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Thread: co2 sensors

  1. #1
    Join Date
    Jan 2005
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    steeler nation
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    i have heard several different opinions when controlling oa dampers with a co2 sensor.

    opinion 1: keep the oa dampers closed (no min position) until co2 sensor demands then modulate to min position.
    this does not take into account other iaq concerns.

    opinion 2: leave dampers at min 10% then open to 20% on co2 demand.

    opinion 3: modulate the dampers up to 100% depending on co2 demand

    i understand the type of equipment and oa conditions have a great influence on the control method but would be interested in hearing any opinions.

    also bringing in outside air in a major city i.e. buses, traffic, delivery trucks idling, trains , homeless using the alleys for toilets etc. how fresh is fresh air !

    i work in the north east.

  2. #2
    If you want to control the dampers directly from a CO2 sensor, I would fit a modulating damper actuator with 0-10v control and a small stand alone controller such as an 'electro-controls' type to suit the sensor and actuator. Set your desired Co2 level and the dampers will modulate to maintain the minimum air quality set.
    This would be the most cost effective option parts around 450 although a good universal controller such as a Johnsons or siemens may be a better option if you want to control other parameter. Such as opening the dampers to 100% during high outside air temp, this will allow 'free cooling' during the summer. Also closing the dampers during frost conditions as well as knocking of the fan.
    All the AHU controls can go on one of these controllers ie, time schedule, Heating/cooling coils, boiler controls etc.

    Can travel to NE if you want a price for the work email peter@picontrols.com




    [Edited by picontrols on 02-18-2005 at 05:48 PM]

  3. #3
    Typically the unit will run at a minimum outdoor air setting setup by the air balancer and speced by the engineer. Then the CO2 sensor signal will be able to override the minimum to allow more fresh air to enter in relation to the CO2, this can be done as modulating or just set to a higher minimum when the sensor dictates that more fresh air is needed. There aren't to many codes that will alow you to close off the outdoor air damper completely, and this can also hurt you in buiding pressure which needs to be in a slight positive typically, the minimums not only provide fresh air but they need to compensate for all of the exhaust in the building.

  4. #4
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    thanks fat eddy

  5. #5
    Join Date
    Oct 2003
    Location
    Minnesota
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    Originally posted by viceman
    i have heard several different opinions when controlling oa dampers with a co2 sensor.

    opinion 1: keep the oa dampers closed (no min position) until co2 sensor demands then modulate to min position.
    this does not take into account other iaq concerns.
    I've never seen this done.


    [QUOTE][B]opinion 2: leave dampers at min 10% then open to 20% on co2 demand.[\B][\QUOTE]

    Never saw this done either. What would be the point. How would one KNOW that just 10% more OA would be adequate?


    [QUOTE][B]opinion 3: modulate the dampers up to 100% depending on co2 demand[\B][\QUOTE]

    Closer to the most common scheme, granting that one is starting at minimum OA position, and/or whatever position one was at while using free cooling.

    Fat Eddy had it right. For MOST applications. There are some variations for special situations.

    I noted that one responder suggested finding some sort of control that was set up to do this task specifically. Which is fine. However, in my experience using specialized controls can end up more costly than just using a small general purpose DDC controller.

    Several manufacturers make such units. And make such units which can operate standalone, needing no other DDC equipment. Controller, small, with 8 inputs and half that many each of analog and digital outputs can be had for $200 to $300. Add one time purchase of local comissioning/programming software.

    The advantage being that once you learn the controller, you can use it for many applications, and it's more versatile that using specialized controllers.

    ie With such, it'd be pretty durned easy to set up controls to control OA/RA and relief dampers to do both tasks of setting up an economizer cycle (free cooling) AND also modulate dampers further open in the case of high CO2.

    Some of the small jobs my department does is precisely this sort of thing. Customer doesn't want full DDC system. But has some special sequence of operation he desires for something. Which can be done using a lot of add-on devices with a lot of complicated inter-wiring. Requiring a large cabinet. Or we just put in a small general purpose controller preprogrammed at our shop to do the task. Sometimes even sticking some small led lights on front door of cabinet with controller, maybe a dial operated variable resistance control for "manual adjustment" of some variable. And perhaps an "Off/On" switch, and/or "Go 100%" switch.

    ie At times our Mechanical department will come to us for this to meet some customer spec that'd be complicated and expensive to implement other ways.

    In other cases, we have a fairly large customer, a property owner/management group which has 18 buildings with 40 to over 100 apartments apeice, who opted to have all their controls this way. Used to be pneumatic. Very little pneumatics are left. Instead, one finds numerous little white boxes with a DDC controller inside programmed for specific task. In several cases, with LEDs and manual adjustment dial on outside.

    Not the way I'd do it. But it's their property and choice. They think it's easier, simpler, and more versatile than keeping the pneumatics. But definitely did not want some system with a front end, where if it crashed, or network wire was cut, a LOT of things were interrupted. Inhouse, the keep a couple new controllers on the shelf. We provided em with copies of all programs. And they have a laptop with basic comissioning/programming tool. So it's no big issue if a controller fails to dump appropriate program to controller that was on the shelf and plug it in, in place of failed controller. This doesn't happen often. Twice, over the past 11 years they've been doing this. Not bad seeing as how they have hundreds of such controllers installed. They also use basic comissioning/programming tool to look inside controller when troubleshooting. To see what's up. Most failures being a sensor feeding an input, or an output device (actuator, relay, etc). Or something in the item being controlled.

    Heck, I even know a small HVAC company which uses this concept. They don't do big, all DDC work. But do use small general purpose controllers in standalone configuration in the ways I describe. To avoid complicated special purpose tasks that'd otherwise involve several special purpose units, box load of relays, timers, etc.

    Not as unusual as it might sound. Numerous times on this job or that I've looked inside piece of equipment direct from manufacturer, which is standalone, having it's on controls, such that all we need to do is enable or disable item, and pick up any desired monitoring points. To find that control inside was a small general purpose DDC controller or PLC. Often with sticker from equipment manufacturer covering up label of DDC or PLC maker. Heck, a lot simpler when manufacturer wants to change something. Instead of doing complete new controls layout, planning wiring runs, relay and timer layout, etc. They just change the program inside.

    also bringing in outside air in a major city i.e. buses, traffic, delivery trucks idling, trains , homeless using the alleys for toilets etc. how fresh is fresh air !

    i work in the north east.
    This is an issue. But almost never addressed by a system design engineer.

    Outside air CO2 content can vary considerable from place to place. Dependent upon a lot of variables.

    But yah just have to live with what yah have to work with.

    As concerns other air contaminants. I've only been involved in one project where system designer and customer spec'd things to account for outside air pollution. And that was a federal government facility. Where they had us installing sensors for and monitoring a lot of other things. But that's as much as I can mention about that. part of the contract, I can't be more specific.

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