Great information and all makes sense to me now being just an end user. I have a Noritz 98% 199K BTU unit in a medium sized townhome. 2.5BA, dishwasher, 2 gas fireplaces, gas range, gas dryer, soaking tub. My unit was installed in the winter in february so lowest groundwater temps. On first use I noticed that my soaking tub turned up to full blast was only about 1/2 the pressure/fill rate of the cold water side. Also affected the water pressure of say the shower running in tandem. I turned the temp down to 115, which is far hot enough and this immediately gained pressure proving the theory of one of the posts that a decrease in deltaT would increase the amount of water that could be heated over the same amount of time when compared to 120. I would try that first off. Also being in PA it starts to heat up in the spring through summer at which point you will notice the ability to use more faucets/showers etc without losing hot water pressure. I think the moral of the story is that if you want to convert to tankless and you are lured by all the sales pitches you really need to take into account proper sizing to fit your demands. If I had to do it all over again i'd have put two in series.