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Thread: Hobart ov 500

  1. #1
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    Hobart ov 500

    So I am an a refer ac tech but I have a question does the Hobart ov500 control the makeup air it looked like it through the schematic but the Hobart tech today told me it was a sparkey issue point is everywhere I check I have 115 going up but not all the way to the mua and the hood is sucking out alot of the stores conditioned air

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  2. #2
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    In general, I've never seen a rack oven control any make-up air unit. All it had control of was the flue draft inducer, and on occation the overhanging hood fan if it was forced exhuast.

    I looked over an earlier schematic, and don't see any provisions specifically for mua, although if you were to put a contactor coil on the flue's draft inducer motor's supply, you could turn on a mua unit when the burner was running, or put a contactor's coil on the oven's main fan contactor to control mua that way, so it runs when the oven is powered up.

    You don't need mua for just the burner though, it's not continious or using a lot of air.

  3. #3
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    Learned something new the other day about venting.

    One of our three gas-heated rack ovens was down for a month or so to repair the rack lift system (Sveba Dahlen ovens). Our 2nd shift mechanical wiz-kid (I'll call him Mech) did all that. I'd merely done a small part by removing the torpedo burner and giving it a cleaning/refresh of new parts somewhere within that timeframe. I intentionally didn't alter any of the burner damper settings during that.

    Those Swedish-made ovens have about 500 lbs of rebar in them as thermal mass, so they never actually entirely cool down from daily use. So, hot air is there when they're turned back on each morning. These ovens are naturally vented, so there's no draft inducer. Just a sizable draft hood to draw in dilution air when the burners fire up. The ovens were installed that way back in the '90s.

    Mech finished his part, looked everything over, called it good and set everything into motion for a test run. They fired up the oven, with Mech standing on top the oven and his helper downstairs at the control panel. With the oven burner running, they both realized that it WASN'T venting properly and they both were awash in hot vent gases.

    They shut the oven down and called our engineering MOD (manager-on-duty). He's a prior boiler tech. After recognizing what they'd described, he fashioned a way to temporarily cap off the flue vent directly at the oven, then had them fire it back up again. When his make-shift cover over the flue vent got hot enough, he pulled the cover off. The rush of rising HOT vent gases had no choice but to go straight up the vent through the roof. That got it primed into properly venting.

    That's what I got from their story. I've crudely tested that oven (and the other two ovens) with a lighter since then and all appear to be drafting as they should.

    Meanwhile, that MOD has gotten other folks involved to determine whether that building is lacking in proper fresh airflow sources (make-up air [MUA] I guess). There may indeed be other issues.

    Their story was quite a learning experience for me.


    "You never know what others don't know." -

    If I can't laugh at myself...then I'll laugh at YOU! -

  4. #4
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    Well not to sound like a total idiot but I am in Florida and like I said a refrigeration/ac tech and I dont have the most experience with gas or hot side period but this oven had a hood over it that was just pulling loads of nice cold air out of it with the doors shut and no heat really coming out if it so like most commercial hood exhaust set up I would assume that it also controlled the make up air vents that are right in front of the hood. I just feel like something is wrong if its pulling conditioned air out without bringing air in causing a negative pressure to building. Also I might add the hood exhaust is a captive aire exhaust fan activated by the oven

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  5. #5
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    Quote Originally Posted by acbean View Post
    Well not to sound like a total idiot but I am in Florida and like I said a refrigeration/ac tech and I dont have the most experience with gas or hot side period but this oven had a hood over it that was just pulling loads of nice cold air out of it with the doors shut and no heat really coming out if it so like most commercial hood exhaust set up I would assume that it also controlled the make up air vents that are right in front of the hood. I just feel like something is wrong if its pulling conditioned air out without bringing air in causing a negative pressure to building. Also I might add the hood exhaust is a captive aire exhaust fan activated by the oven

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    I would have to agree with the Hobart tech. This is a sparky (EC) issue. The exhaust fan should be interconnected with the MAU. CaptiveAire sells electrical control packages or the EC could have built one themselves. It's possible that there is an ECP and the contactor coil is broken for the MAU. There may be safety switches preventing the MAU from operating also. Good luck.
    "Quality is never an accident; it is always the result of intelligent effort. The bitterness of poor quality lingers long after the sweetness of low price is forgotten" --- John Ruskin

  6. #6
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    Aha, I think we are talking about 2 different "hood"s here.

    A typical OVEN hood is directly attached to the oven, extending a short distance over the oven door. It has no previsions for makeup air. It is used only for hot air containment when opening the oven door.

    A typical RESTAURANT hood is mounted near the ceiling, and has, typically, moveable equipment underneath it, and has a row of fresh or treated make up air along the outside edge, or sometimes at the rear wall about 24" from ground level.

    If you have the latter, then, yes there's a mua failure, which has nothing to do with the oven, per se.

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