. Post your layout and appliance choices. And location. Builders are never ever noted for having attention to detail. And this IS far more complicated than you ever thought. Or they think.

I didn't answer the reducer question. One can gag the duct to reduce the flow, but unless the inspector agrees with the reduction, it won't save you from some form of MUA scheme. Normally, the inspectors go by the blower rating in the manufacturers literature.

There is a blower/hood combination (I forget the manufacturer) that has been brought up on this forum where a one-time control board setting can limit the CFM. But unless that hood is the one you wanted, it doesn't solve your problem.

GreenDesigns

We are building in Ohio. I went to a high end appliance store, thinking they would know more than Lowes or HD, that is where I was introduced to the CFM reducer. How much on average does the MAU cost? I've heard it can cost in the thousands, but saw some things online for a few hundred dollars.

It really depends on whether you need heated MUA or whether unheated is sufficient. I live in a very moderate climate. We keep windows open year round. Heated make-up-air would be an absolute waste of money.

On the other hand, 1000 cubic feet per minute is quite a bit of air. If you live in a part of the country that gets down to freezing, you'll be uncomfortable sucking that much cold air into your house. That's when you get more elaborate systems.

On the lowest end of things, MUA isn't really more than just an opening in the wall that opens whenever there is a pressure differential. A permanently open window would do much the same (but not meet the code requirements). This should cost well under $1000 including installation. The next step up is a motorized damper that opens whenever the hood turns on. That's probably what you are seeing for a few hundred dollars. Installation costs add on top of that. But it's not particularly difficult. Depending on site conditions, the overall project (parts and labor) would be between $1000 and $2500.

Where it gets complicated is when you need to heat the incoming air. Now you need a much bulkier appliance. It costs a lot more. And there actually is quite a bit of labor involved. This is when you are looking at several thousands of dollars.

If you are lucky, a hybrid solution is possible. I have seen MUA motorized dampers connected to a forced air furnace. This way, you don't need another heater to bring the outside air up to temperature. I think there are caveats with this type of installation though. So, you'd have to check with your HVAC company whether this is an option

Just curious , what does the range require?

It's unusual for a range to have a requirement that is spelled out by the manufacturer. But if you want to avoid cooking smells, humidity and heat in your kitchen, then there are general guidelines. If you search for postings by @kaseki, you'll find lots of detailed discussions.

But as a rule of thumb, multiply the size of the cooktop area in sqft by a factor of 90cfm. That gives you the desired target flow rate. Then add 50% for inevitable inefficiencies between nominal vent hood ratings and actual observed flowrate.

For typical ranges, that gets you into the 1000cfm range, give or take

Is it gas, electric coil, induction?

The primary hood function is capture and containment of cooking plume grease, steam, and odor. The minimum necessary flow rate is determined by containment requirements, which relate to various cooking parameters. The nominal 90 CFM/sq. ft. volumetric flow rate value refers to the area of the hood entry aperture, not the cooktop surface area. Generally, the hood should be larger than the surface area to capture the cooking plumes that expand as they rise.

We have just such a hybrid setup as M mentions for our make up air. Well, that plus a passive option as well.

Cost was simply additional ducting, vents, and a couple of Broan universal make up air dampers, in our case controlled by relays that pick up on current going to the vent hood, an external vent for the passive option, and a connection to our HVAC air handler for the active option. For a newer Carrier system, this is a simple 2-wire to the G-terminal and enabling it at the control. You can also go with a pressure sensor so that the MUA only comes on if the house approaches neutral to negative pressure. That was the way we intended to go, but for a variety of reasons it didn't work out and we had to tie ours to the vent hood power.

This is not something an appliance place would know anything about. It's more in the realm for an HVAC company, although in our experience many of them don't know makeup air from a makeup mirror.