A word of caution about codes: many cities have their own codes but also adopt the International Residential Code (IRC) and other codes such as the National Electrical Code alongside them. If DC has adopted the IRC, then it does have the MUA requirements for hoods pulling more than 400 cfm. A quick online check indicates that DC adopted the IRC in addition to its own code. Here is a link.

Regulations or no, however, you need to check out MUA as a practical matter. You absolutely want some MUA if there is any risk of backdrafting your combi unit and any other gas appliances you might have (say, a gas clothes dryer.)

You could go with a lower CFM hood vent. That might or might not avoid the practical risk of backdrafting but will not really get you around the code.

Can you get an energy audit (specifically a blower door test) to see how
leaky your house actually is? Once upon a time, I kind of recall
Washington Gas offering free energy audits. That was well before the turn of the
century, though. Maybe they do not do it anymore if they ever did? Worth checking. If you cannot get a free
audit, it will be worth paying for one.

Is your combi-unit a sealed combustion unit? Many sealed combustion units are often regarded as mostly immune to backdrafting.

If you find that you need MUA, what you need might be simple (and relatively inexpensive) or complex (and very expensive) depending on your situation and the layout of your home. There are passive MUA "systems" and home-brewed "snorkel into a bucket" (aka Saskatoon Loops, elephant trunks, etc.) which might work for combi unit and take care of the MUA for the hood. (Though with a 1200 cfm capacity, I kind of doubt it.)

If memory serves, we've also had a couple of threads here discussing the relatively simple MUA "dampers" from the likes of Fantech and Best/Broan.

Do you have windows in your kitchen? (IOW, manually operated MUA intakes?) WIth DC's tropical humidty and heat in the summer and dank chill in the winter, that strikes me as a lot less than ideal solution.

It would be best to start with the range hood size, if that has already been selected, and then work from there for both "required" blower capability and MUA necessity.

kas

Thanks for the responses...

@Stan Z we do some wok cooking, I like to sear steaks on occasion, cook chicken breasts in a skillet, etc. Range top is 6 burners with no grill/ griddle. Each burner is 18,000 btu so that's why we are so high in btu.

We have 9 foot ceilings so lot's of flexibility with height of the hood. However can really only accommodate 8" ducting in the soffit.

@jwvideo not sure on the combi, but this is what we are having installed. Good idea on the audit but unfortunately already in the reno process so can't get it done now. I've read up on the dampers from best/broan and I think that's would be my only way to go if it's something that is needed. It's all open framing still but like I mentioned my contractor has never dealt with this stuff so I would be the one figuring out where to do the direct venting and stuff.

@kaseki since we are going a 36" range I was thinking a 42" hood. I've been looking a lot at the Zephyr Siena Pro and this Best Collone.

They are both 1,200 cfm but looking there since that's where I keep getting steered by the appliance guy. According the calcs on Best's website I don't need a MUA for the Collone but I might be doing the calcs wrong.

That "combi" is a tankless "low NOx" Takagi water heater? Ifso, I believe that you will find those Takagi nits are sealed combustion. If so, it should minimize the the possibility of backdrafting CO, at least. Best to verify for yourself, tho. This is off the top of my head. (I'm more familiar with Rinnai tankless heaters.)

But, now I'm confused. Your opening post mentioned "high velocity air" and also mentioned hydronic radiant heat. Was the "high velocity air" referring to your intended range hood or, maybe, to air conditioning? Is the "radiant" a floor heating system or is it baseboard or other radiators? Will you have a storage tank (maybe a repurposed water heater tank)?

@jwvideo sorry for the confusion. As you are probably learning this isn't my bread and butter. Combi is for a combination of radiators, a baseboard heater, and radiant floor heat. No storage tank. The high velocity air is our air conditioning.

It's an old house. We like to keep things confusing. You should hear me trying to explain our heating/cooling systems to my friends that live in "normal" houses with central forced air.

If you have a 1,200 cfm hood - you need MUA. Without MUA, you could 'pull' air down the chimney, from a fireplace, from a water heater. Any ventilation contractor should be able to recommend a suitable package.

OK, let me go over some recommended parameters.

A 42-inch wide hood is OK for a 36-inch cooktop. Try for deep (front to back) also to obtain at least the same 3-inch overlap at the cook's position. 27 inches is best, 24 is adequate, and less in my view is going to force greasy cooking to the rear burners.

If we assume a 24 x 42-inch hood, and pretend for rounding purposes that this is the aperture size, then the hood aperture is 2 x 3.5 ft, or 7 square feet. This calls for 90 ft/min x 7 sq. ft = 640 CFM actual flow when the upward plume velocity is around 3 ft/sec (typical of hot cooking). This should provide an air velocity at the baffle gaps about equal to the uprising plume velocity, helping ensure good containment of the captured part of the plume (which we hope to be most of it due to overlap).

