Is return air temp the same as room design temp?

[aritafal]

I was just wondering for mixed air calculations how do people determine the return air temps? Is it just the design air temp?

 

[DRWeig]

No, the return air temp will be warmer than the room air at temperature sensor level due to stratification, heat from lights, etc... Add a couple degrees.

Best to you,

Goober Dave

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[aritafal]

Is there a formula to this or is it just a "guestimation" of a few degrees?

 

[lukaiENG]

It depends.
Like Goober Dave stated, stratification and lighting will cause an increase. how much the increase is depends on whether lighting is a proportionally large load in the space. With the advent of LED lights they are become a smaller contributor.
Spaces with high heat loads (data centers, electrical rooms etc) would see a negligible difference between design air temperature and return air temperature.

 

[aritafal]

ok thank you for you responses

 

[EnergyProfessional]

It makes a difference if you have plenum or ducted return and as stated how much of the lighting load goes into the plenum. Also depends on if you have a roof above and how well it is insulated. Most load calcualtions will give the option to take that into account. for lighting it depends if you have recessed or suspended light.and as pointed out by lukaiENG, with more efficient lighting this will be less.

 

[chicopee]

Yes, If you are looking at bulk temperature within the room.

 

[aritafal]

Would I have to make an assumption that a certain percentage of my lighting load will be added to my return air? Is there a guide line for this or do you just state your assumption when doing the calculations for it?

 

[chicopee]

Lighting is part of the sensible heat load in addition to other electrical and electronic equipment, cooking equipment such as electric range, coffee pots etc... and any other type of heat producing equipment.