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Ignore Pressure Differentials At Your Peril

The unintended consequences of monster-size range hoods.


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June 1, 2013 by Robert Bean

Is anyone else noticing an alarming rise in the misguided use of commercial type range hoods inside residential buildings? It has always been a problem but I have recently taken an unusual number of calls from people asking if their already installed monster-size range hoods are going to be OK.

What do they mean by ‘OK’ I ask? 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 mumble mumble…pause…ah no” is what I can hear on the other end. 

So once again with my soft spoken demeanour, I ask, “So now you want my advice, after you bought it and had it installed, am I right? You should have asked a professional that question before you became enamoured with the manufacturer’s glossy range hood brochure.”

For those looking for a way to explain it to appliance addicts in a consumer- friendly way, tell them sticking a 1200 cfm exhaust fan into a typical residential kitchen is not like playing with a hairdryer; it is more like having a home appendectomy with the suction side of an industrial grade leaf blower. In my 40 grit opinion, the subsequent health and building problems created should rest squarely on the appliance manufacturers and their dealers’ shoulders. We have ignorant range hood sales people supported by their just as naïve range hood marketing departments selling range hoods to “doe-eyed” consumers. When it comes to building science all of them put together do not know a water column from a water dispenser. 

The HVAC industry needs to step up and tell these range hood vendors that when you extract way more air than you make up, you are going to create HVAC and indoor air quality problems.

Really, what these hood S & M folks (sales and marketing) need is a simple lesson: when extraction trumps make up, holes and cracks 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. Just what 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, it is all fresh air in those cracks…comments like that give me a headache.

At a time when the federal government is spending our tax dollars on a coast-to-coast “got bad gas” campaign alerting us to the dangers of having radon – an entire appliance industry is flogging kitchen bling that enables environmental contaminates like gases and particulate matter to enter structures freely.

To put this problem into perspective take a hypothetical 20′ x 30′ x 9′ (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. It would only take 4.3 minutes (5400 ft3/1200cfm) to remove all the air such that the pressure would approach 0 psia (assuming the home was at sea level and assuming the fan was capable of drawing down that low). You and I know that it takes longer than 4.3 minutes to cook a pig inside your home and we can all agree homes are not hermetically sealed and that Mother Nature abhors a vacuum.

Over the time the fan is on, replacement air of some quantity 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.

I know that this has been greatly simplified (ignoring the total volume of the home, and so on) but the fact remains that 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. But what if the above grade enclosure and all its corresponding penetrations are, for the sake of argument, sealed or sealed to such an extent that any cracks in, at and around the below grade enclosure became the prime source of relief. What if that air is contaminated with your favourite cancer causing gas, such as radon? This is just one of many examples of where commercial range hoods in residential applications without proper make-up air flies in the face of everything Health Canada wants Canadians to avoid.

What if we could convince everyone that sucking bad stuff into the home is not the smartest of moves and have them put in a proper make up air unit, then what? Well, let’s calculate the winter sensible heating load using:

q = Q • 60 min/hr • ρ • cp • (ti – t0)

Where

q = load, Btuh (kW)

Q = flow, cfm (l/s)

ρ = 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 Btuh (38kW)

Putting this into perspective – with that amount of output you could heat a floor space more than 10 times that of the kitchen it is serving. If you did the same exercise, but for summertime sensible and latent cooling, you would likely find a similar load for dehumidification of incoming outdoor air.

How many people would put in a 10-ton cooling plant just to wring out the moisture from the make-up air in a residence? No one I know, but if they do not dehumidify the incoming air, it is very probable in a conservative-sized home that the indoor conditions would soon represent those of the outdoor conditions. Having a space humidity exceeding 70 per cent RH is just begging for mould to thrive and multiply.

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 outermost surface of the drywall is below dew point then the problem is exasperated further. 

I understand that once again I am taking this to a worst case (but not impossible) scenario. It does need to be stated so that consumers do not find themselves as “in-duct-ees” of Canada’s overburdened healthcare system as a result of the unintended consequences of ignorance over the ramifications of monster-size range hoods installed without proper make up air. <>

Robert Bean, R.E.T., P.L.(Eng.) is president of Indoor Climate Consultants Inc. and a director of www.healthyheating.com. He serves on ASHRAE Committees: T.C.61. (CM), T.C.6.5 (VM), T.C. 7.04 (VM), SSPC 55 (VM). www.healthyheating.com.

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