Online educational resource on achieving indoor environmental quality with radiant based HVAC systems
Not for profit educational resource on indoor environmental quality.
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Fundamentals of indoor environmental quality / thermal comfort and air quality solutions using radiant based HVAC

 

My offer to range hood manufacturers, dealers and sales and marketing people.


"...sticking a 1200 cfm exhaust fan into a typical residential kitchen is not like playing with a hairdryer it’s more like having a home appendectomy with the suction side of an industrial grade leaf blower."


"...what these hood S & M folks (sales and marketing) need is a simple lesson from the Whitney Houston School of Building Science; you know the “Crack is Whack” rule…and when suck trumps blow, holes and cracks in the enclosure become the conduit to contamination."


"...commercial range hoods in residential applications without proper make-up air fly’s in the face of everything Health Canada wants Canadians to avoid."

 


Outdoor air does not equal "fresh air"

Only when the outdoor air is free of contaminates can it be considered fresh. It should never be assumed to be "fresh" when common outdoor pollutants include particulate matter (organics and non organics), ozone, moisture, polycyclic aromatic hydrocarbon (PAH's) etc...

 


The following industry design guides do not replace the need for a design professional - in all cases the authority having jurisdiction must be consulted and where guidance is absent consult with a design professional.

Gas ranges should have at least 100 cfm exhaust air per 10,000 Btu/hr input and a minimum of 500 cfm.

Add 200 cfm for cook tops with grills.

Never have less than 100 cfm per 12" width of cook top surface area. See Schlieren images at different exhaust rates
per linear foot.

The cfm rating is the net flow after allowing for duct and fitting pressure losses, i.e. its not sufficient to buy a fan for a required cfm only to loose its capacity to duct resistance.

Mount hoods between 18" to 24" for standard units and no more than 30" for commercial models.

Install a make up air system equal in capacity to the exhaust air system.

For gas ranges provide combustion air as per the authority having jurisdiction.

Do not use recirculating vent hoods, vent all products to the outdoors.

For quieter operation use a remote fan instead of a fan located above the stove top.

Select the fan using standard selection procedure accounting for duct losses at 500 fpm to 700 fpm.

Use 16 guage or 18 gauge insulated stainless steel ducting, i.e. do not use flex duct.

Pitch ducting downwards back towards the stove top.

Install an appropriate back draft damper at the exterior surface of the enclosure.

Use a hood that covers the entire stove top plus a minimum additional 6" on each side.

Reduce drafts around the range hood from ceiling fans, windows, doors or supply air outlets.

Do not blow make up air directly at the hood or stove top. Let the range hood do its job of exhausting.

Air speeds around the range hood should not exceed 100 fpm.

Preference is given to 100% dedicated make up systems. Alternatively make up air could be delivered from supply air grills within the vicinity of the hood plus air supplied from floor registers used to condition the space.

Provide a dampered make up air duct to the return air stream of the air handler. Interlock the damper with the range hood, i.e. end switch on damper closes contacts on range hood fan.

Conditioning of make up air must account for energy and mass transfer, i.e. heat, moisture, particulate etc.

For additional support on this topic visit our visitor services page.

For sample schematics see our feature page at Houzz.com


"Measured airflows were substantially lower than the rates claimed in product literature for most of the fans evaluated. Only two of the 13 independent models (three installations used
same model) had actual airflows that were 90 percent or greater of the advertised values."
LBNL, 2011


Offer:

Are you a range hood manufacturer, dealer, sales and marketing person?

If you have professional design guidance for residential range hoods that can pass my sniff test I'll give you the opportunity to submit it to me for consideration.

If I like what you have to say I'll include a link from this page to your page at no cost to you.

Limit to the best three manuals.

Instructions:

Post a link to your manual in the promotional tab at our Linked-In discussion group.

Once posted, I'll have a look and if it meets my criteria I'll post the link here.

1.

2.

3.

Readers...if you don't see three samples it's because we haven't received any suitable examples to date...

