[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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