Ideally, one would know the pressure losses in the ducting, hood, baffles, and MUA path as a function of air flow to compare with the fan curves of several possible candidate blowers in order to choose one. (Fan curves are available from Best/Broan/NuTone, but not necessarily from other brands.)

These loss functions will be difficult to know as few are published other than duct losses. A rough approach, usually acceptable with blower configurations of the type Best sells (also for Wolf and Abakka, and probably for Fantech in-line blowers), is to multiply by a factor of 1.5 to get a zero-static-pressure value for blower selection. This gets us to a blower rating of 960 CFM, or around a 1000 CFM.

To allow this to this achieve the desired CFM, the pressure losses in the MUA path have to be fairly low, i.e., mainly duct loss. This would be a passive MUA path with low restriction. Throw in a decent dust filter (like a furnace filter) and you will need either a stronger hood blower (like the 1200 CFM one) and accept some more negative house pressure, or an active MUA with a blower in the MUA path. Best/Broan has some of the former; Fantech some of the latter.

If a water to air heat exchanger is used in the MUA path, this is also a restriction. Coil heaters in the airflow, however, should have low pressure loss. Heating may not be needed in some locales, desirable in others, and in a few locations they are mandated.

As you might infer from all this, there is a degree of recursive engineering involved in selecting candidate parts, evaluating the resulting performance, and revising as needed.

Another dimension is noise. It is best to use a roof blower with an in-line Fantech silencer to keep noise at the cook's ears to a tolerable level when operating at full blower power.

kas

Geoffrey -- This Canadian blog echoes and amplifies your points.

However, you might note note that that OP reports having no fireplaces or chimneys, apparently uses an electric clothes dryer, and is installing (or having installed) a sealed-combustion Takagi tankless combi-water heater. Seems like a fairly minimal risk of backdrafting CO from appliances into the house.

Still, it seems to me that there can be other backdrafting concerns when running such a large capacity venthood. One is the potential for backdrafting through the separate A/C system. Not only bringing in cold air (at the wrong times of year) but maybe dust and who knows what else that accumulates in the ducts. Also, it generally seems like a bad idea to depressurize the house, especially in a densely populated urban area.

Or, maybe increasing radon concentrations or, worse, backdrafting a radon venting system. There may be more awareness of this in places where there is known to be be significant radon, as there is my Montana town, a former coal mining center. If memory serves from when I worked for DOE, a lot of areas in DC and
Virginia and Maryland suburbs were considered a "Zone 1" risk area. My brother, who still lives out in Virginia on the semi-rural fringe of DC, has told me that significant radon concentrations have been turning up in newer and tighter homes with inadequate MUA.

A further potential concern with depressurization could be backdrafting outside air into the tankless water heater on very cold winter days. In places like Montana (where we can have week-long periods of subzero temps), sucking that cold air into the tankless unit has been known to cause freeze ups (basically reversing the heat exchanger when the tankless unit is not running but the range hood is.) Not sure how much of a concern this will be inside DC (a bit of a heat island) but I've heard of this being an occasional problem out in the more rural fringe in Virginia.

In climates with hot humid summers (which certainly includes DC), depressurizing with the rangehood can result in condensation and moisture build up between floors, in wall cavities, and other interior surfaces, all of which can exacerbate mold and decay.

Chad O: Kaseki has written pretty extensively here on venting as well as MUA. You can try searching with a search string like "kaseki + gardenweb + MUA." Also, several years ago, I ran across these two academic publications on MUA concerns from Pennsylvania. If you have not already seen them. you might find them helpful for background and help you with vocabulary in talking to (and evaluating advice from) professionals or contractors on the subject of MUA.

www.phrc.psu.edu/assets/docs/Publications/BB0312.pdf

www.phrc.psu.edu/assets/docs/Publications/BB0412.pdf

Regarding your comment about trying to explain your heating hydronic system to "friends that live in 'normal' houses with central forced air," I fully understand. I, too, have a century-plus old house. In a town of mostly old houses. Some 17 years ago, I was the proverbial "first kid on the block" to retrofit a hydronic floor heating system into his old house. Although floor heat and hydronic radiators are now more common in town, I still find myself having explain my set-up.

@jwvideo / @kaseki: Wow you guy's have enough hot air to power any MUA system.

Your lengthy advice - is it to show what you know? As far as I'm concerned it's just 'flooding' - I guess nobody else will listen to you, so you post these volumes on blah... blah ... blah....

All that your posts do is turn off / confuse people.

If the OP contacts a local HAVC / Ventalition company - there are kit's readily available to solve the problem.

