I am trying to help someone find a good contractor to put in a hood system
for a small cafe, and have a couple of questions to clear up first.
The furnace was just replaced, and someone has mentioned that the furnace
needs exhaust air to be relieved. The contractor said that this will be
taken care of by the hood system, but will need to be ganged with the
furnace to work properly.
Is this true? How does that work?
The contractor said to make sure the hood installer wires up the system to
work with the furnace, the make up air for the hood and the exhaust system.
What exactly are we looking at?
When you put an exhaust hood in, it pumps air out of the building.
That air has to come from some where. Often from cracks around the
door, or windows.
If there isn't enough "make up air", or air coming in, the building
air pressure is lower than the outdoors. Then, air can come down the
furnace or water heater chimney. Drawing the fumes and monoxide into
the building instead of up the chimney.
The question which troubles me. Is there enough provision to bring
fresh air in from the outside?
Christopher A. Young
You can\'t shout down a troll.
Ok, here's what I found. The furnace is supposed to be getting around 1500
cfm fresh air with up to 1800 cfm when the a/c is in free cooling mode (?).
They also said the hood exhausts something like 4000 cfm but will have a
fresh air fan that provides some outside air too.
The contractor is saying that it's better to just make sure that the outside
air for the hood is the difference between 1500 cfm and 4000 cfm, and don't
worry about wiring the systems together. This doesn't make sense to me,
because I think that the systems will run at different times most of the
time. I think they have to put in shut-off dampers when the systems aren't
running, don't they?
Thanks for helping with this.
More important than what the contractor has to say, and more important
than any internet advice is the actual building code requirements for
your local juristiction.
In this neck of the woods, if you are exhausting 4000 cfm, you will
also be providing 4000 cfm of fresh air, and the make up air fan will
be interlocked with the exhaust fan.
You should educate yourself on the differences between fresh air,
return air, and combustion air requirements for any fossil fuel
burning appliance, and the specific rerquirements for each as per the
building codes for your area.
If you're in an area with no building codes, smart money would hire a
mechanical engineer to engineer the specifics of your situation.
Tha makeup air intake can be nautrally aspirating.
Usually teh ducts are mounted near the hood itself,
to prevent unwanted drafts in other areas, like the dining room.
You want to exhaust the immediate cooking area, without exhausting
any other part of the building, as far as the hood fan goes. The
makeup air should take as short a path as possible.
If the intake registers are closer to the stove than the furnace,
than what's the problem? Why the need for forced air intake?
Another problem with poor design or installation of hood venting is
teh development of subsonics. No, I'm not kidding. Though they are
inaudible, they can be harmful. Protracrted exposure to subsonics can
avdersely affect people's nervous systems. This is not good for
business. Nervous customers eat less, and tip less. :-)
Thank you for the tip. I will use this when talking to the contractor. But
let me get this straight ... if the ducting is setup properly, a makeup fan
shouldn't be required, right? Like the path of least resistance concept? If
the ducting is large enough, unobstructed and has few elbows, then there
shouldn't be a problem?
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