Image Credit: Fantech
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Powerful kitchen range hoods like the Wolf 66-inch Pro Island can vent 1,500 cubic feet of air per minute, quickly clearing the room of smoke and cooking odors. But exhaust fans like this also can draw makeup air from unwanted sources, like chimney flues, moldy crawl spaces, and vents for fuel-burning appliances — a dangerous condition called backdrafting.
In the past, providing makeup air for range hoods has often been a haphazard affair. Even companies that manufacture range hoods have not always offered much advice, or seemed unaware there was a problem, as GBA senior editor Martin Holladay described in a blog several years ago.
This is the backdrop for the launch of Fantech’s Makeup Air System (MUAS), a through-the-wall duct kit that balances outgoing air with an equal amount of introduced outdoor air. Incoming air can be heated with an inline electric-resistance heating element, and the ducted system can be installed so it sends fresh air to whatever part of the house the homeowner wants, not necessarily the kitchen.
According to Fantech, a motorized damper opens and the unit’s fan is triggered when the range hood is turned on. Fan speed is automatically synched with the exhaust system’s fan, so the amount of makeup air matches the output of the range hood. The system also can be calibrated to provide a slightly negative or slightly positive air pressure inside the house.
(A similar makeup air system already is offered by a Minnesota company called Electro Industries. )
Three capacities and two heater options
Fantech makes three sizes, with maximum flow rates of 650, 1,600, and 2,000 cubic feet per minute. That would seem to cover most if not all residential range hoods on the market.
Both the MUAS 650 and the MUAS 1600 models are vented with 12-inch metal ducts; the MUAS 2000 uses a 14-inch duct. Kits include the fan, intake wall hood, motorized shut-off damper, filter cabinet with a pleated filter, and a duct silencer. A controller matches makeup air to exhausted air to maintain the building air pressure at the level set by the installer.
In winter, the outdoor air introduced by a makeup air unit can be uncomfortably cold. So Fantech offers three heater options to temper incoming air. All of them run on single-phase, 240-volt current. According to the company, the smallest of the three, the MUAH 10/10, draws a maximum of 10,000 watts of electricity and produces 34,140 Btu/h of heat per hour. The other two, for the higher-capacity exhaust fans, draw a maximum of 20 kW and produce 68,280 Btu/h.
At maximum output, the larger heaters draw 83 amps.
List prices for the fan assemblies ranged from $2,158 to $3,498. Heaters are priced at $1,068 to $1,751.
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20 Comments
Fan types
From the numbers, it's immediately clear that the biggest energy use concern is the heaters, not the fans, but it was interesting to see that the Fantech product uses nice high-efficiency ECM fan motors, that provide something like 2 CFM/W (pretty good!), whereas the other product listed appears to use single-phase induction motors and I didn't see any data on motor energy consumption, which means it is probably bad.
And you can opt to skip the heater, and use the blast of cold air into the kitchen as a reminder to the chef not to use excessive air flow or run it unnecessarily.
Both brands use a current-sense transformer on the power going to the hood to decide when to turn on. The fantech then modulates speed based on air pressure. It appears that the other brand tries to figure out the range hood fan speed from the current transformer signal, and then the installer needs to calibrate it to provide the right air flow at different hood speeds.
So the Fantech does has some nice technology that the other does not.
One thing I don't like about the Fantech is that its controls are powered by a 24 VAC transformer that must be on all the time. Transformers like that draw several watts of standby power, substantially worse than things like cell phone chargers that no longer use 60 Hz transformers. A few watts might not sound like much, but since the controls are on 24/7, they could use twice as much energy as running the fan for half an hour every other day.
Pricey
For $3500 I'll come to your house and open the window.
An HRV sourced option might
An HRV sourced option might be nice, the exhaust air is about room temperature.
However it will be greasy, messy air that will coat the inside of an exchanger pretty quickly. Not an easy problem to prevent :(
Response to Alan B
Alan,
Every time the discussion of makeup air for range hoods comes up, someone mentions including heat recovery. I'm not sure why the topic keeps coming up -- it's a very bad idea, as you realize. You don't want to cool hot grease that has been aerosolized. You want to get it out of your house as fast as possible, without cooling it off.
