What type of exchanger are you using? I have a pipe one on mine and a plate one on my sons? His should like be the pipe one because it keeps the whole tank of water hotter. The plate one is pretty much endless hot water but because he uses so little water the tank cools down a little too much between uses. I have the plate hooked up before the water heater, could hook it after but it was easier this way. My pumps also run 24/7, maybe yours are on demand?