I would like to do some "tuning" of my water heater. Here's the situation.

My heater is gas (propane) fired, and sits up on the roof in a small enclosure. It is a blend of on-demand and tank style heater, in that there are two small (5 liter? 10 liter?) holding-tanks and two burners.

Each burner/tank has its own pilot light and adjustable thermostat. The burners/tanks are connected in series (not parallel) with the first burner pre-heating the water that goes to the second. The heater is rated to provide continuous hot water to two and a half bathrooms. My limited experience with the heater supports this claim

When I shower, the water starts out very hot, with the mixing valve at about the half-way setting. After about three minutes, the water temperature drops, probably about 10-15 degrees F. After about three minutes of this diminished temperature, the water temperature climbs back up to the initial temperature and remains there for as long as I want to stay in the shower.

So, what strategy should I employ with the thermostats to avoid or at least significantly lessen that mid-shower dip in temperature? Right now both thermostats are set to their highest temperature. Perhaps the pre-heat thermostat would be left high, and the output thermostat set lower? I don't have a clear picture in my mind of just what is happening with the current settings. It appears that once the water in the second, or output tank is used, it is replaced by water from the pre-heat tank that is of lower temperature, thus the dip. But why, then, after several minutes, does the temperature in the shower come back up?

Maybe someone here can visualize how this works better than I.

tanstaafl.