Resetting Temperature Setpoints Based On Outside Air Requires… Linear Interpolation?
I always seem to forget that magical calculation that will reset a Supply Air Temperature Setpoint based on the Outside Air Temperature. I figure… Eh, I’ll Google it. Then realize, “WTH do you call that kind of calculation?” Is it a Slope Calculation? with an interception… er Intercept? No… It’s a:
Linear Interpolation Calculation Type of Automation Situation
Yeah, sure… whatever. Believe it or not I failed 3 YEARS of High School Mathematics!
Maybe if I had more HS teachers like this Brazilian Beauty,
I might have paid more attention.
I still ended up loving Math because it’s either RIGHT or it’s WRONG. I also love Excel and figuring out all kinds of spreadsheet formulas as well as trying to incorporate mathematic calculations when writing programming code. Learn that math stuff, it helps…ALOT.
Why Reset the Supply Temperature Setpoint Based on Outside Air
The main reason you want to do this is for Energy Savings by not having to utilize as much mechanical cooling vs. allowing the Ambient Temperature to cool the space.
Ambient – The surrounding area or environment. In HVAC, typically relating to outside or atmospheric conditions.
So if it is pretty cool outside we don’t need to have super cool 52°F supply air shooting down the ductwork. We can allow the supply air to increase up to say 58°F knowing that should be enough cool
air to satisfy the warm areas, but accomplish two energy saving measures:
- Not use as much mechanical cooling (Chilled Water or DX Cooling) to make supply air temperature setpoint.
- Not require the heating or reheating of spaces to overcome the cold supply air. Keeping electric heat stages in terminal boxes from coming on is a huge energy saver.
Now gimme the Calculation or Formula, Einstein!
Down below I even made a downloadable Excel Spreadsheet for ya.
Alright here it is…
Y2 = Y1 + [(X2-X1)(Y3-Y1)] / (X3-X1)
- X1 = Lowest OAT Value
- X2 = Current OAT
- X3 = Highest OAT Value
- Y1 = SAT Setpoint @ X1 OAT
- Y2 = Actual SAT Setpoint
- Y3 = SAT Setpoint @ X3 OAT
Ohhhhh THAT one… yeah I got that already. Easy Cheezy.
Let’s Try an Example Temperature Reset Calculation
I want to have cold 52°F SAT when the OAT is @ 80°F.
………….and I want 58°F SAT when the OAT is @ 60°F.
- X1 = 60°F
- X2 = Current OAT is 77°
- X3 = 80°F
- Y1 = 58°F
- Y2 = What was your answer?
- Y3 = 52°F
??? = 58 + [(77-60)(52-58)] / (80-60)
STOP scrolling back up to the teacher pic and
leave a comment with your answer down below.
Delta Controls GCL and Some Other DDC Controls Make It Easier
I have been using Delta Controls for awhile and they long since came up with a function in GCL programming called ‘SCALE’ and it makes using the Linear Interpolation calculation even easier by typing in the following syntax:
SCALE(<the OAT input>, <bias offset>, <OAT@highest SAT>, <highest SAT setpoint>, <OAT@lowest SAT>, <lowest SAT setpoint>)
SCALE(AI1, 0, 60, 58, 80, 52)
where AI1 = the physical location of the Outside Air Temperature Input.
Most other Controls Systems have similar functions or programming blocks that you can use.
FREE Downloadable Excel Sheet for You to Keep
If you like this Excel Spreadsheet let me know and I will try to make some more stuff like this in future.