In case there is any confusion on the issue: GBA readers should know that all HRV manufacturers and ERV manufacturers forbid a range hood to be connected to an HRV or ERV.
Here's what I wrote on the topic in my Fine Homebuilding article on makeup air for range hoods:
"Many mechanical ventilation systems are designed with a pipe-within-a-pipe system that allows heat to be exchanged between the outgoing and incoming airstreams. Range hoods can’t do that for two reasons. First, the pipe diameters get big fast. If your range hood requires a 10-in.-dia. exhaust duct, then the duct-within-a-duct solution would require a 14-in. or 16-in. duct. It’s hard to find that much room for ductwork in a house. Elbows compound the awkwardness.
"Second (and the biggest problem with the pipe-within-a-pipe idea), the incoming cold air would cool the exhaust duct, encouraging the suspended grease in the exhaust air to congeal and moisture in the exhaust airstream to condense. The cooling effect of the incoming air makes an exhaust duct become dirtier than if it had stayed warm. This process is similar to what happens when woodstoves are vented to outdoor chimneys; cold flues become clogged with creosote much faster than warm flues."
Couldn't this be simplified?
Couldn't someone make a simple makeup air unit that is wired in parallel to the hood range. A simple high efficiency fan with a constant air volume, and a damper that opens when the fan starts and fails closed when the fan and hood range circuit is opened. The damper would prevent air from traveling in or out when the unit was not in operation. Maybee there could be a few different sizes and you could match them with your range hoods high flow ratings. Then it wouldn't draw any amps whent it was not in use. When it is in use maybee it isn't totally balanced with your range hood but how often do people really use there range hoods a day? Also it would have no heater, of which is not very efficient. You are most likely adding heat to your kitchen anyway when you are running your range hood. All ofl it would lead to a unit that would be simple, cheaper to build, and less likely to break. Are there any such units that anyone is aware of? I know there are air exhausters you could use for make up, but I don't know of any with a damper I described.
Response to Dillon Vautrin
Dillon,
You asked, "Couldn't someone make a simple makeup air unit?"
You might want to consider the Broan solution. Although it doesn't include all of the features you suggest, the Broan makeup air unit is simpler than Fantech's.
As I wrote in my article, Makeup Air for Range Hoods:
"If you prefer to install a passive makeup air duct rather than a powered makeup air unit, the duct must include a motorized damper that is wired to come on simultaneously with the range-hood fan. Broan sells motorized dampers for this purpose, and also provides detailed and useful installation instructions. Broan recommends installing an outdoor air duct connected to a grille mounted on the kitchen wall or connected directly to the return plenum of your furnace; the motorized damper is installed in the duct. One 8-inch-diameter duct works for a range hood rated up to 1,000 cfm. If your range hood is bigger than that, you'll need two ducts."
@ Martin
"Every time the discussion of makeup air for range hoods comes up, someone mentions including heat recovery. I'm not sure why the topic keeps coming up -- it's a very bad idea, as you realize."
I think it comes up because its throwing away heated air and replacing it with unheated, what we have been spending years trying to prevent with HRVs.
"You don't want to cool hot grease that has been aerosolized. You want to get it out of your house as fast as possible, without cooling it off."
That makes perfect sense, i assume that grease will destroy the efficiency of an HRV, and you would have to replace most of the system every few years if it were adopted :(
It would be nice to come up with a new option though.
Response to Martin
Thanks Martin, my wife and I plan on building a pretty darn good house using zip board as a continuous air barrier. We also plan on an ERV, small wood stove (with cold air make up) , and most likely a ductless minisplit heat pump. The kicker is I am having serious trouble trying to talk my wife out of a gas range, so the damper seems to make more sense to me because it is simple and more economical.
Response to #1 Fan types by C. Sullivan
The intent of the MUAS is first and foremost to replace the air being removed from the dwelling by the kitchen exhaust hood system. It is required by code in many jurisdictions (International Residential Code versions 2009, 2012 and 2015) for a hood exhaust exceeding 400 cfm. Many local jurisdictions have either not adopted the IRC's wording, or have modified the wording, though.
