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Kitchen Vent Backdrafting Chimney

John Jr
John Jr Member Posts: 210
Has anyone had problems with too large of a stove vent hood creating a backdraft on the chimney? House has a ach of .57.


  • kevin coppinger_4
    kevin coppinger_4 Member Posts: 2,124
    oh yeah!

    some of those thing really suck! Sealed combustion is here to stay. I have run air directly to oil burners even in chimney applications...Restarants have this problem with gas water heaters all the time....kpc

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  • Bob Harper
    Bob Harper Member Posts: 882
    ASHRAE 62.2-2003

    Says they must provide the makeup air for these monsters. May not be code in you area but it is a Nat'l std. now.
    The I-codes call for performance testing upon commissioning, which includes backdraft testing.

    As for sources of backdrafting, consider:
    -leaky return ducts in bsmt or supply ducts in attic
    -upper level exfiltration leaks such as can ceiling lts)esp. non-IC rated), attic hatch, lights, and registers, and open windows including those closed but not latched.
    -powered exhaust fans incl. clothes dryers, bathroomn fart fans, kitchen Suck-O-Matic, whole house vac., whole house fan, and don't forget water heaters, furnaces & boilers, and open fireplaces.
    -local wind depressurization due to geography, trees, etc.
    -Stack Effect

    While this house is above ASHRAE's recommended minimum 0.35 ACH, it is still very tight. The House Depressurization Limits (HDL) are very sensitive in tighter homes--A little force for or against you can have a big impact. Therefore, you might be able to over come this problem without having to re-invent the wheel. Try sealing the upper levels of the house tight as Tupperware, seal the ducts with mastic, then provide a little relief down low in the basement.
  • BillW@honeywell
    [email protected] Member Posts: 1,099
    This has been an issue for a time...

    ever since they've been putting restaurant-sized exhaust hoods into homes. The kitchen designers don't understand that that air has to come from somewhere, and the path of least resistance is usually thru the chimney. Gas clothes dryers, gas or oil fired waterheaters, boilers and furnaces all need adequate combustion air and adequate exhaust to operate effeciently and above all, safely. CO is a constant menace. Sealed combustion, energy or heat recovery ventilator(s) sized to meet the demand are a must. Don't forget about the bathroom fans. Provide the mechanical room with it's own air supply. There are solutions possible, but this issue is best addressed when the house is being built. Unfortunately, it seldom is.
  • Dale
    Dale Member Posts: 1,317
    You are not alone

    Huge problem, even if an all elect house the attached garage CO gets pulled into the house. Only solution is to bring in the amount that's going out. Only reasonable way to do this in a cold climate is with a makeup air unit dumping into a termpering spot like a cold air return, with of course the circ. fan running and probably the furnace, which may be colder than the furnace desigh or an air to air heat exchanger rated for kitchen exhaust for a few thousand dollars. And, they are not 100% effecient so the heating bill will go up. Ducting air in to the top of the range gets a little cold at zero or so outside.
  • Mark Fleming
    Mark Fleming Member Posts: 2

    What about integrating the exhaust fans with the ERV/HRV in a tight house? The idea would be to throw off the normally balanced in/out of the ERV whenever an additional exhaust vent is used.

    When the kitchen fan is turned on, it would work in series with a 120V duct valve (like Famco) in the ERV exhaust. Turn on the kitchen fan, and the duct valve allows the ERV to bring in air, but the exhaust is shut down. If the kitchen fan is huge, the corresponding duct valve could be placed to shut off all the air. Depending on cfm flow of the two units, this might even slightly pressurize the house when the kitchen fan is on. Instead of air leaking around windows and doors, the ERV would filter the air and send it out the normal vent(s).

    When a bathroom fan is used, the duct valve could be in the ERV exhaust for only that room. A simple setup for the bathroom, where grease in the ductwork isn't a problem, would be to use a flap in the duct to direct whether the ERV or the bath fan would exhaust. In a normal position, the ERV would draw air through the same vent as the bathroom fan. When the fan is in use, the pressure of the exhaust closes the vent to the ERV. Makeup air from the ERV continues to flow to the bathroom, helping to keep the pressure nuetral.

