The 3" probe TPR is interchangeable with the 4" probe. The only requirement is that the probe be immersed in the top 6" of water, and that's a tank design criteria. The last time I bought a TPR, the store had two different brands in the same bin, one with a 3" probe, the other with a 3.75" probe. 3 to 4 inch TPR probes are standard in most installations. 8" probes are used in top port installations where no side port is available, and 1" probes are sometimes used in small under-counter point of use heaters.
