[Bldg-sim] Cooling Design Calc Methods

Ery Djunaedy ery.mailinglist at gmail.com
Fri Jul 6 09:53:45 PDT 2012


On 07/06/2012 10:11 AM, Hall, Brendan wrote:
> Apparently, ASHRAE recommends excluding the floor loss in cooling 
> design calculations, which is why it was correctly excluded in HAP. 

Brendan,

ASHRAE recommends excluding the floor heat loss for commercial cooling 
design calculations, not for residential. If you look at the residential 
calculation example in ASHRAE HoF, you can see that the floor heat loss 
is a significant portion of the total load. What I mean is that if you 
exclude the floor loss, and then apply your safety factor, you will end 
up with an oversized AC.

In fact, we have tried the same exercise as what you have done. We 
compared EPlus with whatever the mechanical engineer was using for the 
sizing (in an existing building), and then go to the actual unit itself 
on a design day (i.e. a hot day with max temp higher than the design 
temperature) and ensure that the internal gains are as assumed in the 
calculation. Then we look at the compressor operation during that day, 
and estimate how much oversized the compressor is. We practically nailed 
down everything else, except the floor loss. When I tried using 
adiabatic floor like you did, I got exactly the same sizing result as 
what the mechanical engineer had (I cannot remember what they used, but 
it was not HAP), of course with other assumptions being equal.

Can anybody enlighten me on why there should be any difference between 
commercial and residential design calculation with respect to the floor 
loss? The small commercial spaces that we have worked on are just like 
residential spaces, only with different profiles of occupancy, lighting 
and equipment, plus ventilation requirements of course. For multifloor 
commercial buildings, yes floor heat loss is tiny. But for small single 
story commercial buildings, floor heat loss is significant.

Ery


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