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# Why does Carrier HAP calculate ventilation load relative to exhaust air temperature?

## Why does Carrier HAP calculate ventilation load relative to exhaust air temperature?

(OP)
I was reading the help file that comes with Carrier HAP and it mentions that ventilation load is calculated relative to exhaust air. Whenever I do hand calculations, I calculate the outdoor air ventilation load by taking the delta T between my outdoor air temperature and my cooling/heating coil discharge temperature. Have I been doing it wrong?
I have attached a screen shot of the help file for people to read.

### RE: Why does Carrier HAP calculate ventilation load relative to exhaust air temperature?

Firstly I would say that I have always done as you mention, but thinking about it further, if you view the building as a black box with outdoor air going in and exhaust air coming out then the calculation as stated in the manual would be correct.
This load is is not necessarily what your coil will however, as you rightly point out the air is only raised/lowered to the coil discharge air condition.

### RE: Why does Carrier HAP calculate ventilation load relative to exhaust air temperature?

Yes, you have been doing it wrong.

How much heat does ventilation air add to the SPACE? It's a function of the outdoor conditions and the space conditions.

### RE: Why does Carrier HAP calculate ventilation load relative to exhaust air temperature?

(OP)
Thanks, guys that makes sense

### RE: Why does Carrier HAP calculate ventilation load relative to exhaust air temperature?

Don't forget to include outdoor air latent load and people latent load in calculating total load to be met by the cooling coil. I have experienced new engineering graduate make the mistake of only designing for sensible load. Good thing I caught it in the shop submission check face rather than after construction.

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