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04-09-2011, 11:48 AM #1
Sizing a make up air system question
Hey guys, got a buddy with a small restaurant that has two exhaust hoods and no make up air.
One hood is over a big oven and the other is over a couple fryers. The building is fairly old and has no previsions for any make up air.
Ive only serviced and replaced exhaust and make up's and never sized them.
Im wondering if its just a rule of thumb to match the make up CFM with the exhaust CFM?
Also does anyone know about Florida code as far as material used to duct the system?
The basic plan was to install a couple Greeheck's (or some other brand) and just run flex to some sort of sheet metal diffuser directly above the hood.
04-15-2011, 07:48 AM #2
I would call greenheck for help. Make up air fans only bring in between 70 and 80 percent of whats being exhausted. The rest needs to come from outside air dampers on the a/c's. Where are you located at in Florida?http://acfwb.com/
"The best preparation for good work tomorrow is to do good work today"
04-15-2011, 07:19 PM #3
Older hoods had higher CFM requirements than do the newer ones for several code reasons too long to get into.
If your going to install new hoods than you are looking at big bucks. Don't scrimp on cheesy flex duct for make up air. Do not install it in front of the hood or where it will affect hood operation by blowing air towards the hood.
You will end up two type I grease hoods unless you can get your AHJ to accept a type II over the oven. The type I hood will require welded steel duct and a fire rated shaft or a special fiberglass wrap insulation and a few other specialty items, fire suppression system, emergency gas shut off etc.
Make up air can be sized for anything you want. All the code say's is that it needs to provide basically the same amount as you are taking out... but cant be equal or more. If you have a small rest. then I would do 85-90%. If you under-size it too much you will create a negative building which would create all sort of other problems. The only time I go below 90% is if I have a large rest., or if I can use make up from surrounding areas...think rest. in a hotel.
If he can afford it get the hoods that have VFD's on the fans that ramp up and down based upon heat and amounts of smoke passing through...those are sweet.
smurphy is right...get ahold of Greenheck or Captive Air, they will be more than happy to help you out but you better know what the inspector will say about that hood above the oven....call your building department first. About the only time I have seen a type II hood over an oven is when it was a conveyor belt type pizza oven.
04-15-2011, 07:26 PM #4
Oh I forgot....you will need to know the size of the existing exhaust ducts if you plan on re-using them. I don't have a code book in front of me now but if I recall the air velocity needs to be between 1800 and 2500 FPM. I typically try and go 2000.
04-16-2011, 09:23 AM #5
Thanks for the info guys.
This restaurant is struggling with revenue as are alot of others. He most def will not buy a VFD sys lol....
Well you hit the nail on the head. I was checking over the hvac systems and found that 90% of his problem was the building is in a negative pressure.
As soon as the front doors open hot humid air is QUICKLY pulled into the bar and dining area.
So, in short, im just trying to to help him get through the hot summer coming up. No one wants to sit down and eat in a restaurant thats as nasty inside as it is outside! lol
I just cant see how this place was built years ago, with 3 exhaust units and no make up air???
04-16-2011, 01:55 PM #6
Get a TAB contractor to put a flow hood on the systems so you know how much CFM your dealing with...unless you know already. A simple mau without heat or cool is not much.
04-16-2011, 04:27 PM #7
be very careful here, if you touch this mess they may very well require you to bring it ALL up to code. Don't go giving the customer prices for MAU before you make sure that it will pass.
And you will not be able to measure the air flow with a flow hood, you will need to do transverse readings.I r the king of the world!...or at least I get to stand on the roof and look down on the rest of yall
04-16-2011, 04:45 PM #8
What are you going to traverse, a welded steel duct? That would be a code violation to punch holes in it. Depending up the hood configuration you can use a flow hood. If they can't use a flow hood the under the hood at the duct connection, or make an adapter to the fan outlet, they will use an anemometer and apply the appropriate aK factor.
04-16-2011, 05:12 PM #9
Well guys he knows im a hvac/r tech and i dont get into designing exhaust and make up systems. I was just the first one to get him in the right direction with his store cooling problems.
He had others doing work for him, basically ripping him off not finding the obvious issue.
This is going to get getto.
We talked about it and "he" is going to run a few 18" flex lines to some registers in front of the grills and then to the outside eves of the building. They will basically be static make up air lines to draw the majority of the exhaust air through them instead of the front door.
I know this isnt code, and he knows as well also.
But, he had one of the 3 exhausts not running and the duct was cut off. Just for kicks, we popped the ceiling tile in the middle of the kitchen where the 3rd hood used to be and man, the air was shooting down into the kitchen from the unused exhaust.
We shut off the running exhausts and no more air was being pulled from the unused exhaust. Turned them back on and tons of air again being pulled from the ceiling.
So, we got the idea of running some static ducts above the hoods. I told him its not going to be 100% effective but it should be a hell of alot better than nothing at all.
He may need to eventually have to spent the money on a make up system, but for now he is going to try it due to financial issues.
Thanks again guys for all the info.
04-16-2011, 05:30 PM #10
Wow...that's real sanitary. Just draw in all those flies, dust, and everything else into a kitchen.....and dump it where you cooking. I would let him know that the next time the health inspector is checking him that if he get's caught he runs the chance of fines and getting shut down. For crying out load....if your going to do something on the sly at least....never mind...don't want to give the guy any ideas.
04-16-2011, 05:48 PM #11Professional Member
- Join Date
- Feb 2004
- SW Florida
Alot of, if not most jurisdictions, require any modifications to a kitchen exhaust/MUA system to have an engineer designed, stamped plan to issue a permit for the obvious reasons. This is one area in our trade that if something goes wrong could have serious results, not to mention flies in the soup
04-16-2011, 06:15 PM #12
Guys, the inlets will be filtered! Will be no worse that the usual dirty corroded metal filters normally found in the make up's.
04-17-2011, 05:39 AM #13Regular Guest
- Join Date
- Apr 2010
- southern california
Your buddy should stop and think before doing anything. MUA and exhaust are critical for a commercial kitchen to work properly. Hiring a mechanical engineer would be best. We are a design build contractor and have done plenty . Is his hood up to UL300 rating. This is required of all exhaust hoods serving kitchens. His insurance carrier can refuse to insure his business if not done correctly. MUA can reduce considerably the cost to cool the dining area if engineered correctly. The risk of a fire and smoke damage is real.