 

 

Energy & Indoor Environmental Quality - News

www.healthyheating.com in conjunction with HeatSpring Learning Institute  are pleased to present the following pro-bono webinars:
Relationship Between Building Performance, Thermal Comfort and Indoor Air Quality, Tue, May 13, 2014 1:00 PM - 2:00 PM EDT
Relationship between HVAC system types and energy and exergy efficiency, entropy, thermal efficacy, IAQ and thermal comfort. Tue, May 27, 2014 1:00 PM - 2:00 PM EDT


Uncut and Hoodwinked: The unintended consequences of monster-size range hoods.
Copyright 2013, Robert Bean, R.E.T., P.L. (Eng.), All world rights reserved.
The uncut version - politically correct version originally published in HPAC Canada, June 01, 2013

For additional support visit our visitor services page.


Introduction

Am I the only one noticing an alarming rise in the misguided use of commercial type range hoods inside residential buildings? I mean it’s always been a problem but twice in the last week I’ve had people call up asking if their already installed monster sized range hood is going to be ok?

“What do you mean by ‘Ok’ I ask?” “Do you mean is it going to need therapy?” So I follow up in my most empathetic voice, “Do you know what happens when you try to Hoover up the inside of your home without considering the ramifications of pressure differentials?” “Hmmm mubble mubble…pause…ah no” is all I can barely hear on the other end.

So once again in my famously soft spoken demeanour (choke and cough) I ask, “So now you want my advice, after you bought and had it installed, am I right? Look my new friend – that’s a question you should have asked a building science professional before you became enamoured with the manufacturers glossy range hood brochure.”

kitchen rangehood
Figure 1. The suction power of a commercial range hood is sufficient to put a home into deep negative pressure relative to the outdoors. Such differentials can cause back drafting from combustion based appliances and promotes entry of other IAQ pollutants and contaminates.


For those looking for a way to explain it to appliance addicts in a consumer friendly way, tell them sticking (for example) a 1200 cfm exhaust fan into a typical residential kitchen is not like playing with a hairdryer it’s more like having a home appendectomy with the suction side of an industrial grade leaf blower.


Figure 2. "Trends in Kitchen Design" make note of the range hoods...it's exactly this type of trend that creates building and health science issues for home owners. If the video doesn't play in IE try Firefox.

What we have occurring in the marketplace is ignorant range hood sales people supported by their just as nave range hood marketing departments selling range hoods to “doe-eyed” consumers (as per video above) and when it comes to building science - all of them put together don't know their water column from their water dispenser. In my 40 grit opinion the potential health and building problems created by hood induced negative building pressures should rest squarely on the appliance manufacturers, their dealer’s and the kitchen designers shoulders. The HVAC industry needs to step up and tell these service providers that when you continuously suck way more than you blow you’re going to create problems for the occupants and the building - full stop.

Really, what these hood S & M folks (sales and marketing) need is a simple lesson from the Whitney Houston School of Building Science; you know the “Crack is Whack” rule…and when suck trumps blow, holes and cracks in the enclosure become the conduit to contamination. Why? Because when it comes to buildings you can always count on the inside wanting out, and the outside wanting in; and just what in pray tell do these hood salespeople think is going to come through the cracks when you suck the insides out of a home? According to my conversation with one dealer of hoods, "those are fresh air cracks"…crack head comments like that give me a headache.

negative pressure - outside wants inside
Figure 3. When the outdoor pressure exceeds the indoor pressure, mass and energy will flow from the outside in. The mass could be organic and inorganic contaminates (water vapour, ozone, mould spores, odours, particulate matter etc.) and the energy will be heat and sound.
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At a time when the Federal Government is spending our tax dollars on a coast to coast “got bad gas” campaign alerting our fellow citizens to the dangers of having radon - an entire appliance industry is flogging kitchen bling which enables environmental contaminates like gases and particulate matter to enter at free will. It's like having your own illegal immigrant border crossing problem where the home owner and fan are the unsuspecting smugglers; victims of their own unintended consequences.