    Now, how to avoid complex wiring and ductwork?

    Mark Fleming
  • Constantin
    Constantin Member Posts: 3,782
    I think you're on the right track...but...

    I think the primary issue is not the complex interconnection of HRV's/ERV's its attacking the source of the problem, i.e. the rather ridiculous fans that are being installed with nary a thought as to what is actually required.

    It's fine and dandy to have high exhaust air requirements in commercial kitchens where probably 80% or more of the burners are actually on during operations. In a home, I'd like to see anyone put on more than 3 burners for anything but special occasions... so where does that leave us?

    I think the answer is intelligent fan control. In this day and age of variable-speed blowers in furnaces, air handlers, and modulating boilers it should be peanuts to wire up a variable-speed blower to either take feedback from the appliance (open-loop control), from the temperature under the hood (closed loop control), or a combination thereof. It is my understanding that the Wolf hoods do closed-loop temp sensing, so I'll assume that other vendors have the same capabilities.

    However, I am still shaking my little homeowner head over the 900CFM fan that I am supposed to install for our six-burner stove. That blower outstrips our two HRVs almost by a factor of 2x, so using them alone will not work to bring sufficient air into the home. No, we had to install a second duct that is going into the AC return as you describe (with a motorized damper). Hopefully, all that will be sufficient to keep the place stable. A blowerdoor test will confirm that.

    Meanwhile, I worry about all the holes in the exterior envelope that adding HRVs/ERVs/Kitchen fans/etc. have created. Soon, I won't need ERV/HRVs anymore because the house will have returned to its old leaky self (grumble).
  • BillW@honeywell
    [email protected] Member Posts: 1,099
    You can't intergate...

    kitchen or bath fans into an er/hr ventilator. Kitchen exhaust is full of grease that will coat the heat exchanger in short order. Bath exhausts contain lint, cosmetic residue like powder and hair spray which over time can cause the same problems. The kitchen and bath should exhaust directly outside. ER/HR ventilators pull exhaust air from a central location, transfer heat from exhaust air to incoming fresh air (opposite in summer)and deliver incoming fresh air into the return plenum of your forced air heating/cooling system. They are very subtle...only about 200 cfm, but they operate at low speed 24/7, and on high speed for large gatherings, smoking etc.
  • Constantin
    Constantin Member Posts: 3,782
    I think you misunderstood him...

    ... the way I read his post is that the kitchen vent is not run through an ERV/HRV. Rather, whenever the kitchen vent comes on, the control logic inside the ERV/HRV is supposed to stop the motor that usually pushes air out of the home. That way, the ERV/HRV stops acting like a HX and is just pushing air into the home...

    I note your comments regarding bathroom vent applications with interest. I hope to report back in a year or two to let you know how well the Lifebreath TRV handled being the sole exhaust mechanism for multiple bathrooms. For what its worth, the manufacturers certainly seem to be OK with using their products in this application.

    The Lifebreath TRV series incorporates a ERV and a HRV core. We have a 195 and a 300 model. The names refer to the max. cfm that the units can move. Thay have a 5-speed motor, we plan to be using the lowest speed all year. Occupancy sensors from Leviton then kick the units into high gear whenever people enter the bathrooms. The built-in timer then allows the blower to continue running at high speed for some time afterwards. KISS.
  • C-TEC
    C-TEC Member Posts: 14
    Chimney backdrafting

    Jim Davis, National Comfort Institute, explains how to solve all these problems in his CO certification class. Take some time off, attend Jims's two day class and learn how you can save the lives of your clients! I have been working in the HVAC field for more than fifteen years and it opened my eyes.

    Home Experts Heating & cooling
    Newark, Ohio
  • Bob Harper
    Bob Harper Member Posts: 882
    makeup air kits

    There are now several systems out there for makeup air ranging from the hole-in-the-wall theory to powered fresh air intakes with electrical strip pre-heaters hooked to a slave switch so when the 2 biggest air hogs kick in(usually the kitchen fan and clothes dryer), it energizes the makeup air system. Go to Another passive system called US Plusaire connects a class1 duct from outside to the return trunk about 10ft before the fan. A small warm supply right off the warm trunk is run into a manifold to temper the incoming air.

