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Thread: Make Up Air with Over Sized FCU

  1. #1
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    Make Up Air with Over Sized FCU

    I have a situation where we need to supply 600 CFM of Make-up air. We will be using one of FCU's for this, but the lowest speed on the unit will supply around 1150 CFM. My thought for how to resolve this issue so I'm only pulling in 600 CFM of OSA is to open up the return damper X % so that the FCU is pulling in 600 CFM of OSA and 550 CFM of RA. My real question comes down to what % the return damper needs to be open. My thought is I need to have the pressure drop across the return run (damper, grill, etc) match the pressure drop through the OSA run (louver, duct heater, etc..). Is this approach correct or am I missing something? otherwise the FCU will just pull from the lest restrictive path...??

    thanks in advance

  2. #2
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    First how are you determining the Actual CFM's? Rarely are you actually moving what its designed for!

    Even if you could calculate it that building / area / zone is going to be under high Positive pressure, that is going to screw up the adjustments even more!

    What's going to happen to the supply air temperature as you turn this unit into a 50+% Outside Air Unit?

  3. #3
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    The problems you have to over come are 1. the pressure drop of the outside air damper is less than the pressure drop of the return air duct. 2. The airflow through the dampers is not linear with respect to position.
    Pitot traverse the supply duct to get the total airflow with the outside air damper open about 33%. Pitot traverse the return duct. Subtract the return flow from the total supply at that condition. Adjust the dampers from that point to obtain the airflows you want.
    No man can be both ignorant and free.
    Thomas Jefferson

  4. #4
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    Also , be very careful of pulling down anything on the return / outside side of any fan coil or ahu . If you create too much suction pressure the negative pressure will not allow the trap to drain . Then you have a leaking mess .

  5. #5
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    Depending on the duct layout, or the O/A inlet, if it's difficult, or not possible to do a traverse on the O/A duct, you can probably do a quick down and dirty velgrid, or rotating vane measurement at the inlet.

    The other option, the one I'd choose if the fan is suitable, is a simple sheave change.

  6. #6
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    If you can't do a duct traverse use the heat balance method to set the outside air quantity. The outside air temperature should be at least 30F different than the inside air temperature.
    The velgrid was designed to read filter airflow and must have a correction factor determined by pitot traverse to be reliable for any other use. I have two rotating vanes one of which is a high dollar digital and neither one can be trusted.
    Not trying to upset you Artrose but I didn't even read filters with the velgrid if I could read them with the flow hood.
    No man can be both ignorant and free.
    Thomas Jefferson

  7. #7
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    The more input the better. Education, sharing of information, and a willingness to learn.

    Personally, if I was going that route, I'd probably use CO2 to calc the O/A percentage. Still need the total traverse though.

    Better solution, the sheave change. To each his own.

  8. #8
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    I like CO2 control for minimum outside air but think it is best used along with building pressure control. I balanced a fire station that was designed for 75F at 50% RH. The engineer called and said he was getting high humidity complaints and would I go with him to find the problem. We went and sure enough the RH was 60% plus but the dry bulb was 65F and below. They wanted 65F at 50% not what was designed around. Everything HVAC wise was good except the RH. I had set the OA CFM using mixed air temperature and building pressure because the engineer had incorporated CO2 control in the design. He had also over designed the MAU and could get cold enough air from it to take care of the humidity problem and maintain 65F @ 50% RH.
    To solve the humidity problem we had to have precise control of the outside air. To do that mixed air temperature control wasn't good enough. I had them add a temporary duct to the outside air intake on the roof so I could get a precise setting of the outside air CFM. I set the OA by duct traverse so we could control the humidity and the firemen got what they wanted.
    I added this example to share information Artrose as you indicated because I agree with you. This was a one time thing but the traverse ended up being an absolute necessity. In the event CO2 levels got high the CO2 control would ignore RH and take over the OA. They were lucky the engineer was not only good but willing to work with me to resolve a problem that didn't belong to either of us. His original design was on the money.
    No man can be both ignorant and free.
    Thomas Jefferson

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