The coexistence of ZigBee and WiFi networks is one of the topics which has been investigated both empirically and mathematically. The empirical studies often lack scalability since the networks considered are very small. The mathematical models on the other hand often focus on a stable network, ignoring he dynamic interactions of unstable systems. We provide a real-time analysis method for studying the effect of intermittent WiFi on communication in large ZigBee networks. Using our model we explain some of the common patterns seen in the analysis results. Moreover we establish that in addition to the ZigBee configuration and the clear channel rate of WiFi, the WiFi transmission pattern also has a high impact on the performance of the ZigBee network.