Main reason for the code requirement is to prevent depressurization in the home during kitchen exhaust hood operation. Particularly with larger hood systems (400 cfm and greater), but also for smaller capacity exhaust systems in structures with especially tight building envelopes, the kitchen exhaust system can induce a negative pressure in the dwelling when there is not a provision for compensating makeup air. Depressurization is a concern because it can lead to backdrafting of fuel-burning appliances and chimneys. Depressurization also induces infiltration of outdoor air and its contents through the building membrane.
Depressurization is also an indication that the exhaust system might not be performing to its intended capacity. When properly selected/sized, a kitchen exhaust system has the capacity to remove cooking-generated contaminants adequately for the dimensional and heat output (Btu's) characteristics of the range it serves. For large, pro-style ranges this exhaust capacity can often be well in excess of 400 cfm. If the hood is not working to its intended capacity, it might not be removing the cooking-generated contaminants (heat, vapor, odor, toxins, smoke, etc.) adequately. If any of the readers have a tight home without an adequate makeup air provision, they might have noticed that the kitchen hood does't clear the room of smoke...until a window is opened, and voila! I think most of us would agree that opening a window to allow the kitchen hood to work properly is not ideal.
Although the electric heating component of the Fantech MUAS is optional, many will elect to include it in the system. Yes, it can be a power hog while being used. Keep in mind, though, that the heater does include a discharge air thermostat. The heater only energizes when: a) the makeup air system is operating, and b) when the outdoor air temperature is less than the supply air temperature set point. Also, keep in mind that the heater stages itself to heat up to the discharge air temperature set point. For example, if only 5 kW is required to deliver the air at the temperature set point, then the heater will only consume 5 kW.
The transformer that provides the low voltage power to the controls is a Robert Shaw #620-502 transformer. We powered it up in ourr lab, and found that it consumes about 3.1 watts (70 milli amps) when idle.
The MUAS system is initiated by the sensing of electrical current (amp) draw by the exhaust system's fan motor. The system actually utilizes a current transducer to measure the amount of current (amperage) being consumed by the exhaust system, and then commands the makeup air fan motor to operate at a speed to match the air flow. This results in a balanced pressure scheme (no depressurization and no excess of outdoor air) throughout the exhaust fan's speed range. Works for single speed, multi-speed and infinitely adjustable speed-controlled exhaust systems.
C. Sullivan is correct that the the three MUAS models incorporate a fan with an EC motor. In general, an EC motor is very efficient - about 30% more efficient than a standard, alternating current motor operating at full speed, and the disparity is even greater at reduced speeds.
The fan included with MUAS 650 operates at 4.42 cfm/W at full speed and 0.2" w.c. resistance.
The fan included with MUAS 1600 operates at 4.8 cfm/W at full speed and 0.2" w.c. resistance.
The fan included with MUAS 2000 operates at 5.5 cfm/W at full speed and 0.2" w.c. resistance.
Hope this information is helpful.
Response to all comments re: HRV for makeup air
For the reason that at least one commenter made about exhausting kitchen effluents through either an HRV or ERV, it is simply not permitted.
Of course, it is always a goal to recover as much energy as possible. When dealing with kitchen exhaust, though, HRV and ERV equipment is not to be used.
Perhaps someone will develop a way of doing this one day, but I'm not aware of such a product available today.
HRV and ERV products are absolutely wonderful for regular, continuous or intermittent ventilation for a dwelling, though. They can even be used to provide exhaust from a bathroom or two. Just don't use them for kitchen exhaust.
Response to Could this be simplified, and the passive suggestion
Dillon Vautrin describes a system that is very similar to the Fantech MUAS solution, except one that does not modulate the rate of air supply with the rate of air exhaust. This would of course result in an excess of supply air any time the exhaust hood is not operated at full speed. In climates where the outdoor air is typically comfortable, this may be a very viable solution. In climates where the outdoor condition is often in need of conditioning, this might not be so ideal.
Here's why I say this. The cost of conditioning uncomfortable outdoor air is quite significant. Heating up cold air or cooling/dehumidifying summer-type air requires a lot of energy. If we are supplying an excess of this outdoor air, then the central HVAC system will eventually condition it at the cost of the additional required energy. So, from an energy standpoint it would be better to only supply as much air as is needed to compensate the kitchen exhaust.
