[geenowalker]

Redoing the kitchen and am putting a 600+ cfm range hood in. Neither my contractor, nor the hood vendor mentioned the make-up air damper. If your are not familiar, the make up air damper allows air into the space when the range fan exhausts air outside causing a negative pressure situation. As a former HVAC engineer, I happened to be familiar with why this (and carbon monoxide sensors) are important.

Why is this bad? High CFM fans suck air out of the house causing a negative pressure. The air comes from somewhere, cracks in windows casing, doors, other rooms etc. It will come from somewhere. The tighter the house, the more the chance the air can be in the form of carbon monoxied (combustion materials) from the water heater and furnace.

Got a text from a friend over the weekend that he was awoken to a beeping sound that was traced to be a carbon monoxide detector going off. And subsequently, his 16 year old daughter sleeping in the basement with blue lips and unresponsive (SHE IS FIND AND HEALTHY NOW). This experience including a trip to the emergency room during coronavirus surely took 10 years off his life.

Was this due to a make up air damper?....no likely not, but the CO sensor definitely saves lives. A make up air damper and is something that is little expense when remodeling a kitchen. Code likely requires it in your area over 400 CFM. CO detectors are a must and need to be appropriately located. Leaving a high output fan on by accident and going to sleep can create a problem.

In a time when we are all trying to "be safe", the little things make a difference. I am NOT selling any products and have no vested interest in this other than sharing the ability to avoid a sad drive to another state.

Be safe...

 

[user_12345a]

And subsequently, his 16 year old daughter sleeping in the basement with blue lips and unresponsive (SHE IS FIND AND HEALTHY NOW). This experience including a trip to the emergency room during coronavirus surely took 10 years off his life.

Probably had a ul alarm with a high trip point.

 

[Old Thomas]

That is why my furnace and water heater are direct power vented and I have a CO detector near them.

 

[bfrabel]

Wow. Close call. I'm glad all are recovering and will be OK.

The thing user_12345a alluded to about it being an "UL alarm", is the fact that if the CO detector has a "UL Listed" label on it, it won't alarm until CO levels get well into the danger zone, like what happened here. I believe they made this requirement a long time ago when the detectors were first coming out in order to reduce the amount of false alarms that fire departments were getting.

If you want better protection, you need to search out a "low level" CO detector, which I believe aren't allowed to be sold in retail stores (because of the missing UL certification).

Either way, EVERYONE SHOULD BECOME FAMILIAR WITH THE WARNING SIGNS OF CO POISONING! Here's a list I just lifted from the Mayo Clinic....

Dull headache
Weakness
Dizziness
Nausea or vomiting
Shortness of breath
Confusion
Blurred vision
Loss of consciousness

If you have some of these symptoms, get yourself outside. If you start to feel better then chances are good that an improperly functioning gas burning device is trying to kill you!

By the way, very good point about the kitchen exhaust fan and the make up air duct that's sometimes needed. There should usually also be a combustion air pipe near any gas burning, natural draft water heater, boiler, and/or furnace.

Besides kitchen hoods, there are other things such as bathroom exhausts and clothes dryers that can bring your house into a negative pressure as well, and thus a potentially dangerous situation if you have a natural draft appliance, since the negative pressure can prevent the flue gas from going out the flue pipe like it's supposed to.

Be careful, and like I said LEARN THE SYMPTOMS OF CO POISONING!


edit.... I just looked up some more info about the UL Listing thing, and found this from Wikipedia's page on carbon monoxide detectors. The very last sentence is kind of an eye opener....

"The standards as of 2010 prohibit showing CO levels of less than 30 ppm on digital displays. The most recent standards also require the alarm to sound at higher levels of CO than with previous editions of the standard. The reasoning behind these changes is to reduce calls to fire stations, utilities and emergency response teams when the levels of CO are not life threatening. This change will also reduce the number of calls to these agencies due to detector inaccuracy or the presence of other gases. Consequently, new alarms will not sound at CO concentrations up to 70 ppm. Note that these concentrations are significantly in excess of the Canadian health guidelines, (and also in excess of US Occupational Safety and Health Administration (OSHA) Permissible exposure limits, which is 50 ppm).

 

[beenthere]

At 600 CFM, code requires a MUA unit, not just a damper.

 

[Drachenfire]

Good information there geenowalker, thank you.

 

[geenowalker]

At 600 CFM, code requires a MUA unit, not just a damper.

