Design Conditions

[stern]

Hi all,

I would like to ask why is that in the outside air design conditions for winter there are not included the wet bulb temperature or relative humidity as in summer? In summer with dry and wet bulb temps i go easily to my psychrometrics software and compute some data but for winter I just have the minimum average temp (dry bulb)and no humidity info..

Thanks for your help

 

[KiwiMace]

There should be. You can get this information from the ASHRAE data for the local area or ask the information provider.

However, if it is super cold where you are, it doesn't make a big difference for most heating purposes due to the pinch (low on the low end of the chart. You heat this air up to room temp and you get within a few % of the same RH.

 

[MintJulep]

There is. Typically listed as "mean coincident wet bulb".

However, since heating processes are typically sensible only...

 

[EnergyProfessional]

for heating load it doesn't matter. It is important if you calculate your humidifier equipment, though.
the TMY3 data also should have humidity for all year if you do energy simulation.

unrelated to this, when you design for heating and follow ASHRAE or use the 0.4% percentile etc. this may be a different value than your code requires. In my case ASHRAE would require OAT of -11°F, but code requires -15°F (ironically not the mechanical code, but energy code). this likely has more impact on equipment size than humidity, which in winter doesn't really if you don't humidify.

 

[stern]

Thank you very much for your comments