The passive makeup air solution (such as the Broan makeup air damper) can be a functionally adequate solution, when sized properly. Unfortunately, there seems to be a misunderstanding in regards to the passive solution.
The first thing to understand is that the passive solution utilizes no supply fan. It relies on the kitchen exhaust system to create a vacuum (depressurization) in the home to draw makeup air in through the damper. Of course, the main reason for the building code requirement for makeup air is to prevent depressurization and all of its adverse effects. The passive solution REQUIRES depressurization to work. So, how can the passive solution be used while not letting depressurization get out of hand?
There are many building industry organizations that have determined that 5 Pascals (equivalent to 0.02" w.c.) depressurization is a reasonable limit to prevent backdrafting of fireplaces and naturally-vented, fuel-burning appliances. Some other types of appliances can vent against slightly more, but most codes offices and designers don't want to assume that there won't one day be naturally-vented appliances in the dwelling. Because of this, I could never recommend to design for a depressurization greater than 5 Pa.
So, if we provide a passive opening or openings in the building envelope with enough area to permit makeup air to enter without the dwelling being depressurized beyond 5 Pa, then we've accomplished something. Unfortunately, though, this requires a lot more area than the construction industry realizes. Broan has a very useful tool on their web page that can size a passive system. The tool requires information on the dwelling and the rate of kitchen exhaust. The tool also allows the user to indicate the tightness of the dwelling (information from a blower door test), and a design depressurization limit (this is where you would use the 5 Pa limit). The result might be shocking, since a selection run for systems greater than 400 cfm will usually result in many dampers - not just one.
With multiple passive dampers, the question then becomes what to do with all of these holes in the wall? Yes, this can be done, but it becomes a less attractive alternative to many.
Another thing I want to mention is that it is not necessarily a good idea to duct the makeup air into the return duct of the central HVAC system. Most furnace manufacturers will void the heat exhchanger warranty if air colder than 50-55F enters the heat exchanger. Furthermore, the central HVAC system is designed to maintain space temperature (not discharge air temperature). Residential conditioning systems generally cycle with a call from a space thermostat. Cold air in ducts could result in the formation of condensation on the exterior of the duct. Humid air in ducts could result in condensation on the inside of the duct. Does the central system always run when the kitchen hood exhaust is operated? Just things to consider. This is why Fantech recommends ducting the makeup air to the kitchen, where a portion of the makeup air will actually be exhausted out before it affects the comfort in the rest of the dwelling. And, keep the makeup air system separate from the central HVAC system - the two systems have completely unrelated functions.
I know, it's obvious I don't endorse the passive system. And my opinions on the makeup air topic no doubt are showing through. I just think these things need to be better understood. There are applications where the passive solution makes sense. There are of course applications where the Fantech MUAS does not make sense, too. It is important to understand the basics of the situation, and to make an educated decision for the situation. Hopefully, you will find this information educational, if not helpful.
Help with answering questions and clarifying information
I am a product manager for Fantech, and thought it would be helpful to maybe answer a few questions and provide information where it might be misunderstood. I'm not here to argue makeup air philosophy, but will certainly provide factual information when I know it, and provide help in understanding Fantech's MUAS.
I'm delighted that there is discussion on the subject, because modern construction methods are such that adequate makeup air is becoming a necessity for many dwellings.
Scott Gibson's article is well written. The topic of residential makeup air could easily include enough content to result in many more pages of text, though. I'll try to supplement with additional content, hopefully for the better understanding of all of us. Here's a link to a white paper on our web page that discusses the topic of residential makeup air in more detail: http://catalogue2.systemair.com/FileHandler.axd?id=50124
And, I realize that Fantech's MUAS is not perfect for every situation. It wasn't intended to be.
Hope I can help. I'll reply to some of the comments above.
