Baseline-Heat/Cool sizing ratio

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Folks,

I?m trying to oversize heating capacity by 1.25 and cooling capacity
by 1.15 in order to comply with ASHRAE 90.1 Appendix G.3.1.2.2. If I
specify the same into heat/cool sizing ratio and keeping basic system
sizing as 1.15,I?m not getting any increase in heating/cooling
capacities!!

Any help would be appreciated.

Thanks
GK

GK Pandian's picture
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GK,

My understanding has always been that the G3.1.2.2 oversizing requirement should be met by setting the COOL-SIZING-RATI to 1.15, the HEAT-SIZING-RATI to 1.25 and the SYSTEM:SIZING-RATIO to 1.00 for all baseline systems. The system sizing ratio acts as a multiplier on airflows as well as a multiplier on the cool and heat sizing ratios, so you could accomplish the cooling and heating oversizing by setting system sizing ratio to 1.15, the cool sizing ratio to 1.00 and the heat sizing ratio to 1.09. However, you would then be oversizing the airflow as well. While earlier versions of ASHRAE 90.1 did not elaborate on what exactly should be multiplied, 90.1-2010 specifically says "system coil capacities" shall be oversized. So we should be leaving the system sizing ratio at 1.00 and only oversizing via the cool and heat sizing ratios.

Note that if you are trying to correct unmet loads and you are not restricted to following G3.1.2.2, increasing the airflows via the system sizing ratio is a helpful option.

Regards,
Bill

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