You think you know OA control? Separate min OA volume measurement? - Part 2

[@jnlraboin]

Thanks for your expert advice, I will be consulting a customer on new AHU's sometime this year! This helps gives me some ideas to approach with the customer before the job goes out for bid!

[@coreyr8870]

If the bypass valve does not open to maintain flow the chiller flow will trip on low flow. Our knock on effect is someone has to go reset the chiller, this costs money. Like your videos. If we can find the balance between not tripping and knowing when to open my bypass based on valve demand from my 2port valves vs. pressure control which doesn't seem to react in time to prevent tripping the chiller.

[@owenmilner9700]

Good food for thought as always, I think a significant amount of OA dampers are actually functioning as spill dampers.....

[@davidkemp4557]

Thanks for you’re time on this one Bryce

[@bryceanderson17]

It's a pleasure mate :-)

[@y4lla]

May be overthinking it a little... solar load is roughly 30% of the total heat load....if the SA fan is turning down that much, it is most likely the space is below design occupancy and does not require OAmax.

[@bryceanderson17]

If half the floors are shut down because those companies staff are working from home, the fans will turn down pretty far. Or, if the AHU is running in after hours mode, i.e., there is 1 out of 10 floors enabled, then its likely that both SAF and RAF will be running at minimum speed (the same speed), possibly zero OA. In Australia, usually the RAF's are axial fans and the SAFs are centrifugal fans, they have different characteristics so simply tracking by a fixed 10% probably isn't going to work. Eitherway, we should design and commission control systems to work properly through the full range of the SAF, not just at design speed. "Solar load is 30% of the total heat load". I didn't know that. Thanks 👍

[@owenmilner9700]

In Australia regardless of occupancy there is still a minimum code OA required, usually based on floor area. The code minimum and the design qty of OA are 2 different issues. On a well designed system with DCV the code minimum is the qty we have to preserve when turning down the SAF. DCV will increase to design qty if required.

[@Dogga10001]

Bryce, OA min control is a big energy waster, and I could say that none of them are reliable. There has to be a better way and at the moment I think any system you check will be working incorrectly. The design engineers should have a full proof way of specifying how we should do it, not tell us that we have done it wrong. That’s what the engineers are meant to do. We do it one way and then get told how to do it another way. Big waste of time and money. This is happening too much in this industry. Specifications are written very loosely and we have to make up the slack then get told we are doing it incorrectly. All a waste of time and money. If you want it to work a certain way then tell us how you want it done. If you don’t want to put your credibility on the line then don’t tell us how it should be done. The only full proof way as I see it is maintaining a negative pressure in the mixing chamber that makes sure you have OA coming into the AHU based on the air quantity that the AHU is doing compared to total air quantity which is based on the total number of active VAVs that it is supplying and the air quantity that they are doing. (Maybe AI will work this out) If you don’t like this then tell us in the spec exactly how you want it done….whether that is right or wrong. Vel grids only work when there is not pressure influence in the duct that it is mounted, I am very open to a way of accurately measuring air quantity in an intake duct that is fed by multiple air streams, return, outside air. We only have the tools and inputs to create the correct output, we are not magicians. How else are we to workout that each individual requires 11.5 litres a second of outside air ?

[@bryceanderson17]

Sometimes I read in a control strategy (FD), where the BMS engineer writes that the outside air will be modulated to maintain 11 lps per person. LOL, then I know they copied that out of the mechanical spec and I am thinking, "how are you going to do that" :-)

[@Dogga10001]

@@bryceanderson17 Bryce, in your travels have you come across a good way to measure min outside air ? I have been involved with vel grids and Hotwire velocity probes etc and none give a really good constant reading.

[@Dogga10001]

Still open to an idea on this as knowone has come up with a reliable way to this issue, everyone turns a blind eye.