Thanks Mark
You have some real good points Mark. I plugged some different examples to the Broan make up air specifier using the 5 PA Design Depressurization Limit you suggested and was surprised to see how much passive venting would be required with exhaust hoods rated over 350 cfm. I did know that exhausting fram a tight house could cause back drafting but I never knew the Design Depressurization Limit. I also looked at Building Science Corporations BSI-070: First Deal with the Manure and Then Don't Suck by Joe Lstiburek. It suggested if bringing in low amounts of unconditioned make up air, to do so in places like a basement so that it could be tempered before reaching the living space. It also suggested supplied make up air not passive venting even when using 200 cfm ranges in tight houses. I guess that is why a lot of people have gotten this wrong before they got it right. The fantech MUAS seems like a good product I am just to frugal to spend twice as much on a make up fan and heater as I will for my range. Yet it makes great sense in a higher end home where cost is not as much of a concern.
what am I missing
Why does it seem odd to me that everyone seems to be missing the
obvious solution -- dump the makeup air right there *near the
range*, so it affects the rest of the house as little as possible?
The person slaving over the hot stove will probably welcome a
bit of refreshing blast while working.
_H*
Hobbit
Do you think locating the make-up air in close proximity to the hood makes a big difference to the proportion of warm room air that is removed as opposed to cold supply air? There must have been studies done on this?
Response to Hobbit
Hobbit,
When a person is cooking, the smoke, odors, and aerosolized grease quickly spread out in all directions. A good exhaust fan does its best to collect some of these gases and aerosols.
The main problem with putting the makeup air source close to the range hood is short circuiting. The range hood fan pulls in the outdoor makeup air, allowing the cooking gases and aerosols to drift into the kitchen.
To avoid the short circuiting problem, it helps if the makeup air source is far away from the range hood, not close to the range hood.
Capture efficiency of range hood
There are currently efforts in the industry focused on range hood capture efficiency. Equipment manufacturers and ventilation performance testing/certification organizations are looking into these things. Obviously, if a range hood has a high capture efficiency, then it might not need to exhaust as much air from the home as would a range hood with a lesser capture efficiency.
I did come across a study that was done some years ago in Canada regarding the location of makeup air delivery near the range hood. The study was done for a commercial kitchen exhaust application, but the physics of the situation seemed to transfer to the residential application. I cannot seem to find the study at the moment. If I can locate it, I'll include a link to it.
The study determined that the capture efficiency of a range hood could be adversely affected by the method of makeup air delivery near the range hood. In general, organized columns or curtains of air near the hood were found to be detrimental to a range hood's capture efficiency. More spillage was observed at the hood due to the air pattern disturbance created by the concentrated movement of makeup air. Air diffuser types that were the culprits were slot diffusers and even 4-way types that might send some supply air directly into the exhaust pattern.
When the makeup air was delivered in a very diffused, unorganized manner that did not interfere with the exhaust air flow pattern, much better range hood capture efficiencies were observed. The study, as I recall, recommended that the makeup air delivery be provided via a perforated-type air diffuser at least a few feet away from the front of the range hood. I'm sure diffuser types other than perforated could be effective as well. The gist of the study was that the delivery of makeup air should not be too near the range hood, nor should it be directed towards the range area from nearby.
I wish I could find the study. I'll keep looking...
Capture efficiency - a design guide
I found the document I referenced in the previous comment. It is actually from the California Energy Commission (not Canada as I incorrectly suggested). Here's the link:
http://www.fishnick.com/ventilation/designguides/CKV_Design_Guide_2_031504.pdf
As I mentioned previously, the design guide has the commercial kitchen in mind, but some of the design recommendations can translate to the residential application.
Applications
I have a pretty tight home and it's about to be finished. I have not done a blower test yet but will in a couple weeks. I haven't decided if I need this unit. I have multiple lunos in the house and HVAC Mitsubishi system and a non venting hood. Would this be beneficial for my wood stove or just another whole in my wall?
Response to Clayton Anderson
Clayton,
The makeup air unit described on this page is not intended for a wood stove. The Fantech unit is designed to have an electrical interconnection with a range hood exhaust fan, and such an electrical interconnection is not possible with a wood stove.
Many architects and builders feel that a wood stove doesn't belong in a tight house. Other architects and builders disagree, and have described successful wood stove installations. In many cases, wood stove users end up opening up a window when they first start a fire. For more information on this issue, including suggestions for makeup air sources and conflicting opinions on whether a wood stove belongs in a tight house, see All About Wood Stoves.
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