Cold Water Bleed or Crossover

I just installed a tankless heater. The heater is doing its job, putting out very hot water (according to the remote controller which has diagnostic tools and the fact that the output pipe is too hot to hold on to), yet the water at all the fixtures is warm but not hot enough. The Rinnai folks suggested that there must be a problem with cold water bleeding or crossing over. They mentioned that with traditional (tank) water heaters these problems are often masked or don't make much difference but with the new tankless the problem becomes serious. My question is why is the problem worse with a tankless, and does anyone have suggestions for tracking down the problem? Thanks!

When you have a tank type heater, there are no appreciable restrictions, and you have equal pressure from both sides (hot and cold). On a tankless, in order to get that hot output, they often have a restrictor in the line to reduce the flow so there's enough time to actually heat up enough. My guess is it is that pressure reduction on the hot side that's giving you problems.