CO2 Control - Demand Control Ventilation (BMS)

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[@rakesh.procky5588]

Hey Bryce, I am an HVAC design engineer from India, until watching this video I was misunderstood that when there is more than 1000 ppm , I believed that OA damper will be opened up more than design OA. Appreciate your time and effort for this video. It’s very helpful

[@bryceanderson17]

It's not only you Rakesh, everyone thinks the OA damper opens to 100% when the CO2 is high. I'm glad you got some value from the video, keep well.

[@kidoctane]

Thanks - this has validated my approach to OA / CO demand control for the past 10 years ........ Good vid ....BTW an overloaded floor is really not an issue with a central AHU design

[@bryceanderson17]

Thanks for the comment. If there is a central AHU and it has a CO2 sensor in the return, then yes, an overloaded floor will not make much difference as it will be averaged across all floors. Here in Australia we always have CO2 sensors on every floor, and with a high select, will be the process variable for the outside air damper. If this is what you are referring to. Cheers..

[@tammyghashghaie6242]

you are 100% right and that was exactly my concern. it was years and rears was big question , looks like you have read my mind keep it up thanks

[@bryceanderson17]

Glad that cleared it up. Thanks for the support.

[@rodkington7996]

1. Un ticking is useful for vacant floors or spaces 2. sustainability engineers are/should be focused upon delivering the 'right' ammount of O/A not too much or too little. 3. LEED, Green Star and the like are 'as built' or 'as designed' rating systems, often O/A quantities are dialed back to optimum levels, within the first few years of operation.

[@bryceanderson17]

Agree with all that except the first point. It would be useful, yes, to deselect an unoccupied floor. However, I feel that an optimistic facility manager or young engineer will deselect the highest CO2 sensor to save energy, which it will. But compromise the design outside air to that floor. An unoccupied floor will have low CO2 anyway so won't affect the outside air rates. Thanks for sharing 👍

[@zaheeransari31]

Thanks Bryce.....For explaining this.....I will implement this....however I just want to understand when the damper will open 100% in your case....

[@bryceanderson17]

The damper will never open to 100%, unless the outside air design requirement is 100%, under CO2 control. The damper can open to 100% for free cooling when economy cycle is available. However, if you have a separate economy outside air damper and minimum (design) outside air damper, then the separate (smaller) minimum outside air damper can open 100%. Because it is sized for the design outside air. If you only have 1 (big) outside air damper, then it never opens to 100%, even if the CO2 is very high.

[@zaheeransari31]

@@bryceanderson17 Thanks Understood..

[@aryadas7786]

Man u are so crystal clear in explanation.....it will be helpful u can make a full HVAC videos package on u tube...love from India

[@bryceanderson17]

I'm working on it haha.. It takes about 6 hours to create a 15 minute video. Planning, recording, mistakes, editing, thumbnail, upload, description etc. Thank you :-)

[@aryadas7786]

@@bryceanderson17 cool

[@aryadas7786]

Sir Can u make a video on working of DOAS system (and if u can combine with terminal units it will be better) I am facing some problems where i have to use it in a IT building(2 floors)

[@bryceanderson17]

@@aryadas7786 What does DOAS mean?

[@aryadas7786]

@@bryceanderson17 Dedicated Outdoor Air system (it is used nowadays for better IAQ)

[@ahmedrefaieemara1512]

Thanks, Bryce for this video, and I think you considered the maximum OA damper opened at 50 % because you are considered an AHU with an economizer cycle, but in case someone has AHU without an economizer cycle, this means that the OA damper full opening is selected to pass the design OA flow rate, otherwise this means that the damper is oversized, correct me if I am wrong please

[@bryceanderson17]

This example is for an AHU that only has one OA damper. The OA damper fully opens to provide free cooling when conditions are favorable. The OA damper when not in economy must only open enough to allow the code design outside air or less if CO2 is low. I just made up 50% as an example. I was trying to show that it does not open to 100% no matter how high the CO2 is. I am not sure if this actually answers your question. Note, the main economy damper is not sized for design outside air (occupancy ventilation). I guess it's designed for 100% design supply volume (cooling/heating capacity). This is an assumption, I am not a mechanical engineer. Design outside air is the amount of outside air to satisfy code requirements for people, and building positive pressurisation. This is different to total design supply air required to meet the cooling/heating demand. The outside air damper when fully open I think supplies design supply volume.

[@ahmedrefaieemara1512]

@@bryceanderson17 Thanks Bryce for your reply, regarding the free cooling, you are right the OA damper is sized to cater for the total supply air, this is why I thought you considered the OA damper at 50% to match the total design OA flow (i.e a highly occupied theatre/shopping center with OA/SA fraction of usually 50%)

[@emmadeshabo9819]

Thanks Bryce for this clarification. May I know the relationship with the return air damper when the outside air damper is modulating in response to high CO2.

[@bryceanderson17]

Hi Emma, there isn't a simple answer to this. Two hours ago I was witnessing on site and when we set all the VAV's to Volume minimum. Watched the supply fans slow down, watched the return fans slow down. Watched the minimum outside air damper open more to make up for the reducing supply volume (economy disabled, main outside air damper closed). Eventually the minimum outside air damper opened to 100%. We have asked the mechanical commissioning tech to now slightly close the return damper to force more air to he drawn through the minimum outside air damper. The answer is that in basic terms the return air damper is opposite to the main economy damper (separate to the min outside air damper). But it often needs to be somewhere else depending on the air balance. There are many ways to control min outside air, some companies modulate the return damper to maintain a mixed air plenum pressure (say -30Pa). You need to do what ever the mechanical balance requires the return damper to do. Sorry, iv probably made this even more confusing, but there are probably 5 different ways to do this.

[@emmadeshabo9819]

@@bryceanderson17 Many thanks again. I have learned something new today. I am actually a BMS operator and willing to know more about the equipment sequence of operation. Therefore looking forward to more of you.

[@charlesduffy2033]

Great work Bryce, very informative and good to see you hydrating as usual ;)