how to determine air return temperature

[firmanardha]

Hi...
I have question how to determine return temperature
for example I have supply temperature 12 degree Celcius, 49 % RH, and my room temperature 22 degree Celcius, 50% RH


Thank and Regads

[coolments]

Hi There

Assuming the unit recirculates air from the same space it cools and the room sensor does not get any false readings from being in the wrong location (next to window or in the path off discharged air) then your return temp should be the same as the room temp.

[Toosh]

Hi..
I have question, how to determine temperature at return air?
for example condition

• temperature supply is 13 degree Celsius 45%RH
• room temperature set at 22 degree Celsius 50%RH

Thank and Regards

I suggest you would take it at the return air as close as you can get to the evaperator

Norm

[Tesla]

Hi firmandha
sorry I totally dissagree with coolments answer. The return air temp should be warmer than space/room temp. depending on the load it will vary. Also depends on type of AC - FAV, VAV. For example on a VAV AHU with a warm day of say 28 deg. supply air may be 14, room temp 22.5 and return of 23 or 24 deg. Sometimes due to no suitable location of sensor RA Temp is used to control but the setpoint would be 23.5 instead of 22.5. Don't just take my word for it - go out and measure it in the field.

[wambat]

Hi..
I have question, how to determine temperature at return air?
for example condition

• temperature supply is 13 degree Celsius 45%RH
• room temperature set at 22 degree Celsius 50%RH

Thank and Regards

Measure it

[firmanardha]

Measure it

sorry, I need this data for design i will do psychrometric analysis,
Thnk...

[coolments]

Hi firmandha
sorry I totally dissagree with coolments answer. The return air temp should be warmer than space/room temp. depending on the load it will vary. Also depends on type of AC - FAV, VAV. For example on a VAV AHU with a warm day of say 28 deg. supply air may be 14, room temp 22.5 and return of 23 or 24 deg. Sometimes due to no suitable location of sensor RA Temp is used to control but the setpoint would be 23.5 instead of 22.5. Don't just take my word for it - go out and measure it in the field.

Like I said if the room temp sensor is not in right location, why should the return air temp be higher, im curious.

Surley a room temp sensor should be be used for control and not return air temp sensor, for example controling a remote mounted ducted AHU of a return air sensor would be asking for trouble.

lets face it theres to many variations in design, location and ambient conditions to give a calculation for return air based on supply air and space temp, like mainly where is the retun air sucked from, at high level yeah it maybe slighty higher if the sensor is lower, if at the same level as the sensor and the air does not pick up any heat on the way back to the evap it shouldnt.

The question is for a calculation and I dont know now of any any calculations you could make unless you know the extact conditions it works in and the thermal properies of the building, ducting, how far it runs, through any area the duct runs thats a different temp from the space ect you could go on for ever.
So i supose the best advice is to go measure it, but whos to say it will be same next day unless conditions are exactly the same.

If there is a calc for it, I am intersted to know it.

Next question is after going to the pub, filling myself with beer, coming home and rattling the missus (thats what I call it) am I sat here scanning through reading the all the posts in a quest for knowledge at nealy 4am in morining. I think I did it all in the right order but

Is is dedication to the trade or am I a sad git.