myths about cfm's and mua

musky-hunterJuly 11, 2012

I was going to post this as a response to an active thread about how many CFM�s can be added without makeup air. Since this comes up a lot on this forum, I thought it deserved its own thread so new posters can find it in a search.

For the OP and anyone else considering the tradeoff of wanting higher CFM but thinking they need costly makeup air (MUA), I would like to dispel some myths that get presented on this and other forums. I am currently going through this process and the city building department has given me quite the education about the process so I thought I would share.

We live in Minnesota where the temperature can vary more than 120F in the course July to January. People toss out these generalizations that if you live in a cold climate you have to have heated makeup air to get above certain CFM without MUA.

Generalizations about maximum CFM's that are allowed without MUA are just that, generalizations. Your house is unique to any other house when it comes to determining if you need MUA. Your contractor should work with your local building code department to perform some calculations that will determine if MU is needed. Some factors in the calculation include; number of external fans (bath for example) their CFM's, number and types of fire places, type of water heater and furnace, efficiency of furnace, if an air exchanger is installed, age and materials insulate and wrap the home. With this information they can determine how many CFM's you can pull out of the home without causing negative pressure that could back draft the home. As others have posted, if you back draft, you could literally suck the CO from the water heater, furnace, etc. right back into your home, or even pull embers from your fireplace out onto the floor.

The idea that you have to have heated makeup air (HMUA) is also a generalization. As I said, we live where it gets -20F BELOW zero in a 6 year old very tightly wrapped home with 2x6 framing and crazy high insulated R factor. . We do NOT have to have HMUA to add 600 CFM. All we need is cold MUA. This is a very simple solution if you live in a home with a basement or crawlspace that is not finished. All that is needed is a flexible 6" duct from the outside (where there is no other exhaust ports) that goes into the mechanical room and just hangs there open in the ceiling. The purpose of the MUA is that when you turn on your high CFM hood, if enough other exhaust is also running at the same time (bath fans, etc) and the home needs more air, it will draw from the MUA duct. If is kind of like leaving a window open when there is no wind. If the home needs it, it will draw through it.

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I think it is a myth that you can find out everything you need to know about MUA from absolute strangers on an internet forum. Most of us are very well-meaning, but don't necessarily know any more than an appliance salesman working on commission.

The truth is, codes vary widely. I have often mentioned my $7K heated MUA system because it was very much required where I live. My recommendation has always been to find out what, if anything, is required in your jurisdiction, preferably before you choose your range, unless you don't mind big expensive surprises.

    Bookmark   July 11, 2012 at 7:08PM
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Most senior posters here will give direction with ventilation with the caveat ---check with your GC and local regulations.

People don't come here to read "check your local building codes." They want at least some ballpark figures on what they are getting into when buying a new suite of appliances or a pro range/French range in particular.

    Bookmark   July 11, 2012 at 7:35PM
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cooksnews, What was included in your $7,000 make-up air unit? Was it an all-in-one or was it comprised of several different components?

    Bookmark   July 11, 2012 at 8:33PM
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For $7K,I got 600 cfm of powered and heated incoming air. The ductwork was a bit convoluted as it had to pass through our garage before entering the basement utility room, and into the furnace ducting. The price also included the ducting for the vent hood itself, but that part was pretty much straight up and out. There were also a few controls included so that it would bring in air when the hood was turned on, and a thermostat so the heater would only come on when needed.

The ducting in the basement looks like this when installed in 2008 (we finished up the open walls since then...):

    Bookmark   July 11, 2012 at 11:19PM
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I assume the metal box in the picture is what heats the outside air, is that where the majority of the money is spent ? Sorry if this is a dumb question but I have never seen one before.

    Bookmark   July 12, 2012 at 12:01AM
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Yes, that is the heater, although there is a fan somewhere as well. That duct isn't big enough to passively pull in 600 cfm. And I'm not sure how much extra I paid for the brain-dead electrician who screwed up many things on the installation.

    Bookmark   July 12, 2012 at 12:13AM
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That duct isn't big enough to passively pull in 600 cfm.

Perfect lead-in to a question I've been wanting to ask for a while. Sorry if it's been answered here before, but if so I've not seen it.

