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Old 02-08-2020, 04:40 PM   #1
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Makeup air required for kitchen hood if furnace and water heater are direct vent?


I’m putting in a 660 CFM vent hood in my kitchen. There will be some bends in the output pipe, which I’ve read slow down the CFM of the air, so the output will likely be less than the 660 CFM, but I’m not sure by how much.

I’ve read that they recommend a makeup air hole for vent hoods above 400 CFM for the risks of negative pressure pulling carbon monoxide into the house for furnaces, etc.

My house has no chimney and has a both a direct vent furnace (forced hot air) and a direct vent water heater. Does the makeup air need apply to my house?

I’ll add that my house is not overly sealed/insulated and I consider it to be slightly drafty.
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Old 02-08-2020, 05:26 PM   #2
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Re: Makeup air required for kitchen hood if furnace and water heater are direct vent?


The air has to come from somewhere, so you will be making all those drafty areas worse as apposed to having a supply that you decide where the best place would be to bring air in.
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Old 02-08-2020, 06:24 PM   #3
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Re: Makeup air required for kitchen hood if furnace and water heater are direct vent?


I will assume you have a gas range and oven.
You won't be drawing any exhaust gases in from your furnace or water heater due to the way they are vented. Their air supply is taken care of.
But hours of cooking (with a gas appliance) will deplete oxygen and an exhaust fan will add to that. Some have installed vents into the cold air return that bring in outside air when a negative pressure starts to occur.
That being said, if you house is drafty as you suggest you will most likely be OK.
And you can always crack a window.
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Old 02-08-2020, 06:51 PM   #4
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Re: Makeup air required for kitchen hood if furnace and water heater are direct vent?


I don't think an exhaust fan contributes to depleting the o2. MB if you have a reference please post. The stove top or range may do so, but replacement air will come from somewhere.

It may just be terminology, but direct vent is not the same as sealed combustion. I think of direct vent as being a fan assisted exhaust and not certain that fan can deal with the monster you will be installing. Basically they would be working against each other and the stronger will win.

If they are indeed sealed combustion, then each would have its own intake air source, assuming the installer ran that to the outside. I've seen 2 where they did not.

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Old 02-08-2020, 07:03 PM   #5
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I don't think an exhaust fan contributes to depleting the o2. MB if you have a reference please post. The stove top or range may do so, but replacement air will come from somewhere.

It may just be terminology, but direct vent is not the same as sealed combustion. I think of direct vent as being a fan assisted exhaust and not certain that fan can deal with the monster you will be installing. Basically they would be working against each other and the stronger will win.

If they are indeed sealed combustion, then each would have its own intake air source, assuming the installer ran that to the outside. I've seen 2 where they did not.

Bud
Both the furnace and the water heater have dedicated exhaust pipes run to the outside. But the air intakes are just pulling from the basement room air.
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Old 02-08-2020, 07:07 PM   #6
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Re: Makeup air required for kitchen hood if furnace and water heater are direct vent?


I think you are correct about the direct vent aspect. The OP didn't mention sealed combustion but I took it as such. My mistake.
And my comment regarding exhaust fan.....I re-read my post and I must have been having a brain fart.
All in all....my last 2 sentences, given the information we tend to agree on, seem to be the most accurate.
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Old 02-08-2020, 07:40 PM   #7
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Re: Makeup air required for kitchen hood if furnace and water heater are direct vent?


Check both appliances and maybe talk to the mfg to see if they can be adapted to have an outside supply of combustion air. My oil fired furnace has a kit option, not familiar with gas furnaces or water heaters.

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Old 02-08-2020, 08:10 PM   #8
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Re: Makeup air required for kitchen hood if furnace and water heater are direct vent?


