Wearable sensor systems are widely used for vital sign monitoring in hospitals, and at homes in recent years. However, most of them are designed for bedridden patients in hospitals. Using wireless sensors to capture physiological data from patients for long term health monitoring in a health institution has become increasingly popular these days. Some previous studies recorded complaints from the users about the discomfort from wearing the devices. In addition, such a system sometimes can be interfered by the existing Wi-Fi traffic. Therefore, a sensor system that is comfortable to use and does not interfere with the existing wireless traffic is required. In this paper we present the system design that includes the probe, sensor, radio, and the receiver, for a long term heart rate monitoring system in a senior center. Our system has been deployed in a senior center since May 2012, and 63 seniors had used this system since then. The senior center has its own wireless network and we made no change to the existing network. Our monitoring system runs together with the existing network, and achieves a 97% packet delivery rate.