The air exiting the fan into the duct is under pressure (or, in the case of a remote blower, the air in the duct is under partial vacuum) so what's the relationship between CFM and MUA duct size for a passive system to effectively achieve neutral pressure? In the case of the OP, I find it impossible to believe that a 6" duct alone is providing enough make-up air for a 600 CFM fan (let alone other incidentals such as bath fans.) Even a tight house isn't 100% air-tight, so the rest must be coming from other leaks if the interior is really not under negative pressure.

    Bookmark   July 12, 2012 at 2:39AM
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foodonastump raises a good question, that I will be following for years to come. I am puzzled too, by the makeup air that magically appears when air is sucked out of a house.

I have often wondered why I have never seen an air curtain with new outdoor-air being fed directly to the spot where it is needed: at the edge of the cooktop. Then, this unconditioned air that is not optimal for your house will get used to carry effluent out of the house.

I would love to have a clearer sense of the optimal setup that can be implemented in any climate. In the NE and in eastern Canada, summer air can be so humid that it's uncomfortable, and also not good for your building when cooled. So air conditioning is used to _dehumidify_ this air, and to keep the house interior drier than otherwise. Dehumidified. This is once again a good example of air grom outside that one does not want to bring inside to get mixed together with house air in general. It would be better to channel it to the countertop/cooktop and use it to funnel the cooking exhaust out by suckage up the hood.


    Bookmark   July 12, 2012 at 7:51AM
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I'd like to hear more about the mythical contractor that understands MUA, how to bring it in and where to put it.

    Bookmark   July 12, 2012 at 10:03AM
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To answer davidro1's second paragraph first: It seems in MUA/hood experiments for commercial setups that such MUA feeds tend to cause disturbances (turbulence) in the upwelling effluent that causes some of the effluent to miss the hood aperture or spill out of it. Searching the web for kitchen ventilation related Schlieren photographs may lead to whatever article I read this in.

I also recall that if the air is introduced towards the floor at the back of the cookstove it spreads out enough to minimize the disturbance. In test kitchens the MUA is usually introduced through large perforated panels in a distant wall to try to keep it non-turbulent when it reaches the cooking appliance.

The ventilation system's pressures could be more easily explained using electrical analogs if we were all equally versed in electrical engineering. But we aren't, so let me try something different -- a garden hose and tank analogy.

Imagine a sealed tank of water with some air at the top. It also has a hose at the bottom connected to a lake. Another hose at the bottom leads to a pump which pumps water to the lake. (In this case the pump has to not be positive displacement, but more like a propeller.)

When the pump runs, water is pulled from the tank and pushed to the lake, and replaced from the lake through the passive hose. In steady-state conditions, the flow in each hose is equal. The air pressure at the top of the tank has to be less than ambient or else water would not be pulled from the lake. How much it drops when the pump is turned on depends on how much leakage there is around the pump blades (analog of cfm achieved), and what the restrictions are in the hoses (analog of pressure loss in ducting).

This negative pressure is what the make-up water is trying to reduce, and given enough hose restrictions, the only way to keep the air in the tank near ambient is to add another pump to the hose bringing water from the lake to the tank.

So, one way of keeping the pressure near ambient in an MUA system is large MUA ducting with minimal blockage from such devices as air filters. Another way is adding a fan/blower. Another is a house so leaky that the MUA ducting is actually carrying only a portion of the actual MUA.

When the specifics are considered, to keep the house differential pressure at zero under all conditions requires the differential pressure be measured and the MUA blower controlled. To do this closed loop is complicated; to do it open loop extremely difficult because all the losses and all the leakages, and all the fans various speeds and fan curves and filter losses (vs. cleanliness) would have to be known and used to calculate and control the MUA blower.

Approximate MUA blower control or use of balancing of restrictions in ducting can be used to keep the house pressure no worse than -0.03 inches where backdrafting of the most sensitive combustion appliances begins.


    Bookmark   July 12, 2012 at 10:26AM
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Pretty ideal solution with MUA right to range, abetted by range directly on exterior wall.

