Prije nego što instalirate napu, pročitajte ovo

Ventilatori kuhinjskih napa mogu biti stvarno snažni. Ta je snaga odlična za izbacivanje dima, mirisa i pare iz vaše kuhinje tako da ništa od toga ne cirkulira vašim domom.

Ali kao što nas podsjeća ujak Ben, velika moć sa sobom nosi i veliku odgovornost (vidi dolje).

Ispostavilo se da moćne kuhinjske nape mogu uzrokovati probleme ako ih građevinari, izvođači radova i vlasnici kuća ne postavljaju odgovorno. Neki od ovih ventilatora pokreću zrak brzinom većom od 1000 kubičnih stopa u minuti (CFM). Kao rezultat toga, povećavaju negativni tlak zraka i prisiljavaju "nadopunjeni" zrak da uđe u dom kroz praznine i pukotine u ovojnici zgrade.

Drugim riječima, moćne kuhinjske nape mogu nenamjerno unijeti prljavi zrak u vaš dom, stvarajući nezdravo - i potencijalno opasno - okruženje.

Prvo najprije:Što je pozitivan, a što negativan tlak zraka?

Za svaku kubičnu stopu zraka koja napušta zgradu, još jedna kubična stopa ulazi u nju:

• Zrak koji ulazi u prostoriju je pozitivan tlak; praznine i pukotine u ovojnici zgrade unose pozitivan pritisak u vaš dom. Kao i otvoren prozor za vrućeg dana.
• Zrak koji izlazi iz sobe je negativan tlak; ventilatori za kadu i kuhinju mehanički su sustavi koji izvlače zrak iz vašeg doma, stvarajući ravnotežu negativnog tlaka dok rade.

U klimatiziranim prostorima želimo kontrolirati gdje zrak ulazi i izlazi. Zato nastojimo što inteligentnije postaviti kanale, dovodne registre i povratne rešetke. To je i razlog zašto u neke domove dodajemo sustave mehaničke ventilacije.

Zašto razlike u tlaku čine razliku

Općenito govoreći, pozitivan tlak je "dobra vrsta" tlaka u svijetu HVAC-a. Kada su domovi pod blagim pozitivnim tlakom, ustajali, "stariji" unutarnji zrak izbacuje se iz kuće vanjskim zrakom. Pod idealnim okolnostima, taj vanjski zrak je svježiji i čišći od zraka koji zamjenjuje.

Uravnoteženi tlak zraka također je dobar. Ovdje na jugu domovi visokih performansi često sadrže ERV-e za zamjenu starog zraka za svježi, čisti 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.

Postaje gore. 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.