How does chf cause pleural effusion?

Leaky lungs: The pulmonary lymphatic system can hold about 4 liters of fluid per lung (8 liters total) before they start to leak. In heart failure, there is back pressure from the left side of the heart into the lungs; if there is enough pressure to result in more than 4 liters backing up on one side, the lungs will leak into the pleural space leading to an effusion.

Complex mechanism: When both ventricles fail (& it does require both), there is hydrostatic back pressure in the pleural and pulmonary vascular beds causing transudation of fluid from the vascular compartment to the interstitial space. Moreover, hormones are secreted such as aldosterone which act on the kidneys to retain fluid and engorge the central circulation: nature's way of attempting to improve cardiac output.