Project Report


Residential Cooling Load Calculation
Methods Analysis

Report prepared for: Pacific Gas and Electric Company, Products and Services
Report Dated: January 18, 1995
Prepared by: Proctor Engineering Group
Contributors: John Proctor, P.E. and Zinoviy Katsnelson, Ph.D.

Abstract

In 1994, Pacific Gas and Electric Company (PG&E) undertook a study entitled, "Residential Cooling Load Calculation and Air Conditioner Selection Methods Analysis". This study was the outgrowth of concern over the coincident peak effect of residential air conditioners. Residential AC coincident peak load depends on (among other factors) the size of the unit. In 1994, PG&E began requiring a cooling load calculation as a condition for residential AC rebates.

An air conditioner selection process consists of two stages. First the building sensible and latent load at the design conditions is calculated and then an equipment selection method is applied to choose a particular unit from the manufacturer's catalog. If errors occur in either one of these stages the units would be sized improperly.

The two parts of the study mirrored the two stages of equipment selection. In Part One, forty-one cooling load calculation methods submitted by over fifty contractors and distributors were compared against ACCA Manual J, an industry accepted standard. As submitted, ten of the methods calculated loads within 20% of Manual J. With revisions, another ten methods came within 20% of Manual J.

In the second part of the study, equipment selection methodologies were compared based on how they actually sized units to the expected indoor design conditions. A method of predicting indoor conditions specific to each piece of equipment was developed. Existing equipment selection methodologies can oversize units on houses in hot dry climates by 50% or more

Executive Summary

Various research projects and field testing performed for Pacific Gas and Electric Company by Proctor Engineering Group and others have indicated that residential air conditioners are substantially oversized (Lucas 1992, PG&E RACER 1992, Florida Solar Energy Center 1994). HVAC contractors often size air conditioners by rules of thumb that have developed over the years. This leads to substantial over sizing and a higher diversified electric peak load.

In order to solve this problem the AC sizing should be performed in two stages. First, an accurate cooling load calculation method should be used to estimate design sensible and latent loads. Second, an equipment selection method should be applied to choose a particular unit from the manufacturer's catalog that just meets these loads. If errors occur on either one of these stages, or if the load calculation and sizing methodologies make different assumptions, the units could be sized improperly.

In 1994, concerned about the high coincident electric load of residential air conditioners PG&E's Products and Services Department began requiring a cooling load calculation as a condition for residential AC rebates. At the same time, PG&E commissioned an investigation which had the following primary goal:

In the first part of the study contractors submitted load calculations These methods were compared against a liberal criterion, the method must not produce loads differing from Manual J estimated loads by over 20%. The analysis of these submissions is summarized in Figure 1-1. By the end of the process, half of the methods were approved (one quarter as submitted, one quarter with revisions).

Also see Energy Efficiency Research

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