static pressure reset

[drafty888]

Hey guys I wanted to get some feedback on your experiences with duct static pressure reset routines.

When I am done with the TAB contactor, I'll already have a duct static setpoint that correlates to the largest volume VAV being about 80% open with all VAV's at maximum CFM setpoint. Then in low cooling/ventilation load days the duct static will be reduced incrementally up to xx% (??) until the VAV is back to 80% open. I don't see any value in increasing the duct static setpoint beyond the TAB setpoint. Do I have this correct? What is the maximum % used as a static pressure setpoint reduction?

Thanks!

[maxpower]

Maybe its just me but I like to keep things on the simply side. Usually a constant 1.5 inches w.c. 2/3 down the duct works fine.
Then again I'm not a programmer.

[digo]

"reduced incrementally" doesn't mean increasing the setpoint. It think they mean to lower the setpoint by increments of a negative number.

Where I'm located the diversity factor varies greatly from engineer to engineer, but in most cases there's no way in hell the air handler can provide enough air to satisfy all boxes at max cfm.

What we usually do is reset the duct static pressure setpoint between 0.75" and 1.5" based on the worst damper position being at 95%.

The idea is, if the worst zone can satisfy its max cfm requirements at 95% open, why try to maintain a higher duct static? The trick is setting a pi loop slow enough to adjust this setpoint, thus maintaining the worst zone at 95% without hunting all over the place.

The controlled variable is the max damper position, the setpoint is 95% (or 80% in your case), and the output of this direct acting PI loop is linked to a ratio or reset block so that 0-100% = low spt - high spt (e.g. 0.75" -1.5")

[crab master]

I like Digo's thinking. Generally I am resetting about the same ratio's, a few I was able to go down to 0.6" w.c.
I've reset duct static based on OAT and on terminal load as well, depending on application and data available.

maxpower - simple is nice but when you get your first building tuned to a duct static reset I've found the building occupants will definitely notice - 'my office used to be so loud in the winter' and 'reduced energy bills'

[maxpower]

I like Digo's thinking. Generally I am resetting about the same ratio's, a few I was able to go down to 0.6" w.c.
I've reset duct static based on OAT and on terminal load as well, depending on application and data available.

maxpower - simple is nice but when you get your first building tuned to a duct static reset I've found the building occupants will definitely notice - 'my office used to be so loud in the winter' and 'reduced energy bills'

that makes sense.

[freddy-b]

I feel static reset is a waste of time unless the system is A: WAY oversized or B: some genius thought it was smart to oversize one unit instead of adding a separate unit for the the Occasional occupied High Load area ...such as a Auditorium or something simular.
So whats the savings..... the Chicken or the egg?

[digo]

I like Digo's thinking. Generally I am resetting about the same ratio's, a few I was able to go down to 0.6" w.c.

crab, great minds think alike!

On a recent job, I was able to drop the static to 0.4" while still maintaining the worst zone at 95% open. I was told however, never go below 0.5" in order to avoid the Coanda effect

Right now, 3:00pm in SoCal, 72F Outside air, CHW valve at 8%, I'm at .80 in. w.c., with the SF VFD at 56%, supplying around 64F air.

I'm also resetting DAT based on max terminal load.

[digo]

I feel static reset is a waste of time unless the system is A: WAY oversized or B: some genius thought it was smart to oversize one unit instead of adding a separate unit for the the Occasional occupied High Load area ...such as a Auditorium or something simular.
So whats the savings..... the Chicken or the egg?

Yes, I've seen these over-sized monsters before, but that's not the only reason to implement Duct Static Reset.
When the load is minimal, there's an opportunity for savings. I've heard the number to be about 20-30% savings on the energy used by the VFD.

ASHRAE Standard 90.14 and California Title 24 Energy Standards both mention this strategy should be implemented.

[drafty888]

The controlled variable is the max damper position, the setpoint is 95% (or 80% in your case), and the output of this direct acting PI loop is linked to a ratio or reset block so that 0-100% = low spt - high spt (e.g. 0.75" -1.5")

exactly what i was querying... thanks. i think that a very slow PID calculation rate (1-2 min or greater), coupled with a small integral value will work here. got to give the VAV boxes enough time to settle out at the new duct static setpoints before re-calculating another duct static setpoint based on the max open VAV box position.

[dave2142]

I feel static reset is a waste of time unless the system is A: WAY oversized or B: some genius thought it was smart to oversize one unit instead of adding a separate unit for the the Occasional occupied High Load area ...such as a Auditorium or something simular.
So whats the savings..... the Chicken or the egg?

I agree with freddy on this one. If the system is sized and balanced right then this should be a moot point. Afterall if boxes throttle back then static goes up hence drive speed goes down. If boxes go open static goes down and fan speed goes up. Why does it have to be so complicted and add reset of the setpoint? Comes back to the usual controls being respoinsible to fix poor design and engineering.

[crab master]

Let's see -
Eastern Washington weather
- 20-30 Deg F OAT ave for 2 months
- Parallel Fan powered boxes w/1st stg reheat being fan only, 2nd & 3rd stgs electric reheat.
- Supply and return fan's on VFD's with economizer and DX cooling only at AHU.
- Reset static and DAT based on OAT and terminal load. Supply VFD went from running about 45-50Hz in the winter average, to 30-35Hz from memory, and return was running at around 20 Hz. Prior to changes return was tracking supply exactly, but added bldg static and offset to return.

BTW server room had its own unit so didn't have to 'compensate'

I really wanted meter data but this building was not sub-metered. Of course VFD speed reduction is a cubed savings value, so why a moot point?

Granted there are systems designed around the fan curve, where yes it is a moot point and shouldn't be a vfd on it anyway. I haven't worked on one yet, but I had a pretty good discussion with an air balancer about a building setup this way.

[markman]

Controls should be able to fix `poor design and engineering' even when it really is is poor. Design is basically predicting the future and we know how successful you can be with that. Building dynamics are COMPLICATED and there is no way even the best design can account for all the uncontrolled variables in their occupancy, local weather and construction. That's one of the great things about modern controls - they can make the system work better in spite of all the unknowns in the design process.
But only when control contractors step up and do their job right.

[crab master]

Yeah but sometimes it's difficult when you get a poor spec! Then it's our fault (controls contractor) and we have to go to all ends of the earth to prove it's not!

BTW - Welcome markman