The age of the hood should not matter as long as it has been reasonably well maintained. A new hood will probably be more efficient but an older one should still work. (By efficient, you always have to supply a bit more exahsut than a theoretical calculation would predict. How much more is - in effect - a measure of efficiency.)
How effective your hood will be is a function of several things. First the air flow. If you have it tested to ASHRAE 110 by a qualified testing firm who knows how to do it right, then it should be satisfactory if used correctly. If you test it yourself or have an unskilled firm do the test, my experience says you will probably have much less certainty. Even if tested properly, improper use can invalidate the hood's effectiveness. Issues include working too close to the from, moving the sash too fast or too far, and doing abrupt operations or operations involving larger items being moved in and out of the hood.
I completely agree with another comment that the hood should be mechanically pinned at 15 inches and not rely on a marking. Personally, I think 15 inches is a totally unrealistic working height that he operators will almost certainly ignore with some frequency. My thoughts on this contentious subject can be found at https://www.linkedin.com/pulse/limiting-hood-openings-bad-idea-richard-palluzi/.
NFPA 45 Fire Protection for Laboratories Using Chemicals is an excellent standard on this subject. Contrary to what another post suggested It can - and should - be applied to all laboratories. Your local fire code may limit your quantities of flammable materials more than NFPA 45 and so be determining but the rest of NFPA 45 is still intended to be applicable. (section 220.127.116.11 was added by the committee to address continuing to use NFPA 45 even if local fire codes are determining and do not reference NFPA 45.)
Proper use of hoods is a complex subject. For more information you may want to consider Pilot Plant and Laboratory Safety 2: Real Hazards - Proven Solutions give by the University of Wisconsin which has a major section on the safe use of hoods and ventilated enclosures. More information on this course can be found at https://epd.wisc.edu/course/pilot-plant-and-laboratory-safety-2-real-hazards-proven-solutions/.
Pilot plant and laboratory consulting, safety, design,reviews, and training
Richard P Palluzi LLC
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Basking Ridge, NJ 07920
From: ACS Division of Chemical Health and Safety <DCHAS-L**At_Symbol_Here**PRINCETON.EDU> On Behalf Of Margaret Rakas
Sent: Friday, November 16, 2018 12:12 PM
Subject: [DCHAS-L] Operating a fume hood w/Sash Height 15 inches max...
I wanted to check whether there are any health and safety concerns or any 'lessons learned' that would preclude using a continuous volume fume hood with a maximum sash height of 15 inches (rather than 18) if it passed ASHRAE 110 testing and had an average face velocity between 80-120 fpm. We would train users regarding why the sash has to be at the 15 inches and there would be a sticker indicating maximum sash height. It is the sole fume hood in a lab, and would be used for mixing aqueous acidic solutions and dissolving rocks rather than complicated organic chemistry setups...I realize manufacturer's specifications are likely 18 inches (this is an old fume hood, probably from the 1980's) so looking up documentation on this is probably not going to be possible and might not be helpful, anyway.
A renovation several years ago turns out to have futzed the ventilation in this room; we're planning another renovation in the next year or so, but in the meantime it would be great to be able to use this fume hood if there are no regulations/guidance which would indicate it should not be used.
Margaret A. Rakas, Ph.D.
Manager, Inventory & Regulatory Affairs
Clark Science Center
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