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  • How much CFM is required to suck out from a late model Combi Master PLUS?  THIS UNIT HOLDS TEN PLATES

    guest created 2 hours, 34 minutes ago 9 Members · 13 Posts
  • guest

    June 1, 2017 at 12:00 am


  • john

    June 1, 2017 at 4:24 pm

    The Food Service Technology Center tested a Rational CombiMaster Plus 61E (CMP61) using a 4-ft deep canopy hood with an exhaust rate of 300 cfm per linear foot. Check out the link here: 


    I’m not sure what the minimum would be; though if you’re adding that amount of ventilation, you may need makeup air to compensate.


    We have a great user, ectofix, who has a vast amount of experience with Rational. I’m hoping he has some useful insight.

  • badbozo2315

    June 1, 2017 at 6:02 pm



    Why do you have a fire (smoke?) alarm in a commercial kitchen at all?

  • ectofix

    June 1, 2017 at 7:05 pm

    Local codes, I’m sure.  We got ’em in our kitchens too.  One REALLY large kitchen in particular – five hood systems abreast – when we lost one ventilator (directly over the smoky griddle), became a constant nuisance regarding the smoke alarm.  Had to set the VFDs for the other four ventilators in bypass, so they’d all run 100% to compensate.

  • fixbear

    June 1, 2017 at 7:15 pm

    Yes codes demand fire alarms in the kitchen now,  but they are supposed to be temp alarms, not smoke.  And there are two different types of smoke alarms.  But just grease vapor and steam vapor is enough to set off either one of them.  They should not be installed near the line, but on the end walls or oppisite wall as the line.  Ceiling mounts will false trip much faster.

  • ectofix

    June 1, 2017 at 7:19 pm

    I’m no a hood expert.  I don’t install them.  I don’t spec them.  I don’t (and will never) work on them.  I only work on the equipment UNDER the hood.

    However, I can share some of the criteria Rational spells out in their installation manual regarding ventilation.  They do NOT mention anything regarding CFM, though:


    Ventilation, technical data, heat dissipation

    On-site ventilation:

    • When installing an externally vented exhaust hood, observe the following:
    • Comply with all local regulations and standards (NFPA 96; Gas combi or electric combi where applicable)
    • The exhaust hood should protrude 1-1.6 ft  [300-500 mm] over the front of the unit.
    • If using a VarioSmoker the unit must be installed underneath an externally vented exhaust hood (CO!).
    • Install a grease filter into the protruding part of the exhaust hood.


    • Re-circulation hoods (UltraVent) are available for most single table electric units. They can be retrofitted. Consult the hood installation guide for information on connecting the hood.


    There’s other data concerning latent and sensitive thermal load, which varies dramatically between oven model numbers.  Since we don’t have a model number, I can’t offer the specific data.


    If the OP can look at the specification label located on the left side at the lower-front corner, maybe our guest would be so kind as to provide that information.


    The only other thing I can suggest?  Every instructional video that Rational has published shows that, when initially opening the oven door, to only open it a few inches for about five seconds so that escaping steam is directed straight up under the hood’s canopy.  OTHERWISE – if they’re just flailing the door wide open from the git-go, then NO hood system would be sufficient enough to capture the wafting clouds of steam that they’re letting loose.

  • ectofix

    June 1, 2017 at 8:58 pm

    Here’s some FWIW and maybe some TMI for everyone regarding our present-day combi-ovens.  I was inspired to write this because…well – it so happens that the number of PLATES that the OP’s oven holds has absolutely no meaning to me with regards to the oven’s size or model number.  Anyways, here goes…


    Rational letters in their model numbers (for instance…the CMP) merely serves as a simple description of the oven’s FEATURES.  A CMP (Combi-Master Plus) doesn’t have anywhere near the same features that a SCC (Self-Cooking Center) does. That’s neither here-nor-there, though.


    The NUMBERS in Rational’s model numbers are what’s important regarding the oven’s SIZE.  Specifically, the numbers are based on the quantity and orientation of standard-size HOTEL pans the oven is designed to hold.

    • For instance, a CMP61 is a 6×1.  So, it will hold six levels of standard-sized hotel pans x one pan deep.  The rack support is wide enough to put the short (narrower) side of the pan in first.  As such, one pan fits for each level or slot.   It so happens there ARE more than six sets of rails in the rack supports (for half sheet pans and shallow hotel pans), but STANDARD hotel pans is what the size standard is based upon.
    • A CMP62 is a 6×2.  It also holds six levels of standard-sized hotel pans, but at two pans deep (Long [wide] side of pan goes in first.  The oven is wider and the rack supports are spaced so that each level will hold two pans side-by-side per slot (one to the rear and another in front).

    So, the 62 will hold double the amount of hotel pans as the 61.


    • Taller than a 61 or 62, another counter-top model is a CMP101 – a 10×1, which holds one hotel pan per each of its ten slots.  Its big brother – a CMP102 – is a 10×2, thus holding two pans per slot.  AGAIN, twice the number of pans.
    • A floor-model CMP201 is 20×1.  You guessed it – one pan per its twenty slots.  A CMP202 (the largest oven) holds 20×2, so two pans per slot.


