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Direct fired MUA

Direct fired MUA

Direct fired MUA

Our customer has purchased four direct fired MUAs for a bakery building, constant speed each 50,000 CFM. There are many exhaust fans in the building mostly integrated with process equipment. B149.1 requires these MUAs to be interlocked with exhaust fans. Each MUA cannot be interlocked with more than one fan. In BC, MUA can be interlock with a fan that represents the 1/20 of total exhaust rate, is there a similar regulation in Ontario?
How can I operate these units and meet code requirements.

RE: Direct fired MUA

You should consult the local Ontario code and talk to JHA.

why does a customer just buy 4 MUA before talking to an an engeineer and thinking about code?

RE: Direct fired MUA

Client just saved a bagful of money (he thinks) buying cheap direct fired MUA's. Now the reality of Code Compliance will require spending probably triple what the Client saved, on interlocks and new controls, plus your time in proving to the AHJ that the localized direct fired air quantity is being fully exhausted by the locally interlocked exhaust fans. For a few bucks more, the Client could have bought nice indirect fired MUA's and saved a lot more than he will spend making the direct fired units compliant. In answer to your post- the only Code Compliant way I can see (knowing Canadian Building Codes) is to spend a lot of time and adding direct interlocks to the local exhaust fans in the zones where the MUA is serving and doing the calculations and spreadsheet showing that the exhaust rates vs the MUA airflow meets Code for the four MUA zones.

RE: Direct fired MUA

Thank you for responses,
I'm reviewing two options:
1- Installing static pressure controller for zone served by each MUA and control air pressure at almost neutral level
2- Adding airflow switches on each exhaust fan and get the signals from these switches and control MUAs.

Code doesn't clearly mention that an electric interlock is required and these are kind of mechanical interlocks.
Please let me know your thoughts,

RE: Direct fired MUA

I can't speak to Canadian Code requirements.

But, 50,000 CFM at constant speed? That's a lot of 'all or nothing' air.
1. Trying to control with zone pressure controller is going to be problematic, I would expect the pressure to be all over the place as the controller cycles the MUAs on and off.
2. Not sure I see how this would be a better option; surely simple interlocks are cheaper/easier to install.

Where is the 'requirement' that "Each MUA cannot be interlocked with more than one fan." coming from? I've seen examples of where exhaust fans out number supply fans and vice versa.

I'm guessing your bakery has a lot of small exhaust fans for hoods and direct vented appliances, and there is a large diversity. If this is the case, I think your best solution would be to go with your option 1 PLUS add VFDs on the MUA units. This assumes the burners in the MUA units can be turned down.

RE: Direct fired MUA

MUAs are constant speed and no turn down capability;
Code asks to interlock EFs to MUAs, although we need an intermediate controller to make sure all fans are on to turn on MUA which doesn't provide us the best operating conditions. As you're saying there are many EFs, for instance in each oven and proofer there are between six to ten fans.
Since I have one big room with four MUAs, the system can already be controlled in 25%, 50%, 75% and 100% of full capacity. Maybe I should stick to SP measurements and find proper spots for placing these sensors. The most effective way both in operation and cost.

Again, what's your understanding from this clause in B149.1, do you think I'll be fine?
Except as specified in Clauses 7.20.4, 7.20.12, and 7.20.13, a DFMAH shall
(a) have an air discharge capacity that does not exceed the total discharge capacity of the exhaust
system by more than 10%; and
(b) be interlocked in such a way as to prevent its operation unless the exhaust system is functioning.

Thank you all and let me know if you have any other thoughts.

RE: Direct fired MUA

I personally don't like direct fired MUA equipment. For whatever reason someone from maintenance modifies the installation not fully knowing how the system should work, you can have a big problem.

RE: Direct fired MUA

Each MUA cannot be interlocked with more than one fan. In BC, MUA can be interlock with a fan that represents the 1/20 of total exhaust rate, I could not understand it clearly, can you give an example, Table B149.1 is not mentioned in Ontario Building Code I belief but need double check.

RE: Direct fired MUA

B149 is not in OBC, however we should meet its requirements for TSSA.

To provide interlock between make up air and exhaust system, it should be all done thru control system. When even one fan out of a group of six or eight is not running, one MUA shuts down. Basically, I'll tie each MUA with a series of exhaust fans. This is not the way I'd like to do it, but apparently the only way.

Even though, I'm trying to avoid this and get a more accurate control by static pressure measurements.

Still considering various possible options!

Ideas are welcome,


RE: Direct fired MUA

"This is not the way I'd like to do it", what would you like to do? you want to use static pressure, but it looks like you want to run MAU with capacity more then 10% of the exhaust capacity, otherwise there is no reason to try to avoid table B149 requirement.

RE: Direct fired MUA

Static pressure controller = gravity operated backdraft damper relieving air outside?

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