    As with any such system, you'd need to measure your Delt T's &Ps. Also, the intake will need a damper so you don't breath in soggy summer air unless you really have to. All fittings need to be non-corroding due to condensation.

    The bottom line is, the whole house is a system and must be balanced to work. Lot's of cause and effect.

    I know Tjernlund and Field Controls make some systems, too.
  • Tom_35
    Tom_35 Member Posts: 265
    We're faced with this on a job right now.

    We have a 8,500 sq. foot home that is under construction and the homeowner has bought a Viking 1500 cfm variable speed exhaust fan for her Wolf range---which is 42" wide. Normal fan guidelines state that a 100 cfm per lineal foot of cooktop surface is what is required for this instance.

    This home also has 4 fireplaces to draw air oout of the home. We have install Field Controls metering air systems for make-up on each of the air handlers, but they are still not enough in the event that she cranks up the exhaust fan above the cooktop and has the fireplaces going as well...not to mention the exhaust fans for the bathrooms and the clothes dryers.

    The 1500 cfm fan calls for a 10" duct (which equates to a little of 1.0" of static pressure) on the fan. We mocked up a duct system as it would be on the home complete with the 45 degree elbows and the length of duct that the fan would actually see. After installing a filter, we measured just over 1,000 cfm that would be exhausting out of the home.

    Normally, we would look at installing a 900 cfm make-up air fan and introduce the air into the hood, which would give the slightly negative 10% that is recommended. The problem is that with variable speed, the low setting is 300 cfm, and moves up from there.

    I talked to the company that sold her the fan and they said that we were the only ones that had ever come to them with the question about make-up air. The manufacturer was no help either.

    We have asked to see the actual Viking hood so that we can come up with some sort of integreted wiring to hopefully work with the control switch.

    Is it no wonder that homeowners that have the incredibly huge expensive homes have smoke backdrafting from the fireplaces, dust all over the place because of the home being negative and sucking in dust everytime the doors are open, and possible CO issues?

    Tom A.
  • BillW@honeywell
    [email protected] Member Posts: 1,099
    Hi, Constantin!

    I may have read the post too fast, you could wire up a conrol system to do just that, in fact, someone in a post below mentions a pre-packaged combustion air unit.

    Kitchen exhaust is tough to handle, and potentially flammable if grease is allowed to build up. Bath exhaust is nearly as bad, just ask anyone who has done sevice work on beauty parlor hvac systems. Believe me, if women knew what they were breathing in in most beauty parlors, they'd run away screaming! Ammonia from hair dye, particulate consisting of skin flakes, fingernail dust, dried hair spray (a protein), solvents from nail polish, vapors from various cosmetic get the idea. Granted your house won't have the levels of a beauty parlor, but if you have teen-age girls, it may come close!

    The arrangement you have for the ER/HR's is pretty slick. Did you design that yourself?
  • Constantin
    Constantin Member Posts: 3,782
    Hi Bill!

    No question, there are environments that are downright awful to work in, and the kitchen flue vent has to be one of them. I also agree with your worry about beauty parlors... some of them are scary places, and not just because of the environmental hazards.

    At the end of the day, I think I'll have bigger worries than the ERV's once my kids reach that troubling puberty stage. Small indoor riots are peanuts compared to changing a core, cleaning a filter, etc.

    As for the control mechanism, I can only take partial credit. At first, the German side of my heritage tried to break through and mandate a humidity sensor in each bathroom. Said humidity sensor would then engage high-speed mode whenever the conditions warranted it, with the low-speed HRV/ERV mode there to exhaust the usual smells.

    Then reality hit as I became downright dismayed how UGLY the average humidistat is. No one seems to make units that can talk intelligently to a central humidity computer that compares average house humidity to the bathrooms and engages the HRV/ERV accordingly. Or perhaps I don't know where to look...