Here is a link that might be useful: Make Up Air for Commercial Ranges in..Home

    Bookmark   July 12, 2012 at 10:28AM
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A make-up air system for the range hood isn't going to run constantly like a normal HVAC system that heats/cools/filters your internal air throughout the day. It also cannot be compared to a commercial MUA unit for a vent hood over a range in a restaurant because they cook from the time they open to the time they close. So a residential MUA system will have much less of an impact on internal comfort levels than what most people are envisioning.

Most HVAC systems are designed to run with slightly negative air pressure to begin with. That's why things tend to blow into your house rather than out. Ask yourself, "how often do I turn on my range hood compared to opening the back door/garage door/front door of my house?". Every time you open a door to your home you are letting in a large amount of unconditioned freezing cold or hot and humid air. And when enough outside air is mixed with the internal air to affect the temperature, your HVAC system will activate and begin exchanging it for conditioned air. The same process will occur with a passive make-up air system.

Everyone seems to be afraid of the air outside their homes, but who ever says, "I'm gonna step out for some dirty hot and humid air" one. Everyone always says, "I'm going outside to get some fresh air." It really is as simple as allowing your house to "breathe". And like foodonastump said, no house is sealed 100% airtight so your home is going to find a way to bring in outside air. You are just giving it an extra set of lungs by adding a make-up air system during operation of a high CFM range hood to avoid creating enough negative air pressure to stall or backdraft combustion vents.

foodonastump, the 2009/2012 IRC M1503.4 code provision reads "Exhaust hood systems capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be provided with makeup air at a rate approximately equal to the exhaust air rate". So the relationship between the exhaust opening and fresh air intake need to be symmetrically proportional at minimum. And the building inspectors will default to the commercial code which reads that make-up air needs to be supplied at 80% of the exhaust rate. And because there are residential range hoods that can exhaust 600 CFM through a 6" or an 8" round duct (several manufacturers exhaust up to 1000 CFM through an 8" duct while most use a 10"), the question becomes even more confusing. And based on the fact that any exhaust vent in your home, combustion or not, is fitted with some type of backdraft damper/flapper, the reasonable path of least resistance for fresh air intake would still be an open and clear 6" duct, even at a 600 CFM exhaust rate.

davidrol1, An air curtain is a great application for commercial settings but is just too uncomfortable for a residential homeowner.

Hope this helped a little.


    Bookmark   July 12, 2012 at 10:43AM
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SparklingWater: I would like to see Schlieren photography of that MUA system in action to be sure that it is not disrupting the rising effluent.

On the subject of unconditioned MUA. I can attest that if I turn on to high both my nominal 1500 cfm cooktop hood fan and my nominal 1000 cfm wall oven fan, and then open a distant window in the NH winter, the house will cool off faster than the hydronic room heating can possibly make up the temperature. (In this case the actual air flow rate is probably around 1500 cfm given the window has a screen and there are other sources of pressure loss.) Only with a dedicated "radiator" heat exchanger in the path rated at nearly the full power of my oil burner furnace (ca. 180 kBTU/hr) can I heat this air fast enough to not chill down the house and the cook.

At 600 cfm (or whatever the actual flow rate is with a six-inch MUA duct -- maybe 400 cfm), the cooling effect will be reduced, but not eliminated. It will also matter whether winter is in San Diego, TX, NH, or MN.


    Bookmark   July 12, 2012 at 6:39PM
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I believe someone already pointed out earlier that the installation in the video violates the manufacturer's explicit instructions about openings in the floor.

    Bookmark   July 12, 2012 at 7:27PM
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"...Schlieren flow-visualization system to verify capture and containment. This system is a major breakthrough for visualizing thermal and effluent plumes from cooking processes. "Schlieren" is derived from the German word for "smear." A schlieren system presents an amplified optical image (see Figure 2) due the different air densities, similar to the mirage effect we see over hot pavement" (page 3 of link).

kaseki-While I can't provide you with your request, I can provide you with another well written article referencing your point of effluent venting escape, and I link it below. It is written, as many are, addressing industrial kitchen venting yet it's science is helpful in residential kitchens. You, as do the many other knowledgeable contributors here, help us all to better grasp the issues, concepts and proposed solutions of proper venting and MUA for the modern day power ranges/cooktops/rangetops that are available to us. As a homeowner studying up on our existing but old ventilation system I thank you all very much.