    FWIW, this way of doing model numbers seems to be an “unofficial” standard amongst most of the other combi-oven manufacturers as well (Alto-Shaam, Covo-therm and others).  Still, there are still deviations outside of this norm – such as Rational’s newest and more compact XS oven.


    I hope this trivial bit of info helps y’all a little.

  • badbozo2315

    June 2, 2017 at 5:35 am

    > Yes codes demand fire alarms in the kitchen now,  but they are supposed to be temp alarms, not smoke.


    Thar ya go.  Also, we’ve run into smoke sensors in kitchen hvac units that go off now and then. Few and far between though…

  • bush

    June 8, 2017 at 9:09 am

    This may be a another example of a TechTown post where the guest drops in a grenade (question) and runs away.

    When only a fraction of the necessary information is furnished, and others are expected to weigh in making assumptions or to “pull teeth” to gain answers from the submitter, mis-diagnosis will always be the order of the day.

    That said, here are the questions:


    What model?

    Gas or Electric?

    Single oven or combi duo?

    Height and depth of canopy hood?

    What make-up air method is used?

    Where is combi located within hood?

    Heat detector or ion detector?

    What is ceiling height?

    What cooking mode causes the nuisance?


    Feel free to provide unneeded info such as, what you paid for the unit, how loud the alarms are, how it “really is an inconvenience”, or that you have an important upcoming event!

  • rico

    June 14, 2017 at 5:29 am

    It may depend on the style of hood. As someone else stated, you should only crack the door for a second or two. Some hoods have an air curtain that is supposed to help contain this sort of thing. Also the hood make-up air needs to be set correctly. It should be about 10% less of the hood air flow. I would also check to see what the codes say reguarding just how close the detector should be mounted to, what typee of equpt.

  • rationaltechnician

    June 25, 2019 at 3:35 pm

    Your best source is the Rational designer’s manual.  Rational does not state what it must be so this is a guideline only.  Your local authority will have last say.

    Rational bases it on a global criteria the combi-ovens fall under “mixed ovens”.


    I’ll assume when you say unit holds 10 plates you mean 10 trays.  If so, you have a 101 or 102.  Regardless, I cover all models in my answer for everyone’s benefit.

    Like many others here, I am not a hood expert, but I read their manuals and do my best to understand the theory behind the given information.


    So here goes.

    I converted what I found in the designers manual from ft3/h to CFM since that’s just the way it is here in America.

    The below apply to all Rational index H and I units (5th position of serial number) regardless if it is a SCCWE, CMP, gas or electric, 120V, 208V, 240V, 440V or 480V, it does not matter.  This has to do with heat output of the unit and how much exhaust air is needed to remove it. 


    Of course you still need to follow requirements for grease filters, fire suppression and the new crap they are starting to unload in some areas such as requirements to remove odours in certain installations.  Gas units must be vented outdoors anyway.  Electric units…well that depends on what you are cooking and if your local authority thinks your cooking is a fire risk (cooking of raw proteins and fats).


    Add CFM requirements of other equipment under same hood to below figures.  Then add an additional 25% to allow for unfavourable conditions.  If you come up with a crazy number, talk to your hood manufacturer or venting expert.  Could be that over calculations led to this.  Again this is just a guideline.

    Model 6-2/3

    Model 61

    Model 62

    Model 101

    Model 102

    Model 201

    Model 202

    Airflow requirement – unit freestanding in room (100%)

    115 CFM

    221 CFM

    450 CFM

    375 CFM

    740 CFM

    746 CFM

    1171 CFM

    Airflow requirement – one side of unit against a wall (63 %)

    73 CFM

    141 CFM

    284 CFM

    237 CFM

    467 CFM

    470 CFM

    832 CFM


    Lastly, some Rational electric models can use a Rational ultravent instead of a hood or at least be in a kitchen where there is a hood located somewhere.  Not designed for you to vent outdoors (void warranty I believe).

    Ultravent is a recirculating hood that filters out grease and steam but returns the air into same room.

    OK when steaming, baking and cooking precooked foods.

    Raw proteins and fats, your inspector may not allow and I don’t think all states allow ultravent.

    Not good for low ceilings and small rooms due to heat increase in the room.

    They recently introduced an ultravent plus which uses an additional filter to also filter out odours and smoke.  Haven’t seen any of those so I don’t have any customer feedback.  Again, I think some states will not allow.

  • MaiaLenart

    October 23, 2019 at 1:04 pm

    Hi…I have two of the Combi Plus CPC-61E. I am thinking about upgrading to the new SCC-61E.

    It is ashame that there isn’t more information about Rational ovens. There is quite a bit you can do with programming etc. The documentation only scratches the surface. The trouble is Rational does not seem to have a bulletin board on its site, and I don’t think there are enough people on eGullet that have them.

    • ectofix

      October 23, 2019 at 4:53 pm

      I can only be partial to the oven’s capabilities from the technical aspect, since I’m a technician who repairs them.  I also have a solid grasp on the features of BOTH – your old CPCs and the the new SCCs.  Do you have any specific questions about them?
      Otherwise, perhaps some videos demonstating the SCC’s use and care might be helpful:

      Notice that there’s a playlist for twenty-six videos.  The icon for the pull-down list is in the upper right-hand corner.

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