    ... anyway, in dismay I called up Lifebreath. After a lengthy interview process to ascertain that I wasn't a blundering idiot, the technician there suggested looking into occupancy sensors instead of using humidistats. KISS.

    With that hint, I recalled seeing such sensors in a Leviton catalog when my wife and I designed how the hallways would auto-light at night. Thus, I re-dove into the literature at Leviton and discovered that they make very neat, ceiling mounted, low-profile occupancy sensors that tie into a so-called power-pack. Said power pack takes 120VAC and produces low-voltage power for up to three occupancy sensors. The sensors themselves can be adjusted for sensitivity (i.e. turn certain sectors off, they are PIRs), length of timer, etc.

    Three-small guage control wires go out to the sensor, with the third being a signal cable back to the power-pack. The power pack has two leads that are attached to a 120VAC-rated contactor. Well, the Lifebreath has an over-ride mode with a provision for dry contacts. In other words, this is a match made in heaven.

    Attach the two red leads from the powerpack to the lifebreath and every time a occupancy sensor trips, the unit goes into high speed mode. The larger one serves three bathrooms and exhausts into the return trunk of the top-floor AC, while the smaller unit in the basement deals with 2 half-baths and the rest of the house.

    Incidentally, both units are the same size cabinet-wise. The only apparent difference must be the ECM blower motors used.
  • Mark Fleming
    Mark Fleming Member Posts: 2
    Backdrafting, humidistats, dryer vents, etc.

    My ERV can't compete with a 900 cfm fan. The design I'm working on has an electric range, so giant fans aren't required. I think I can get by with a 150 cfm. My ERV is rated at 200 cfm. Even if the complex ducting on the ERV can't meet the flow of the straight kitchen exhaust, the hope is to keep pressure difference close enough that the wood stove doesn't backdraft, even though it might slow down the stove's draft.

    The ERV and kitchen fan ducts are separate. I've heard not to connect an ERV exhaust in the bathroom because of towel lint, baby powder, and airborne hair from the blow dryer. I clean this stuff off the bath fan blades about every 3 years because you can hear the fan wobbling and the motor struggling. Still, most people hook up the ERV to the bath and I probably will, also.

    I haven't been able to find a good looking humidistat, but that is what really fits the bill. My biggest worry is humidity in a tight house and a motion sensor doesn't address the issue. Humidity in our bath can drop after a shower, but in another 15 minutes, it's back up because of the wet stall and towels. That's after I'm out of the bathroom (and maybe the house).

    My work around is to put the ugly humidistat inside the ERV exhaust ductwork. I set it and (hopefully) forget it. If the humidistat senses high humidity, from whatever source, it kicks up the speed on the ERV. If it kicks down only to have the wet towels/shower stall drive the humidity back up, it kicks up again.

    I haven't yet found a solution for the dryer. Seems like there would be a market for this. The dryer could exaust through a simple aluminum air-to-air heat exchanger that has a small intake fan and a filter. The heat exchange takes place outside, where the condensation drops to the ground. Even if the gizmo were only 50% efficient, that's good enough given that the exhaust is 120 degrees. Turn it off in the summer or have a thermo switch that doesn't operate the fan at 70 degrees and above.

    More gadgets to tinker with.

    Mark Fleming
  • Constantin
    Constantin Member Posts: 3,782
    Towel lint, et al...

    I've seen enough plugged return plenum filters on furnaces to know that bathrooms aren't the only places to produce filter/HX clogging dust... Could you point me to a study that definitively shows how bathroom exhaust is much worse, pollutionwise, than most air in a home? Any living being produces lots of dust as skin flakes off, hair falls out, etc. so there cannot be a "safe" area for an ERV to suck air out of, unless it's a clean-room.

    Perhaps adding a coarse particle-filter at the intake may be the way to go.

    Adding a Humidistat into the plenum sounds like a great idea if it can be implemented well. In my home, I don't know where I'd put it to be servicable. Perhaps your home has home runs to the ERV in a accessible area.

    I think a occupancy sensor with a timer will achieve 95% of the functionality while being much easier to service, in my application
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