Commercial hood and furnace

[MindOvrX]

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?

Thanks

[Stormin Mormon]

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?

[MindOvrX]

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.

[aka-SBM]

Ummm....you mean MOVING?

Some? Better provide 4000CFM when its on.

Bullshit.

If you dont know, then no. Sure dont.

The closest Chris has gotten to a make up air unit, is when he goes to Wendys for a burger.

[gofish]

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.

[~^Johnny^~]

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. :-)

[MindOvrX]

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?

Thanks