Quote:
Originally Posted by Missouri Bound View Post
I will assume you have a gas range and oven.
You won't be drawing any exhaust gases in from your furnace or water heater due to the way they are vented. Their air supply is taken care of.
But hours of cooking (with a gas appliance) will deplete oxygen and an exhaust fan will add to that. Some have installed vents into the cold air return that bring in outside air when a negative pressure starts to occur.
That being said, if you house is drafty as you suggest you will most likely be OK.
And you can always crack a window.
not really - that's the point. it's pretty easy to overcome the draft up a chimney in a tight house with a strong kitchen fan.
Quote:
Originally Posted by ed3120 View Post
Both the furnace and the water heater have dedicated exhaust pipes run to the outside. But the air intakes are just pulling from the basement room air.
and the basement room air is coming from... ?
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Old 02-08-2020, 08:23 PM   #9
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Re: Makeup air required for kitchen hood if furnace and water heater are direct vent?


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Originally Posted by u3b3rg33k View Post
not really - that's the point. it's pretty easy to overcome the draft up a chimney in a tight house with a strong kitchen fan.
He doesn't have a chimney.
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Old 02-08-2020, 11:12 PM   #10
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Re: Makeup air required for kitchen hood if furnace and water heater are direct vent?


You need makeup air for the for the furnace and water heater that are not using outside air for combustion. That is, unless you have a leaky house. Codes spell out how many square inches of inlet for each unit of BTUs.
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Old 02-09-2020, 12:36 AM   #11
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Re: Makeup air required for kitchen hood if furnace and water heater are direct vent?


Quote:
Originally Posted by Missouri Bound View Post
He doesn't have a chimney.
missed that. IF the installer didn't cheap out and you have supply air piped properly it might be OK.
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Old 02-10-2020, 08:26 PM   #12
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Quote:
Originally Posted by u3b3rg33k View Post
Quote:
Originally Posted by Missouri Bound View Post
I will assume you have a gas range and oven.
You won't be drawing any exhaust gases in from your furnace or water heater due to the way they are vented. Their air supply is taken care of.
But hours of cooking (with a gas appliance) will deplete oxygen and an exhaust fan will add to that. Some have installed vents into the cold air return that bring in outside air when a negative pressure starts to occur.
That being said, if you house is drafty as you suggest you will most likely be OK.
And you can always crack a window.
not really - that's the point. it's pretty easy to overcome the draft up a chimney in a tight house with a strong kitchen fan.
Quote:
Originally Posted by ed3120 View Post
Both the furnace and the water heater have dedicated exhaust pipes run to the outside. But the air intakes are just pulling from the basement room air.
and the basement room air is coming from... ?
Basement has a door separating it from the upstairs, but the door is usually open. I have a return vent on the main floor as well as one in the basement. Both returns run into each other and feed into the furnace/AC.

So if the basement is short on air it will pull from the upstairs either through the door or through the upstairs return sucking air in.
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Old 02-10-2020, 11:12 PM   #13
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Re: Makeup air required for kitchen hood if furnace and water heater are direct vent?


Your makeup air is coming from upstairs. In a well sealed house, you can only suck a limited amount of air from a box. In reality your makeup air is coming from putting your living space under negative pressure forcing outside air in through every leaky spot. It would be better to put in a vent to allow your appliances to have makeup air directly from outdoors.
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Old 02-11-2020, 03:50 AM   #14
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Re: Makeup air required for kitchen hood if furnace and water heater are direct vent?


Quote:
Originally Posted by ed3120 View Post
Both the furnace and the water heater have dedicated exhaust pipes run to the outside. But the air intakes are just pulling from the basement room air.
They are not actually direct vent then.

It is a code requirement for make up air for hoods 600 CFM and above. Even in an all electric house.
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Old 02-11-2020, 12:25 PM   #15
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Re: Makeup air required for kitchen hood if furnace and water heater are direct vent?


Direct vent means the furnace and water heater have a exhaust fan which pushes the fumes out to the outside thru a pipe.

They have a pressure proving switch in the burner area which is supposed to prove there is enough air for safe combustion.

However that is tested under Normal house conditions which I imagine is with a range hood under 600 cfm.

Point is direct vent is a term BUT for safe clean proper combustion you need lots of combustion air and relying on a pressure switch for that is living very dangerously.

If you have sealed combustion then you have a intake and exhaust pipe ( 2 pipes ) going outside. Then your appliances won't be affected by a exhaust fan.

If you don't have a makeup air pipe then dirty outside air will be sucked in under your doors/around windows and can even be sucked thru electrical outlets on your wall and leave black stains.
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