I have to give some credit to the GC/builder in the video I posted. He is at least attempting to address the issue make up air (MUA) of a high BTU residential range. I can't address Marcolo's well made point, but I see through his video's that he might respond to this question put to him. I thought to ask if there is anyway a HEPA filter might be added to the MUA intake system. Sitting so low on the outside, it's close to grasses and pollens etc, which might be avoidable by placing a round cut out of an efficient HEPA filter and inserting it near the air intake flow.

Here is a link that might be useful: Design Guide: Improving Commercial Kitchen Ventilation System Performance

    Bookmark   July 12, 2012 at 8:51PM
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kas, If you added a separate heater that is equal in heating capacity to your already installed HVAC furnace, then why couldn't you bring in the outside air through your existing oil burner furnace powered by your air handler fan and heat it as it enters the house? It would still heat in the direct path of the replenishment air and would save a lot of expense. I understand that the volume of air an HVAC system can heat/cool is dependent on total conditioned living area and unit tonnage, but if you are running your vent hood at no higher than 600-1200 CFM then the air flow rate should be sustainable. And do you really run your hood at the max 1500 CFM every time you cook?

I am all for spending money on upgrades for better insulation and more efficient HVAC units in your house to reduce energy costs. If you operate your range hood on high fan speeds once or twice daily or if you live in an area with extreme temperatures, then your home may require an upgraded/custom MUA system specific to your needs. But unlike upgrading your appliances to increase the value of your home, an over-built make-air system will never yield ROI (return on investment). And the reality for the greater majority of homeowners is that they rarely cook something that warrants turning their hood on the highest setting.

Many tightly sealed, energy efficient homes have already been built without a MUA system installed and I have never heard of anyone complaining to the hood manufacturers that when they operate their high CFM range hood the house gets really cold in the winter. And without a working MUA system the replenishment air entering the home has no chance of getting heated or cooled.

And please don't misunderstand my intentions or comments directed at kaseki, I have major respect for this man and have learned a lot from his posts and he truly does understand this subject on an expert level. I just feel that the main body of discussion for MUA systems is focused on the exception and not the rule. Not every homeowner needs to spend an excessive amount for a working MUA system that is safe and reliable.


    Bookmark   July 12, 2012 at 9:01PM
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SparklingWater: You have found it; the California Energy Commission paper I read a long time ago (it seems now) but forgot the name of.

I wasn't attacking the builder. My impression was that the characteristics of the MUA system at the range are those included by the range manufacturer, and since I haven't seen any imagery from that particular setup, I can't assume that it will work well. If it does, great. It may work well enough that it is a step forward from no MUA.

A HEPA filter will have a significant pressure drop across it, at least enough to backflow some combustion appliances. With a filter a blower is needed to make up the pressure loss.

Nateman: My home furnace is a hot water boiler that feeds hot water via pumps to individual hydronic heater circuits (baseboard heating). The only air involved is that used to burn the oil. One of those circuits is via a Taco 013 to a radiator heat exchanger in a ceiling above a 3 x 3 ft diffuser. MUA would flow through this heat exchanger from a roof mushroom vent via an axial fan and filter pack.

Ultimately, the MUA system I am building is to allow simultaneous cooking and fireplace use, independent of fan settings in the kitchen and elsewhere. Nothing I do in this house has an ROI better than 0.3, I would estimate. It is done for my interest in constructive activity and/or DW's demands. :)

While the boiler can handle all the hydronic circuits in the house, even simultaneously if the MUA circuit is not included, the MUA is sized on using the entire furnace capacity. During such - as you note brief events - if no MUA is being pulled from other rooms, temporary lack of hydronic heating will not be a problem.

Full fan power is used for wokking and similar frying operations. I also use it for effluent removal after broiling meat in an oven and opening the oven door.

If a house is tightly sealed without any deliberate MUA, the kitchen ventilation fan won't pull any more air than it can squeeze out of the tightly sealed house. There is always MUA and it always matches the outflow of the hood, and vice versa.


    Bookmark   July 13, 2012 at 10:33AM
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