HVAC Ducting (Black Steel) for Kitchen Hood

I think our friend is new to the business.

Your thorough answer is appreciated. It is indeed a commercial hood, but nonetheless the approved shop drawings call for it to be fabricated from black steel (most likely due to cost considerations). The specs also call for the gaskets between its mating flanges to be rated for 1500 degrees F. But the specs are silent about the exact details about the paint and this is why I reached out to experts in the field with more experience than myself (as correctly sensed by another respected commentor). The duct has been painted red both inside and out and it obviously is a blunder. However, to pin down the matter in a professional manner the details and procedures were requested in my question.

Anticipating further elucidation about the requiremments.

I believe the specs are calling for blackened steel ductwork, which does not involve paint. Here's a link to give you an idea of what it is.

http://www.ehow.com/how_5810102_blacken-steel-prevent-rust.html

Another:

http://www.epi.com/pages/black-oxide-library

Some more research material for you:

http://www.acitydiscount.com/Restaurant-Grease-Heat-Hood-Sizing-Guide.31.1.htm

Our OP tells us the hood and duct are painted red inside and out; red oxide primer, done by the fabricator is my guess.

I cannot think of a paint coating that I would feel good about. As the coating fails (and it will), chunks of it will fall from the hood onto the cooking area. Yum!

Oops..........I didn't read far enough to see that.

If it's red, and it's a stove hood, it wouldn't be red oxide primer............it would probably be a baked on powder coat.

The OP has two choices, get rid of it, or convince the owner that red is good. There is no paint that can be applied to a baked on enamel that's going to hold up to the heat.

He needs to stay away from paint!!!!! I think it was supposed to be a blackened steel finish and they got red...........big difference.

Painting it will destroy any resale value/ chance of returning it.

If it truly is, "paint", on the inside of the hood, or ducting, my guess is that it's now unusable garbage. While I'm no expert on commercial range hoods, I do know that I've never, ever seen one that is painted on the inside.

Other than that, I'm out of ideas. I wouldn't throw good money after bad though, by either installing it, or painting it, without knowing for sure that it is as specified and will meet any applicable codes.

The links provided by Karamarat are deeply appreciated. Likewise are the inputs from Doorman and Lyn. The novices somehow felt it was necessary to paint the inside out of the black steel duct with Dulux (ICI) Gloss Enamel that is good only for upto 90 degrees celcius. Now the smart team will have to remove it all and sand blast the duct and do it all over again for some sort of rust prohibition. Given the fact that the grease will have to be scraped from the interior every now and then, any attempt to paint or even hot dip galvanize the duct would at best be of a very transient life!

As smart as we think we are, things go really wrong, all the time.

When they do, we get a little smarter.

The ones that learn from those mistakes, will ultimately succeed.

In the end, the job will be done properly. No matter how many times it has to be done.

My goal is to do each job only once. I'm getting there. The worst anyone can say is that I'm slow. Especially my wife.................I'm eight years into a fixer upper that I swore would only take one.

Beer helps me through the hard times.

Most municipalities and other governmental entities follow the lead of the NFPA.

The answer to this question and most others you will encounter will be found somwhere in these specifications. You should get a copy.

List of NFPA codes & standards

Then, you won't have to wait for us to give you the answer.

On this site we are reaching out to the experts with a working knowledge of the codes. Mere reference to the NFPA codes that is about 13,000 pages is not what we are looking for in this forum.

Yes indeed in the end the answers are based on the NFPA, but the book does not speak. This forum does!

Whether domestic or commercial there are ducting kits on the market to suit all sizes, Most of which are made of plastic and do not need painting, these kits go up to 8" (200mm) Dia. over this go to galvanised which again is available in kit form,& either flat,square or round section in both materials.

I've never seen a commercial stove (not bathroom moisture removal) duct kit made from anything but SS, Steel or copper. All are usually unpainted. A commercial unit will have grease on the interior surfaces, after about 6 minutes of use. Isn't the function of the hood to remove noxious gas and odor, and if it catches on fire, to contain the fire momentarily so you can get the hell out, and maybe put out the fire?

After I saw the comment in #19 about plastic, I went and looked at the vent of my (home) range. It is plastic (with wire "reinforcement to keep it from collapsing). I wonder if:

• that met code 32 years ago when it was installed (USA)?
• if it still meets code?
  

Yes and NO. Residential kitchen exhaust systems are only designed to remove cooking ordors, steam, etc; and are considered very low use. Reference below.

SECTION 505

DOMESTIC KITCHEN EXHAUST EQUIPMENT

505.1 Domestic systems.

Where domestic range hoods and

domestic appliances equipped with downdraft exhaust are

located within dwelling units, such hoods and appliances shall

discharge to the outdoors through sheet metal ducts constructed

of galvanized steel, stainless steel, aluminum or copper.

Such ducts shall have smooth inner walls and shall be air

tight and equipped with a backdraft damper.

Thanks!

(I'll change the plastic to metal--on further examination, the plastic duct is sort of a transition--the horizontal piece going to the outside is metal (haven't yet tried to reach in and see if it is aluminum or galvanized).)

I don't want to look at the inside of mine. I'm scared. It's shiny and clean SS on the outside. I'm going to believe it's lovely, fireproof and shiny inside. I installed it 5 years ago.

Look up www.i-sells.co.uk/ducting they sell tons of the stuff, or for Europeans tonns of the stuff.

Although I see pictures of plastic ducting here, with the name of the page being: Dryer and Cooker Hood Ducting Kits, I don't immediately see anything that states the plastic ducting is suitable for use as hood ducting in commercial kitchens.

I'd be very attentive to codes here.

In 25+ years of installing commercial kitchen exhaust systems, I have never seen a plastic enclosure. Code requirements state 18" minimum clearances to combustables on a commercial type-I duct system. We have used 2 layers of 5/8" fire rated drywall to make a shaft enclosure per building code and attached stainless steel sheeting (28Ga) for appearance in areas that the exhaust duct could be seen where the specs required black iron and not stainless steel. This shaft enclosure only needs 6" clearance.

I thought you were pulling my leg about plastic duct and teasing about the use of paint on the inside of duct. The overwhelming response that followed gave credence to the existance of plastic ducts. Wow! Even the gaskets that are called for must withstand 1500 F, so I wonder if it is some sort of superplastic or what. We had a heck of a time to make gas turbine components with inconel alloy to withstand those sort of temperatures, so I guess the superplastic used in ducts could be used in gas turbines also!

I wonder what lyn's NFPA library says about plastic ducts for kitchen hoods!

Let me look that up for you.

The paint will simple fall off of the inside because of the grease, so no need to worry about that. On most commercial installations, none of the grease duct cam be seen, and even on 20 year old installations, I've never seen enough rust on the exterior to worry about. If the ceiling is so high that part of the duct is visible, I usually wrap it in Stainless for appearance sake only, if the customer wants. -- JHF

If the duct is black steel interior is sand blasted and painted with 1500 F rated painted, I dont understand how it will simply fall off, unless it is scraped off by periodic cleaning?

Definitely not, the paint will act as an insulator. Since periodic inspection is strongly recommended and often required by code, paint will tend to conceal leaks. Finally, most codes will not allow painting of hood vent ductwork.