Hi, Paul,

I'm not sure a passive system is going to be able to meet all your objectives; you already know it won't filter the make-up air. That said, with regard to design of passive make-up air systems, Broan/Nutone has an on-line tool you can use https://bnt-makeupairspecifier-tools-prod-2023.azurewebsites.net/MUATemplate.aspx I suggest you use zero air leakage as the input for air leakage and the full capacity of the range vent hood as the required make up air flow.

Good luck.

Why such a low CFM hood? Most pro style range require MUCH more CFM for venting.

The Broan calculator is excellent, easy to use, and answers my question, thanks. Neither passive nor powered make-up air seem a perfect solution. Unless there is some way to auto balance powered make-up air to the various speeds of a kitchen hood (if there is, please mention as this seems like a really strong need in the market), I believe passive will result in better overall air quality. Passive make-up air that enters close to the cooking hood, though unfiltered, will mostly go straight out the hood and hopefully what doesn't will sufficiently find its way out the natural flow to the kitchen ERV vent. Passive won't create pressure imbalances that force outside air to the rest of the house, nor will passive (when properly sized) exceed acceptable house depressurization limits. Passive is quieter and requires fewer components to buy and maintain (no filter box, no duct fan, no duct silencer).

As far as sizing for the capacity of a pro kitchen hood, that's a good idea for future proofing the design, but in this case impractical because a make-up air duct for that kind of air flow must be over 2 feet in diameter. In this house situation a minimum 400cfm hood is suitable and I earmarked a 500cfm unit. This already requires an 18 inch diameter make-up air duct which presents some design challenges.

"Unless there is some way to auto balance powered make-up air to the various speeds of a kitchen hood (if there is, please mention as this seems like a really strong need in the market)"

There is. Check out Fantech's powered make up air system https://shop.fantech.net/en-US/muas--1200--makeup--air--system/p111242

Your math is wrong. If an 8” duct can exhaust a 1200 CFM fan, an 8” duct can also supply enough MUA to replace it. No 24” duct required for either. Your lack of experience is showing. Your HVAC specialst can set this up quickly and efficiently, with no issues.

For a passive system to provide 500 cfm of make up air in a "zero leakage" home, the Broan calculator indicates you have two options: install a single 10" diameter duct or the combination of a 6" duct and an 8" duct. The ducts can be up to 25 ft long and have (2) elbows.

To Charles Ross Homes: wow, the fantech make-up air products are very well done. Thank you.

To Lomo, I am leaning induction here. According to broan “Electric and induction cooktops release less heat and as a result, they require a less powerful range hood. Often, a ventilation power varying between 350 CFM and 500 CFM is sufficient, depending on your cooking habits.” This statement is consistent with calculators and other sources I have come across that indicate 400cfm is most suitable for this particular kitchen. Can you provide any references that supports this >500cfm requirement?

To Monique, perhaps it is you that does not yet grasp the large difference between passive and powered duct sizing. I simply answered questions and the broan calculator did the math. if you disagree with the math, I suggest taking your issue up with broan and reporting back what you find.

PJB. The Broan calculator did not answer the same question for me. I have a max 1200 cfm exhaust fan. 10 inch exhaust duct. Florida Building Codes says the make-up air must equal the exhaust air. A 10 inch passive make-up duct may not be not sufficient since the make-up air is not being forced in like the exhaust air is being forced out. At any point to double the exhaust duct you must multiply the size exhaust duct by the square root of 2 or square root of 3 for triple the size. I'm installing a pleated air filter with a carbon filter which will restrict the incoming airflow. The only way to know what size duct you need for the make-up air is to have a blower door test. That will tell you how much air leakage your house has already and what you need to balance the exhaust cfm.

@imsloan,

At 1,200 cfm you've exceeded the exhaust rate for which a passive make-up air system is appropriate, in my opinion.

A blower door test at ACH50 measures the leakage rate at 50 Pa pressure. The natural air infiltration rate is inferred, not measured.

Many answers related to this may be found on the Appliances forum where hood system issues are routinely discussed.

