Ballistocardiography (BCG) is a non-invasive technique measuring heart and circulatory activity, proposed as an alternative to ECG for long-term heart rate monitoring. This study evaluates a custom multi-node piezoelectric BCG sensing mat for heart rate measurement compared to single-lead ECG in post-cardiac surgery patients. 14 patients (10 males, mean age 63.47 ± 9.35) postcoronary artery bypass (N=8) or heart valve repair (N=6) were enrolled at Maastricht University Medical Center. Patients in the cardiac ICU used a bed with a custom BCG sensor mat (10 PVDF-TrFE piezoelectric sensors, 90x50 cm²). BCG and ECG were recorded simultaneously for an average of 15 hours per patient. BCG heart rate values were extracted over ten-second epochs using a multinode rule-based algorithm. Performance was assessed using mean absolute error (MAE) comparing BCG and ECG heart rates. Heart rate values derived from the piezoelectric BCG mat resulted in a weighted mean MAE of 1.41 bpm with 66.14% coverage compared to ECG. Piezoelectric BCG shows promise as an accurate heart rate measurement method in post-cardiac surgery patients, offering a non-invasive alternative to ECG. These findings support further development of this technology for clinical use in remote heart rate monitoring.