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Ventilatorji nape so lahko res močni. Ta moč je odlična za odvajanje dima, vonjav in pare iz vaše kuhinje, tako da nobena od teh stvari ne kroži po vašem domu.

Toda kot nas opominja stric Ben, velika moč prinaša veliko odgovornost (glej spodaj).

Izkazalo se je, da lahko močne kuhinjske nape povzročijo težave, če jih gradbeniki, izvajalci in lastniki stanovanj ne namestijo odgovorno. Nekateri od teh ventilatorjev premikajo zrak s hitrostjo, večjo od 1000 kubičnih čevljev na minuto (CFM). Posledično povečajo negativni zračni tlak in prisilijo "dopolnjen" zrak, da vstopi v dom skozi vrzeli in razpoke v ovoju stavbe.

Z drugimi besedami, zmogljive kuhinjske nape lahko nenamerno prinesejo umazan zrak v vaš dom in ustvarijo nezdravo – in potencialno nevarno – okolje.

Najprej:Kaj je pozitivni in negativni zračni tlak?

Za vsak kubični čevelj zraka, ki zapusti zgradbo, vanjo vstopi še en kubični čevelj:

• Zrak, ki vstopa v prostor, je pozitiven tlak; vrzeli in razpoke v ovoju stavbe v vaš dom vnesejo nekaj pozitivnega pritiska. Enako velja za odprto okno na vroč dan.
• Zrak, ki zapusti prostor, je podtlak; Kopalniški in kuhinjski ventilatorji so mehanski sistemi, ki odvajajo zrak iz vašega doma in med delovanjem ustvarjajo ravnovesje podtlaka.

V klimatiziranih prostorih želimo nadzorovati, kje zrak vstopa in odhaja. Zato se trudimo kanale, dovodne registre in povratne rešetke vgraditi čim bolj inteligentno. Tudi zato v nekatere domove dodamo mehanske prezračevalne sisteme.

Zakaj so razlike v tlaku pomembne

Pozitiven tlak je na splošno "dobra vrsta" tlaka v svetu HVAC. Ko so domovi pod rahlim pozitivnim pritiskom, zunanji zrak potisne ustaljen, "starejši" notranji zrak iz doma. V idealnih okoliščinah je zunanji zrak svež in čistejši od zraka, ki ga nadomešča.

Tudi uravnotežen zračni tlak je dober. Tukaj na jugu visoko zmogljivi domovi pogosto vsebujejo ERV za zamenjavo starega zraka za svež, čist zrak. In these airtight homes, not much air enters or leaves on its own, so the ERV is needed to maintain good indoor air quality.

Lots of negative pressure, on the other hand, isn't good for any indoor environment, no matter how tight or leaky the building envelope. When a home - or even a room - is under negative pressure, large amounts of air are being sucked out of it. That air has to be replaced somehow, and what happens is that air enters the home via:

• Gaps and cracks between the attic or crawlspace and the living space
• Unsealed spaces around windows and doors
• Chimney and water heater exhaust flues
• Dryer vents and bathroom fans

Needless to say, air that enters your home through the attic or crawlspace isn't the kind of air you want to breathe. And in some situations, negative pressure can lead to carbon monoxide, or CO, backdrafting. When air enters the home to replace the air being removed, some of it might contain CO that was supposed to leave your home through an exhaust vent.

This situation is more common than you might think, and it's dangerous.

Which brings us back to range hoods. That powerful new kitchen fan might be making your home "go negative" by sucking out more air than your home passively takes in. When that happens, indoor air quality doesn't improve actually when you run the fan.

It gets worse. Lots worse.

According to Section M1503.4 of the International Residential Code, you should install a make up air system with motorized controls if your exhaust fan is rated at 400 CFM or higher. Many kitchen fans move air at a rate of over 1000 CFM. Do you think that most people whose fans are that powerful are also installing make up air systems?

They're not. But they should be.

What happens when you run an 1100 CFM kitchen fan in a high performance home?

One day, we got a call from a family with an air quality problem. Despite investing in an energy efficient home, they weren't breathing healthy air. Every time they ran the first floor kitchen fan, air would enter the home through bath fans and gaps in the attic envelope on the third floor .

In other words, the kitchen fan was too powerful. It was causing a severe negative pressure imbalance in the home.

Large quantities unfiltered air were entering from the outdoors and the attic. Not only was the air unclean - it was increasing the heating and cooling loads of the home. The homeowner even installed a ventilating dehumidifier to introduce positive pressure and remove the humidity entering the home when the kitchen fan was on. It was all to no avail, of course.

When we went to the client's home to scope things out, we performed pressure measurements with the fans on and off. We also compared the amount of air the hood was removing with the amount of air being brought in. The results were 100 CFM being brought in to 1100 CFM being removed.

That's eleven times as much air going out than coming in! No wonder there was air entering the home from all sorts of weird places.

The solution? We designed and installed a make up air system sized just for the home, which included:

• Variable speed fans, variable speed controllers, and large exhaust ducts
• Duct elbows and air mufflers to manage the CFM as needed
• Backdraft dampers on the bath fans to prevent air entry
• New ductwork for the ventilating dehumidifier to optimize airflow

We also performed blower door-guided air sealing to address air leakage that wasn't mitigated when the home was built.

As a result, the client was able to balance the negative pressure produced by the range hood with positive pressure from the new ventilation system and ductwork. Now the family can breathe healthy air. It doesn't matter how often they run the kitchen fan.

CO wasn't a factor in this case, but it can be.

Thankfully, this client didn't have an issue with CO entering the home.

But when a kitchen fan creates a negative pressure environment, CO backdrafting from gas appliances is a potential problem. That's why it's important to insist on a combustion safety test as part of any energy audit, blower door-guided air sealing work, or ventilation-related renovation you pursue.

The only way to know there's a CO issue is to test for it after making the home improvements.

Recommendations for kitchen ventilation

So, how can you ventilate your kitchen without creating a harmful air pressure imbalance? And if you've already got a powerful kitchen fan and a really tight home envelope, what should your next move be?

Here are our recommendations for different situations:

You're about to install a new range hood

Have a professional measure the amount of air currently entering and leaving your home under various conditions. That way, you can make an informed decision about how powerful your new kitchen fan should be. PV's whole home assessment includes this test.

You've got a fancy new range hood with a 400+ CFM fan

Depending on how tight or leaky your home is, that new fan could be generating excessive negative pressure. If you're in an older, pre-2000s home and haven't performed any major energy efficiency improvements, you might not have a problem. But if you're in a newer, tighter home, it's possible that you're bringing in dirty air that you'd rather not breathe.

Hire a professional to perform a combustion safety test to see whether any CO enters your home while the fan is running. Again, PV's whole home assessment includes this test.

You don't cook a lot, don't fry things very often, or just aren't worried about kitchen ventilation

Go for a sub-400 CFM range hood or use a recirculating fan instead of a ducted range hood.

Recirculating fans don't exhaust any air, but they do contain a filter to capture the greasy stuff inside your cooking fumes. They're better than nothing, but if you've got one, consider cracking a window while cooking.

Though rare, it's possible for these little range hoods to suck natural gas out of your stove's innards and blow it into your kitchen. It's a good idea to perform a combustion safety test, just to be sure.

In spite of these problems, range hoods are pretty great.

Believe it or not, we like range hoods. We've even got them in our own homes! What we don't like are negative pressure environments with polluted air.

You probably don't either, which is why it's a good idea to know what you're getting before you install a powerful kitchen fan.