The long posts are not to show what we know but to try to share it. The main point I was making in my last post was that there can be important reasons to get MUA even if (as with Chad O's home) there isn't a fireplace or water-heater that will be backdrafted by the vent hood. I'm sorry our efforts offended you.

jwvideo, I admire your, er, delicacy in responding to geoffrey_b's personal attack; I won't be sorry until geoffrey_b demonstrates that he can better inform our readers using fewer words. My working assumption, evidently misguided in at least one case, is that the readers here have adult attention spans and can skip material that they believe that they don't need to read.

On the other hand, I admit that we could all save a lot of time just answering each HVAC question with "contact your local HVAC company and accept whatever you are told." Tailored to the thread topic and widely applied, this approach could save houzz a lot of server storage space.

kas

As the OP I do appreciate both the quick responses as well as the more in depth. Each his own but for me @jwvideo and @kaseki responses are exactly the kind of things I was hoping to get. It's surprising how little folks in my area really know about MUA. I think it's largely because a) they are old leaky houses and b)it's not something that comes up in inspections. The salesman say do it, and builders say skip it. I'm trying to make the best informed decision as the homeowner and the detail helps.

@jwvideo thanks for the post. While I had quit worrying about backdrafting I can now move on from that and worry about radon! I joke.

@kaseki super super helpful.

In any event this whole 1200cfm thing is becoming mute since I learned today that the soffit that was built can only accommodate a 8" duct. And since it's such a short run no getting around it. So maybe I dial back the CFM and have a hood that can handle everyday cooking (two or so burners) and be under powered in the unlikely holiday event that I'm pumping all 6 burners on full blast?

No matter what hate the idea of sucking in a bunch of stuff I don't want, but also hate the idea of a passive MUA (my only real option if I go the MUA route) letting in hot air in the summer. Even I do go lower CFM, still a lot to think about with the MUA.

I just recently returned to the Appliance and Kitchen forums - I've
been more active in the Decorating side. But we are downsizing into an
over 55 in the spring and gutting the kitchen and I for one am
absolutely thrilled to find some familiar names here who willing to
share their knowledge. Especially in the more technical areas where I am
somewhat lacking - but I can readily show these posts to DH who has a
strong science and technical background - and he gets it - I'm not sure
about the last 5% or so but he can reasonably follow the logic and
equations and he appreciates the facts.

So thanks to all of you who have been sharing ....Maire

Well, seems to me that Chad has pretty well narrowed his choices, then.

Since there is no code that requires you to get a 1200CFM hood for your rangetop, you can pretty much get whatever hood suits your preferences and your house. We work with the kitchens and homes we have. The ideal, optimum choice may not be feasible for a lot of different reasons. FWIW, the 1200 cfm recommendation is just a rule of thumb that ignores other aspects of hood design. Kind of like recommending a car only by horsepower or mpg ratings.

If you can, look for a 42" wide hood with an actual collecting canopy design as opposed to a flat-surfaced base. The canopy can help a lot with collection of vapors and cooking effluents. It will not be as dependent on sheer CFM pulling power for effective venting.

And don't forget that there is always MUA, whether you want it or not. No air goes up the hood and out that hasn't leaked into the house through some path. Switch boxes and ceiling lights are common paths where there are no deliberate other venting paths. For the blower types used in kitchen ventilation, the lowest the house pressure can get is only a few inches of water column at most, and a few tenths of inches more typically. This is fortunate, because otherwise with perfect house sealing one could pull an ever increasing vacuum until the weakest wall or window failed.

"You must understand this completely." Miyamoto Mushashi, Go Rin No Sho

kas

Reading all this, could a system like this work: Have maybe a 6" diameter metal duct penetrate the outside wall in a basement, then have the duct go around the perimeter of the basement, as much as possible, then end with a grille. This way, the taken in air from the outside would not be so brutally cold, having gone through this duct and picked up some heat. Also, the turns and the length might prevent drafts. Or would this arrangement use too much energy or not be enough air?

Length and turns just add pressure drop, thereby reducing house pressure for a given hood blower flow rate. It is probably better to just expand the duct so that the register at the end is large enough to reduce the air speed. The MUA duct, if without its own blower, should be commensurate in diameter with the hood duct.

In general, whatever BTUs are needed to warm the air where you want it will have to be supplied by either the house heating system or by auxiliary heating. Because high MUA flow occurs for only a small part of each day in residential cooking, it is least complex, I believe, to just electrically heat the air in the duct or in the basement if that is where the air is supplied.

If a hydronic heating system is in use to heat the house, then a fan blown water to air heat exchanger can be used for supplemental heat. A thermostat near the air register would control heating. For example, in one approach an air thermostat controls the hydronic loop circulation pump and a water thermostat at the heat exchanger controls the heat exchanger fan.