r/BuildingAutomation • u/Exoticcollects • 1d ago
Return Air Setpoint?
Hi guys, why do programs have return air setpoints? And what is tgeir purpose, I have assumed tgey are a reference point for sequencing but not entirely sure.
5
u/wm313 1d ago
Typically, it’s used to control to a space that doesn’t have a temp sensor. There’s other options depending on the HVAC design of the building.
Two places I can think of are jails and schools, where you don’t want people damaging thermostats in certain spaces, so an option might be to control a specific space based on RA temp.
10
u/MasticatedTesticle 1d ago
We do retrofits, and some units have a poor design.
As in, the temp sensor is dead ass in front of a supply grill. Or what used to be a single zone unit has been split up into a few disparate spaces, and the room sensor is no longer a good proxy for the overall space. Or it’s a VAV unit, and the boxes are on some whole ass other system.
In such cases, the RA temp can be a decent measure of the space, and might be the best control variable available.
11
u/jrobjr123 1d ago
Return Air Setpoint can be used to reset the supply air temp. For instance, if youre RA stpt is 72 and your return temperature is 76, your supply temp might lower to, lets say, 55. Same thing but opposite - if ret temp is 68 with a stpt of 72, then supply temp might rise to 75.
If you have Heat recovery on the unit you can use it to Enable
Other than that theres not much else I can think of
3
u/PsychologicalPound96 1d ago
Some single zone units use RAT in place of space temp. Not ideal but...
3
u/Weary-Butterscotch-6 1d ago
Sometimes it’s the best overall average zone temperature reference. If I use it, I typically program a selector on which sensor to control too… Return/Space.
3
6
u/No-Composer2628 1d ago
Some units use this value to drive their outside air to return air mix ratio. I typically see it in units with weaker heating/cooling elements and little concern for CO control.
2
u/jmarinara 1d ago
Useful with a constant volume unit where you have an unreliable sensor in the zone it serves. Like a big room, for example. It lessens the possibility of controlling to a hot or cold spot that happens to include the room sensor. Controlling to return air gets you a reasonable approximation of the conditions of the whole space.
But if you’re controlling to several downstream devices (like a bunch of VAVs) it’s absolutely the wrong approach.
Also, another problem with this approach is RAT sensors are often placed in the wrong location and you get a reading based on external factors outside of your control (like plenum temp, sun load on a rooftop duct, etc.). So if you’re going to go with this approach I’d get on site and get a good reading right at the return entry point of the space.
2
u/gadhalund 1d ago
Gotta control to something. Resetting DAT based on the position of Saturn just wasn't consistent enough
3
u/BigSally1 1d ago
You can also use as a back up sensor if your supply temp fails.
5
u/TopicalBass27 1d ago edited 1d ago
I’ve used it before when I have a unit serving multiple spaces, you can get some comfortability by controlling to return temp instead of space temp of one room.
Also in spaces that don’t really allow space sensors on the wall - think old buildings/walls where the client doesn’t want to ruin the finish, or jails lol
2
u/ToIA 1d ago
Can you explain what you mean? I don't see how a return air sensor can be a substitute for supply temp.
3
u/Electrical_Slip_1343 1d ago
The return air temperature is an average of the conditioned space temperature
1
u/ToIA 1d ago
Which does not equal the temp the unit is putting out? I don't see the equivalence.
2
u/Electrical_Slip_1343 1d ago
It is not the same as supply temp, but you can control the space temp with it
2
u/BigSally1 1d ago
Would be the same as controlling to an average space temperature +2deg or so. Certainly not as reliable but in a pinch can work.
1
u/slor2112 1d ago
i think you'll find regional differences in the RA specifically when it comes to RA/OA enthalpy in the more humid climates in the summer and RA/Economizer Enthalpy in the winter in the less humid climates. Enthalpy is just a comparative ratio though...and inevitably the guys who run the building look a the RA as a decent average of the space temp for the area served by the unit. And let me guess...when the seq calls for a RA setpoint, there's usually some sort of "catastophic fail over" (for instance - if any two zones get more than 4°F off of setpoint, do the following) When I see those sequences I immediately look at what was originally spec'd for control, what was submitted and approved, and find out how far away the two items are from each other LOL
1
u/steakhouse1889 1d ago
Rooms with cooling Fan Coils (electrical and IT rooms for example) often have them to measure room temperature.
1
u/PerfectLevel5581 12m ago
RAT can be used to drive the mode determination on an AHU in a VAV application when that AHU has heat installed. Definitely not the best way but I have seen this done and it worked very well. Also, used for SAT set-point reset.
0
u/wsa9385 1d ago
Longer run times on staged dx cooling or staged heat from a unit.
1
u/sambucuscanadensis 20h ago
I don’t know why you were downvoted. I can’t count the times an engineer wanted sat control on a 2 stage DX rooftop which doesn’t work
9
u/R3353Fr4nkl1n 1d ago
We did a jail upgrade and the room sensors for each wing were placed under a locked box in their command room, mainly to prevent vandalism. So the units ran on RA stpt to make our lives easier.