So let’s put this problem into perspective by taking a nice sized hypothetical 20 ft. x 30 ft. x 9 ft. (6m x 9.14m x 2.74m) kitchen and let’s play my favourite game of, “what if?”

What if that 5400 ft3 (153m3) kitchen was sealed up and the 1200 cfm (0.57m3/s) kitchen exhaust fan activated. Well it would only take 4.3 minutes (5400ft3/1200cfm) to remove all the air such that the pressure would approach 0 psia (assuming the home was a sea level and for giggles assuming the fan was capable of drawing down that low).

Now you know and I know that it takes longer than 4.3 minutes to barbeque a pig inside your home and we can all agree homes are not hermetically sealed and Mother Nature abhors a vacuum; so what must happen is over the time the fan is on, replacement air of some quantity and quality must find a way through the enclosure into the space.

If the replacement air were supplied at the same rate as the exhaust air it would be equivalent to 14ACH or almost 70 times more than its natural rate for a well-built home. Now I know this has been greatly simplified (ignoring the total volume of the home, run time, size of pig, etc.) but the fact remains if the supply air volume were less than the exhaust air volume, then interior space pressures must drop below atmospheric pressure which motivates the outside to move inwards (see Table 1 below).

But what if the above grade enclosure and all its corresponding penetrations are for arguments sake “sealed” or “sealed” to such an extent that any cracks in, at and around the below grade enclosure became the prime source of relief and what if that air is contaminated with your favourite cancer causing gas – like radon? Or what if the combustion venting for heaters, stoves and fireplaces became a back drafting relief connection between the outdoors and indoors? No one survives an extended exposure to carbon monoxide. This is just one of many examples of where commercial range hoods in residential applications without proper make-up air fly’s in the face of everything Health Canada wants Canadians to avoid…time again to rewrite Oh Canada into Woe Canada.
 

Table 1. Approximate total exhaust airflow (cfm) needed to induce depressurization

Envelope Air tightness

-2Pa

-5Pa

-10Pa

Air-sealed older home: 8 ACH50 (Terrible home)

300

540

850

Typical CA new home: 5 ACH50 (Traditional home)

185

335

530

Very tight new home: 2 ACH50 (Transitional home)

75

135

215

Passive House: 0.6 ACH50 (Terrific home)

25

40

65

Notes: assuming cfm is delivered flow at design static pressure, and ignores cycle time:

Example 1. A 215 cfm exhaust fan is capable of inducing -10Pa depressurization in a transitional (very tight) home constructed to 2.0 ACH50.

Example 2. A 185 cfm exhaust fan is capable of inducing -2Pa depressurization in a traditional (typical) home constructed to 5.0 ACH50.

At 5 ACH50, 150 cfm range hood + 200 cfm dryer fails combustion safety test

Adapted from: Singer, B., Kitchen Ventilation Solutions to Indoor Air Quality Hazards from Cooking,  Lawrence Berkeley National Laboratory, October 10, 2013

Table 1. Provides an estimation of how much airflow and at what ACH50 to induce depressurization. At -5Pa its enough to cause a health and safety risk due to back drafting of naturally vented appliances. If you don't want to induce negative pressures you will have to provide relief air - and in many climates this air will need conditioning.  Bookmark and Share

All right what if we could convince the crack head hood dealers and their innocent users that sucking bad stuff into the home isn’t the smartest of moves and have them put in a proper make up air unit, then what? Well let’s take calculate the winter sensible heating load using;

q = Q * 60 min/hr * p * cp * (tit0)

Where

q = load, Btu/hr (kW)

Q = flow, cfm (l/s)

p = density, lbm/ft3 (kg/m3)

cp = specific heat, Btu/lbm F (kW/kg C)

ti = air temperature inside, F (C)

to = air temperature outside, F (C)

Assuming an outdoor air temperature (to) of -30F(-34C) and an inside (ti) of 70F(21C), and standard air conditions, the winter sensible load (q) at 1200 cfm (0.57m3/s) becomes in IP units;

q = 1200 * 60 * 0.075 * 0.24 * (70 - -30)

q = 1200 * 1.08 * 100

q = 129,600 Btu/hr (38kW)

 

 

 

 

Figure 4. For the non-technical person - imagine 129,600 wooden matches burning per hour because that's a visual heat approximation for one hour of pig roasting time.

