Seems you'll need a comprehensive long-term plan and way to fund or diy it. When you cure the air infiltration and add insulation the conditions should change. Then you'll need MUA. Your plan would install it at that time.

You always need MUA. Or it will pull the dirty air from inside your walls, and doors, while trying to pull the hot/cold outside air in through all of those leaky spots. And that leads to moisture problems inside your walls.

and as of now we cannot get new windows for the house because there are 14 non standard sized total and would have to be custom made.

Windows are custom made. Window companies do not stock "standard sized" windows. The windows you see in places like Home Depot are simply ordered by them in certain sizes.

@HU-0228123141598721

would I be able to use the existing vent? we are moving the cooking area to the opposite side of the kitchen so the exhaust will vent directly out the exterior wall so the original vent will still be there. Can i add a manual vent damper or will i still need something with a motor to draw in air? adding a picture of what our supposed vent is now.

@PPF

Well i know nothing about windows and thats were we went so you are probably spot on. the other issue is we wanted to try and keep the same vintage asthetic for the windows so maybe thats why they told us that.

@kaseki

thank you for your detailed explanation. I do not have any other combustion appliances in my home. So hopefully if I’m undersatanding correctly since this is the case a 42” hood with a blower of 800-900cmfs with a manual dampner installed in the exsiting vent hole that exhausts out the roof should be okay but is not ideal?

That will work for now. But once you tighten up the air sealing and add more insulation to the home, and replace windows, you will likely need powered rather than passive MUA.

Stef: please clarify. The existing ceiling hole is to be used for incoming MUA, and the hood is to be exhausted through the wall?

What is the size of the duct from roof through attic to the ceiling hole?

A manual damper won't do. Get a spring loaded one that wants to be closed but will open with very little pressure.

Do you live where an uninsulated house is OK in the winter? If not, you will likely want to plan to heat the MUA. Code won't let you depend on opening the window, and in northern USA climates most likely nearby humans and pets won't let you do so either.

I think a lot will depend on how airtight your home is and how big your kitchen space is. We opted not to do MUA after lots of research and a couple of HVAC consultations. We had a grill vent like yours but closed that up and have a 750 cfm vent hood that vents through the roof. Our 1915 home is very leaky (original windows) and the 200 sf kitchen is unconditioned space with uninsulated plaster walls (though attic is well insulated). I'm sure those factors all contribute, but we haven't had an issue with it over 14 years--vents well with no back draft. You'll also want to check your local code since it may be required (it was not in our city).

Hi, Stef,

First, a fun fact: If your local building code is based on the International Residential Code (IRC)-- you aren't required to have an exhaust hood over a range or cooktop. That said, if you choose to install one (and I definitely would,) it must meet code requirements. With respect to make-up air, the code doesn't differentiate between "tight" and "leaky" homes. From a practical standpoint, your "leaky" home may allow adequate make-up air at this time, but it will likely be insulated and better air sealed at some point and an unsafe condition could result from the change.

The passive make-up air duct you currently have may provide adequate make-up air, but it doesn't meet IRC requirements. Make-up air ducts are required to have a damper which meets the requirements of M1503.6.2 Typically, it will be a motor-operated damper. Broan/Nutone has an online calculator to help with design of make-up air systems https://bnt-makeupairspecifier-tools-prod-2023.azurewebsites.net/MUATemplate.aspx

@kaseki

Yes that was my intial idea when i started reading about Mua’s. We live in Socal so no major temperature issues and no other conbustion appliances. I haven’t had a chance to look up code since this is all new to me. I haven’t specifically been up in the attic to measure duct for that vent but ducts can be altered if needed right?

@artemis78

our house is definitely not air tight, raised foundation, wood floors on top of planks and 10 original windows that you can noticeably feel a breeze from in just the right conditions in our kitchen, dining and living room alone. We also have uninsulated plaster walls that we don’t plan on doing anything with unless its a wall that needs to come down for design purposes. Our kitchen is also smaller but we plan on taking down the wall with a pocket door that goes into the dinning and living room.

OK, in principle, then, you can increase the diameter of the MUA duct from roof to hole in kitchen ceiling as required. You don't need MUA heat. You don't have a back-draft issue.

The ceiling interface should be a diffuser (in the fullness of time) so you can direct the MUA where you want it to go, which is away from the hood if the distance is "short" and toward the hood if the distance is "long." Long and short are qualitative intended to suggest that your goal is non-turbulent air at the hood.

Generally, it is less than desirable to have MUA pulled in through walls and cracks around windows because of dust and moisture. However, in your case maybe there isn't any significant amount of dust (or insulation particles), and the walls are not rejecting moisture anyway. Next to [image below] your configuration is one of the few described here where passive and not particularly pressure balanced MUA can be tolerated.

One test to perform after the new hood is installed is to see how much air flows via the duct and how much via a window at various openings. Obviously, the window will dominate when widely open as it will exceed the duct area and not have more restriction than its screen (some pressure drop). If the window dominates at an open area equal to, say, 1/10 the area of the hood capture area (entry aperture), your duct or its roof interface may want to be larger because the ratio is also the ratio of duct velocity to hood aperture intake velocity (900 ft/min vs. 90 ft/min) if the window is closed. High MUA duct velocity will correspond to higher pressure loss.

Another way to look at it is: If slowly closing the window leads to a condition where the duct flow seems to not increase at some point, then the MUA pressure loss is countering the hood blower's fan curve drop in flow rate with pressure loss. You ideally want the hood to manage at least 2/3 its rated flow rate, and that primarily limited by the pressure loss in the hood baffles, not the MUA system.

Perfect (zero pressure loss) passive MUA.

@kaseki welp since my home although close isn’t quite that open I have been looking into some systems. These two below are what i found. Aside from the huge price difference one if i understood what you and others suggested is what would be ideal because its fancy and motorized and brings in almost exactly what the hood takes out in real time. The other more budget friendly basically just turns on while needed and brings in a set amount of air flow there is also one for a 10in duct too. Please tell me the Broan is suitable.

"... and brings in a set amount of air flow..." Actually, the flow will be determined by the pressure drop between outdoors and indoors, which is in turn determined by the restrictions in the MUA path, wall leakage, and hood flow rate -- itself a function of power setting and blower fan curve.

I think that with your fairly unique conditions (relative to others who have raised the question here) you can get away with the pressure controlled damper. I would go for the 10-inch size. You also need duct, a roof or wall cap with screen (something finer than condor size grid) intended for incoming air, with a better interface device in the ceiling, e.g., a diffuser, with a duct to diffuser transition.

Normally, one would want a dust filter in this path, with attendant duct transition and register/diffuser interface or transition. To minimize pressure loss, one could go as large as 36 x 36 inch with a 4-inch pleat in an appropriate "cassette." The pressure loss at that size for your flow rate (600 CFM, nominal) would be very low compared to the hood baffles, but not negligible. For your house the pressure loss for some filter size may compete with the leaky walls, which won't be filtered, negating the filter's value.

I recommend you have a "door blower" test performed to see what the "wall" pressure loss is as a function of flow rate so the utility of a filter can be assessed, and, if suitable, what size filter would be sufficient to have freer flow than the walls. Perhaps a filter size widely available in hardware stores would be sufficient. As an example, a small 14 x 20 x 1 Filtrite 600 would have about 0.2 inches,* w.c., of pressure loss; larger area filters will have less; higher "MERV" level filtering will have more.

As far as code goes, if applicable in SoCal, I think (but your AHJ has priority over me) that so long as the MUA duct diameter is larger than or equal to the hood duct diameter and you have the damper, you will pass.

*Edit: 600 CFM at 300 CFM per sq. ft.