Arent they the same? I don't recall a fan providing positive pressure to the space, just the damper to open when the range fan is on. Is an MUA just an inline damper opening and closing based on static air pressure (or end switch to open when ever fan is on) or is a fan part of this?

 

[user_12345a]

Makeup air unit has a fan and pushes air into the space to stop it from going into negative pressure. They can and often do include heating and or cooling

 

[geenowalker]

Makeup air unit has a fan and pushes air into the space to stop it from going into negative pressure. They can and often do include heating and or cooling

Can you share a link. Thanks.

 

[user_12345a]

usually only for commercial/industrial - https://www.cambridgeair.com/make-up-air

most residential kitchen don't need a 600 cfm exhaust fan.

i would just open windows for a house and forget the code.

 

[beenthere]

Arent they the same? I don't recall a fan providing positive pressure to the space, just the damper to open when the range fan is on. Is an MUA just an inline damper opening and closing based on static air pressure (or end switch to open when ever fan is on) or is a fan part of this?

No, its a fan forced air providing unit.

 

[Bud9051]

Sorry, but I always hate to hear the "open a window" solution. yes, it needs to be a forced air intake and NOT just a damper. A 600 CFM fan can create a significant negative pressure and can be in addition to other fans being used, called a worst case. Many naturally drafted appliance, especially a hot water heater, cannot tolerate that negative pressure and will easily backdraft. Emphasis has lessened as more direct vented appliances are being introduced. But don't be fooled, CO is a killer and a proper make up air system is necessary.

Bud

 

[user_12345a]

whole house fans move more than 600 cfm and people know to open enough windows to stop the pressure from becoming too negative.

it's not an ideal solution but really - who's going to put a makeup air unit in a house?

 

[beenthere]

whole house fans move more than 600 cfm and people know to open enough windows to stop the pressure from becoming too negative.

it's not an ideal solution but really - who's going to put a makeup air unit in a house?

Someone that wants to be able to sell the house later, and not get ding by the inspector for any other work that was done without a permit, or inspection.


And people that actually like things done the right way, instead of a hack job.


In reality, most homes with a 600 CFM exhaust fan or larger. Dial or have them cut back to 400 CFM or less. After they find out how much it cost to heat or cool all the fresh air that needs to be brought in.

 

[user_12345a]

hoods/exhaust fans can be changed to comply before selling the house.

really, i don't see the need for a 600 cfm hood in a residential application. better to get have something more modest which is less likely to cause back-drafting and can be legally installed without a make-up air unit.

 

[u3b3rg33k]

I think that's the reason you don't see proper makeup air units, they're either really expensive to operate, or really expensive on capex. it's much more profitable to sell a big fancy range hood and drop a one-liner in about "proper makeup air", even if you don't make or sell one.

nothing like heating 600CFM from 0F to 70F in the dead of winter - that's 45k BTU if i have my figures right. not happening with an electric MAU in most homes. 15kBTU of cooling for 600cfm of 90F outside air to 72F temp WITHOUT dehumidification.

are people going to spend an extra $10k on a proper MAU to support their $1000 fancy range hood? unlikely.

 

[beenthere]

hoods/exhaust fans can be changed to comply before selling the house.

really, i don't see the need for a 600 cfm hood in a residential application. better to get have something more modest which is less likely to cause back-drafting and can be legally installed without a make-up air unit.

Little to no need in a residence.


Same as there is little to no need for anyone to own a luxury SUV.

 

[geenowalker]

Little to no need in a residence....

citing? Range hood sizing is based on the highest number of room size air changing, range size, or range btu output.

The average Samsung Home Depot gas range puts out 50K BTU per hour on the 4 corner burners. 50K / 100 (to get CFM) requires a 500 CFM hood.

 

[user_12345a]

How often do you really have all four burners on high?

What about the oven then? Guess you would need more cfm.

*btw i thought the typical gas stove burner is 5-7000 btu, not 10 000.

 

[geenowalker]

How often do you really have all four burners on high?

What about the oven then? Guess you would need more cfm.

*btw i thought the typical gas stove burner is 5-7000 btu, not 10 000.

I routinely have all 4 on, as well as the oven. The new ranges ($550 samsung at HD)

Burner No.1 BTU-17000
Burner No.2 BTU-15000
Burner No.3 BTU-9500
Burner No.4 BTU-9500
Burner No.5 BTU-5000

Just using the corners would only warrant a 320 CFM fan.

Not going with forced MUA just damper as my inspector said damper was sufficient. Upsizing the fan to run mid speed and reduce the noise. My current 330 CFM fan barely keeps up with the smells and is on most of the afternoon.