Putting this into perspective - with that amount of output you could heat a floor space over 10 times that of the kitchen it is serving. If you did the same exercise but for summer time sensible and latent cooling you would likely find a similar load for dehumidification of incoming outdoor air. How many people would put in for giggles a 10 ton cooling plant just to wring out the moisture from the make-up air in a residence? Not likely, but if they don’t dehumidify the incoming air, it’s very probable in a conservative sized home that the indoor conditions would soon represent those of the outdoor conditions, and having a space humidity exceeding 70% RH is just begging for mould (mold for my U.S. readers) to thrive and multiply.

Furthermore if adequate relief is not provided, outdoor moisture will be pulled into the enclosure cavities where it will also likely encounter a vapour barrier which serves as a capture plane. If the home is air conditioned and the outer most surface of the drywall is below dew point then the problem is exasperated further.

Ok I get once again that I’m taking this to a worst case (but not impossible) scenario but it needs to be stated so that appliance dealers and their unsuspecting users don’t find themselves as an “in-duct-ee” of Canada’s overburdened healthcare systems from the unintended consequences of monster sized range hoods installed without proper make up air.

For additional support on this topic visit our visitor services page.
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Additional hood design information

Figure 5. Schlieren images at different exhaust rates per linear foot (lf)20
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Figure 6. Range hoods: kitchen pollutants hazardous to your health
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Resources

  1. Singer, B., Kitchen Ventilation Solutions to Indoor Air Quality Hazards from Cooking, Residential Building Systems & Indoor Environment groups, Lawrence Berkeley National Laboratory, California Air Resources Board Research Seminar, October 10, 2013

  2. Dennekamp et al., 2001. Ultrafine particles and nitrogen oxides generated by gas and electric cooking. Occup Environ Med 58: 511-516.

  3. Fortmann 2001. Indoor air quality: residential cooking exposures. Final Report, ARB Contract 97-330.

  4. Hu et al., 2012. Compilation of Published PM2.5 Emission Rates for Cooking, Candles and Incense for Use in Modeling of Exposures in Residences, LBNL-5890E

  5. Logue et al., 2013. Pollutant exposures from unvented gas cooking burners: A simulation-based assessment for Southern California. Environ Health Persp; Provisionally accepted

  6. Gerstle, W.D., New Rules For Kitchen Exhaust, ASHRAE Journal, November 2002

  7. Sizing exhaust hoods - air volume flow and capture velocities - online exhaust hood calculator, www.engineeringtoolbox.com

  8. Bailes, A., 2012, Recirculating Range Hoods — As Effective as Recirculating Toilets, Energy Vanguard, < http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/56716/Recirculating-Range-Hoods-As-Effective-As-Recirculating-Toilets>

  9. Singer, B.C. et al, 2011, Pollutant Removal Efficiency of Residential Cooking Exhaust Hoods, Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, LBNL Paper LBNL-4902E, http://www.escholarship.org/uc/item/16f896xz

  10. Livchak, A., Schrock, D., Sun, Z., The Effect of Supply Air Systems on Kitchen Thermal Environment, ASHRAE Transactions 2005, Vol 111, Part 1.

  11. Yik, F.W.H. , Au, P.W., 2002, Flow Rate and Capture Efficiency of Domestic Kitchen Exhaust Hoods for Chinese Households, Research Centre for Building Environmental Performance, Department of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong, China

  12. Singer, Brett C., 2011, Experimental Evaluation of Installed Cooking Exhaust Fan Performance, Lawrence Berkeley National Laboratory, LBNL Paper LBNL-4183E http://escholarship.org/uc/item/8pv0317x

