One of the issues to consider when adding a range hood, or replacing one, is whether it will cause depressurization problems in the house. If the exhaust flow of a range hood (or any other fan) causes high levels of house depressurization, there are some possible negative consequences:

  1. Backdrafting or spillage of vented combustion appliances, including fireplaces and woodstoves
  2. An increase in radon entry from the soil
  3. An increase of air entering the house from attic spaces or wall cavities
  4. Drafts from the outside

The bigger the fan and the tighter the house, the more depressurization that is likely to occur.

Have you encountered this in your work? Have you seen instances where a newly installed range hood requires some monitoring or the provision of make-up air? What levels of depressurization have you seen created?

Back when I was testing houses in the eighties and nineties, we saw several houses exceed 20 Pa of depressurization. Are we seeing that today?

The gradual disappearance of natural draft appliances (those with chimneys) and the availability of consumer carbon monoxide alarms have reduced some of the dangers.

We would be interested in hearing about your experiences.

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Replies to This Discussion

Don,

My concern is one of liability. 

Even with sealed combustion devices the manufacturers (for example) of gas fireplaces have in the "fine print" warnings for potential back drafting with CYA clauses and advisement for MUA...no one reads these but they are there in the manuals.

Then there is the issue of attached garages...self closing doors have to be adjusted to close off against the potential negative pressure but the hardware installers don't know enough to put the house into negative pressure to test the setup. Have seen this a lot with fire doors in high rises.

Also there is the introduction of risk when ownership changes hands and the new people put in "sealed wood stoves" which never are... 

I would suggest most HVAC/electrical and kitchen contractors and the wholesalers they work with who take on the design and installation of these systems fail to appreciate the risks. Likewise for homeowners who have little understanding of this subject matter.

We put in our spec the option for depressurization testing but have a hard time convincing the generals and their clients to  carry this out...too bad because in about 60% of the projects where problems of all types (mainly moisture related) show up...a test would have would have discovered the flaw(s)

Cheers,

RB

Robert,

Thanks for that. It is a significant problem with house ownership changes and the new occupants are unaware of the workarounds. I have been in houses that have been owned for several years and the owners were unaware they had an operating HRV (operating poorly by that time). It is counterintuitive to a new owner to open a window when they operate the range hood, if that is the recommendation made to the previous owner at the time of hood installation.

Don, can you help us understand the ventilation needs for induction cooking?  There are no gas combustion byproducts, but the water vapor, and fine particulates from cooking oils likely still need to be removed from the conditioned space.  Is there a specific CFM for this kind of cooking?

Thanks for your thoughts on this,

Alice

Alice, there is probably not a specific ventilation requirement for induction cooking. The ROCIS experience with particle monitors shows that a bit of oil on a burner will create lots of particles (as will a snuffed out candle). Induction cooking should alleviate that particular risk. However, we have not quantified how much of the particle creation we have seen comes from the burner and how much from the pan. Do you have a particle monitor? If so, I would simply fry up an onion in an oiled pan and see what transpires. Most of the time, a single fried onion will create a decent particle spike in a kitchen.

Brilliant; I will ask Santa for a particle monitor for Christmas now.  Thanks!

Induction definitely produces fewer pollutants than gas - because you don't have the combustion products - and it appears also less than electric coils. But we also know that heating oil and many other types of cooking releases particles and VOCs even when cooking with an induction stove; and those pollutants should be vented. And while several low cost IAQ monitors are great at detecting particles from lots of cooking and other sources, they simply can't see very small particles. They are thus mostly blind to some cooking-related particle emissions. Berkeley Lab paper on low cost particle monitors   

Have you done any more experiments to estimate combustion gas particulate while cooking?

Presuming that the appliance isn't defective -- e.g. the flame is blue not yellow...

If the carbon soot generally forms if the combustion process is too rich and incomplete.  One possiblity is to wipe bottom of cooking pan afterward and measure soot if any?  I'm guessing that it will ineligible... and instead is more what CO might be produced while cooking with gas.  I'd bet that most of the particulate we see from cooking comes from that process itself.... 

Hi Don.

Yes, we are still doing depressurization testing. We use the CSA F326 test criteria. Now-a-days (as opposed to the '80's), new homes do not have spillage susceptible combustion appliances so we apply the 50 pa limit. As larger range hoods tend to go into larger homes we rarely/never encounter a problem there. We use the Ontario Building Code 9.32 rules to exempt solid fuel combustion and hope that the CO & Smoke alarms will result in appropriate user behaviour modification. The problem we do have is gas log sets in open masonry fireplaces. We apply a 5 pa limit (or observed spillage) to these and a large range hood will almost always cause more than 5 pa of depressurization. Some clients will opt for replacement of the log set with a direct vent insert, others want the full powered make-up air with re-heat system. We use a gas-valve interlock so that if the range hood is on and the make-up air system is not, the gas to the log set(s) will be cut off..

That is good information, Dara. Too bad we were not able to establish real gas appliance depressurization limits before we defaulted to -50 Pa. I am confident that many gas appliances are good to -50 Pa but some will not be. Specific depressurization testing as part of the performance tests in gas standards would have been preferable.

I agree that is a gap in combustion appliance certification testing. There is a logical disconnect between combustion appliance standards and ventilation/depressurization standards. Being the first on the block combustion appliance producers have tried to shift the focus onto the ventilation systems without taking responsibility for their appliances' performance in the range of depressurization that can be reasonably expected in modern homes. This will be an interesting topic in the upcoming CSA F300 standard review.

Do you see the same problems in modest homes or are the householders in small houses less likely to call in a specialist?

I agree. For modest homes a specialist is not likely to be engaged unless the Building Official flags it during construction. If it is done after the fact (conversion from wood to gas) then the only knowledgeable person who could stop it would be the gas fitter. At the same time, we find modest and merchant built homes use factory built direct-vent fireplace units that should not have those problems. Gas log sets in masonry fireplaces tend to be found in the pretentious homes of people who do not care about efficiency and have no intention of actually burning wood. If TSSA (the gas safety folks) would make a regulation to ban those things retro-actively (the way they have for natural draft boilers) we would all be much better off.

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