The pleural fluid in CHF originates from increase filtration of plasma across the capillaries of the visceral pleura and, more importantly, excess fluid in the interstitial spaces of the lung, both related to the increased hydrostatic and capillary wedge pressures (1).

It is postulated that leak of edema fluid into the pleural space may serve as a “safety valve” to mitigate overflooding of alveoli (2). 

Interestingly, although the pleural effusion is commonly bilateral in CHF, when unilateral, it is more likely on the right (1). The reason for this finding is unclear but several hypotheses have been put forth including compression of the azygous vein (which drains a portion of the parietal pleura of right lung) due to the dilatation of the right heart, and compression of the right pulmonary veins by an enlarged right atrium (1).  

Bonus Pearl: A minimum of 50 ml and 200 ml of pleural fluid are required for visibility on lateral and posteroanterior views of a chest radiograph, respectively (3).

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References

1. Natanzon A, Kronzon I. Pericardial and pleural effusions in congestive heart failure—anatomical, pathophysiologic, and clinical considerations. Am J Med Sci 2009;338:211-216. https://www.ncbi.nlm.nih.gov/pubmed/19574887
2. Zocchi L. Physiology and pathophysiology of pleural fluid turnover. Eur Respir J 2002;20:1545-1558. http://erj.ersjournals.com/content/20/6/1545.short
3. Mammarappallil JG, Anderson SA, Danelson KA, et al. Estimation of pleural fluid volumes on chest radiography using computed tomography volumetric analysis: an update of the visual prediction rule. J Thorac Imaging 2015;30:336-339.https://www.ncbi.nlm.nih.gov/pubmed/25811356