  13. Brown, S., Dedicated Outdoor Air System for Commercial Kitchen Ventilation, ASHRAE Journal, July 2007

  14. Kitchen Exhaust and Make Up Air, Technical Papers and Seminars, ASHRAE Winter Conference, Dallas, Texas, 2013

  15. Holladay, M, 2010, Makeup Air for Range Hoods: If your kitchen has a powerful exhaust fan, it may be pulling air down your chimney or water-heater flue, Green Building Advisor < http://www.greenbuildingadvisor.com/blogs/dept/musings/makeup-air-range-hoods>

  16. Gibson , S., 2011, Dealing with a High-Capacity Range Hood: Big fans need lots of makeup air, but where does it come from?, Green Building Advisor < http://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/dealing-high-capacity-range-hood>

  17. Seville, C., 2012, Why Range Hoods Don’t Work, Green Building Advisor < http://www.greenbuildingadvisor.com/blogs/dept/green-building-curmudgeon/why-range-hoods-don-t-work

  18. Farrish, K., Range Hoods, B.C. Building Info <http://www.bcbuildinginfo.com/display_topic.php?division_id=13&topic_title_id=157&topicid=741>

  19. Lstiburek, J., 2013, First Deal with the Manure and Then Don't Suck, BSI-070: Building Science Insights

  20. ASHRAE HVAC Applications Handbook, 2011, Chapter 33, Kitchen Ventilation

  21. Pollution in the Home: Kitchens Can Produce Hazardous Levels of Indoor Pollutants, Berkeley Lab research on indoor air quality finds previously overlooked dangers. July 23, 2013

  22. Kuehn, T.H., et al. 2001, Effects of air velocity on grease deposition
    in exhaust ductwork
    , ASHRAE 1033-RP Final Report

  23. Halton design guide for indoor air climate in commercial kitchens

  24. Gerstler, W.D., Kuehn, T.H., Pui D.Y.H., Ramsey, J.W., Rosen, M., Carlson, R. P., Petersen, S.D., Identification and Characterization of Effluents from Various Cooking Appliances and Processes As Related to Optimum Design of Kitchen Ventilation Systems, ASHRAE 745-RP Phase II Final Report, University of Minnesota, revised February 9, 1999

  25. Liescheidt, S., 2002, Makeup Air Effects On Commercial Kitchen Exhaust System Performance, California Energy Commission, PIER Report P500-03-007F

  26. Supplemental Research to ASHRAE 1202-RP, Effects of Range Top Diversity, Range Accessories, and Hood Dimensions on Commercial Kitchen Hood Performance, Final Report - January 2006

  27. Alter, L., 2013. The most screwed up, badly designed, inappropriately used appliance in your home: the kitchen exhaust, TreeHugger

  28. Kitchen Ventilation Systems: Part 1, Evaluating the 2009 IRC Requirement for Makeup Air. Builder Briefs. Pennsylvania Housing Research Center, March 2012

  29. Kitchen Ventilation Systems: Part 2, Providing Adequate Makeup Air . Builder Briefs. Pennsylvania Housing Research Center, April 2012

  30. Wallace et al., 2004. Source strengths of ultrafine and fine particles due to cooking with a gas stove. Environ Sci Technol 38: 2304-2311.

  31. Wan et al., 2011. Ultrafine particles and PM2.5 generated from cooking in homes. Atmos Environ 45: 6141-6148.
     

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Related reading:

Infiltration: Leaky buildings are lousy buildings
Particulate Matter
Do I need an engineer? A Guide to HVAC/Indoor Climate Design Service Providers
Where will your indoor climate system score?
How to "ball park" your budget for indoor climate control.
Indoor environments: Self assessment
Built to code: What does it mean for consumer thermal comfort?
The Total Comfort System - The "Un-minimum" System
Thermal Comfort: A 40 grit perspective for consumers
Thermal Comfort: A Condition of Mind

Do-It-Yourself HVAC - Should you do it?
The Cost of HVAC Systems - Are You Paying Too Much for Downgrades?
Radiant Installations - The Good, Bad and Ugly
Thermal Comfort Surveys - Post Occupancy, Part I
Thermal Comfort Surveys - Post Occupancy, Part II
HVAC does not equal IEQ


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