There are code requirements for MUA, and a safety requirement. The safety requirement drives the code requirements, in a somewhat crude manner in most cases.

The safety requirement is to not back-draft combustion appliances. This is moot if the appliances have their own make-up air and have sealed exhausts, or are in a space sealed from the kitchen or otherwise isolated. Code might apply anyway. The image below provides examples of the differential pressure between inside and outside that can cause back-drafting. Inches of water mean inches of water column, where atmospheric pressure can pull a water column about 407 inches high against gravity. Click to enlarge.

A passive (no blower, large duct) solution should still have filtering, and in most cases heating. The filtering alone will cause some pressure loss. See table in the image below. Feet per minute (fpm) are per square foot of filter and thus equal CFM per square foot.

So passive MUA is a potential solution where back-drafting risk is negligible. A duct diameter equal to that of the hood duct would probably pass code. In this case, it is better to ask (the AHJ) for permission rather than forgiveness.

Where the pressure loss in the MUA path is too high, then an active (with blower) MUA system is needed. These can be controlled by various schemes to balance indoor pressure relative to outdoor pressure.

It is worth noting that the actual flow rate of the hood system is likely only 2/3 of the blower's rated flow due to pressure loss in the hood system, particularly in the hood filter (mesh or baffles) vs. the blower's "fan curve."

In order that the MUA not cause turbulence at the cooking zone that can cause failure of the hood intent -- capture and containment -- MUA needs to be introduced "quietly." That is, MUA flow should be expanded into a diffuser much larger than its duct size. If introduced near the hood, the MUA flow should be directed away from the hood (across the ceiling), and if introduced at the floor aimed so that doesn't interact with the cook moving about. Commercial kitchens can sometimes get away with dumping the MUA under the cooking zones, all of which are raised above the floor, but otherwise typically introduce the flow at the ceiling. Test kitchens use perforated walls without intervening equipment.

Passive MUA is generally self auto balancing, but the balance is not too good as may be seen from the filter table above. Assume for convenience a 2' x 2' x 4" cassette pleated filter. For example, a MERV14 4-inch filter at 1200 CFM (300 fpm per square foot) would have a pressure loss of 0.29 inches, or 73 Pa, requiring a blower unless there were no combustion appliances.

(Estimating 0.2 inches, w.c., at the 800 CFM a 1200-CFM rated hood blower would likely deliver against the hood pressure loss, or 200 fpm at the MUA filter, we are still at 50 Pa, and combustion appliances, if present, really need to be in their own room with its own MUA to get away with passive MUA.

Active MUA where the only significant flow is via the kitchen hood can use hood system blower motor current sensing, duct flow sensing, or actual pressure sensing if the hood blower uses variable speed control. With multiple household exhaust blowers and with fireplaces (not sealed having their own MUA, and intended for use during cooking), pressure sensing based motor control may be the only practical scheme.

For contrast, large buildings are kept pressurized to aid door opening against winds, passive bathroom exhaust flow, etc. They run constant rpm blowers that feed ducting that includes a bypass duct to the blower intake. This bypass is controlled by building internal pressure vs. external pressure. Passive pneumatic damper control is feasible for this.

While the IRC allows for both passive and active make-up air systems, be prepared to prove to the code official that the system installed provides the required make-up air. They may require performance testing to verify the design works. If they surprise you with that requirement at final inspection, an inadequate make-up air system could delay your certificate of occupancy.

Hopefully they test that the house internal pressure differential doesn't drop too much. MUA from wherever (including wall leakage) will ALWAYS match the air flow out of the hood system. If the MUA is strangled, so is the hood system. No house has ever imploded from insufficient MUA, because residential hood system blowers generally can't pull more than a couple of inches, w.c. (see figure).

Nonetheless, I would go farther than Charles Ross Homes and suggest the home owner initiate having it tested for safety reasons in the context of the pressure limit for the appliances that can be back-drafted. A CO sensor is a good idea in the vicinity of any combustion appliances.

Our local code official hasn't demanded performance testing of make-up air systems yet, but we implemented it in the last year anticipating that he will in the near future. We measure actual make-up air flow at full range hood blower speed and document it.