A question: If I keep an indirect water heater at, say, 125 degrees (just bear with me for a minute) and run 180 degree water through the tank's internal heat exchanger, what will the return temperature of the water generally be coming out of the water heater, on its way back to the boiler? I'm sure that this is very dependent on the specifics of the tank, but in generalities, will the temperature be close to 125 degrees, or rather much higher than that? Has anyone ever measured this sort of thing? Thanks, Jeff

The return water temp will be closer to 125F than 180F- the actual temp depends on the flow rate on the boiler loop and the heat exchanger's efficiency at that flow rate, as well as the dynamic stratification in the tank from potable water flow. Return water temps can be well below 125F when the potable water flow rate is high and the temp at the bottom of the heat exchanger is very low due to high dilution by the cold water supply. When the potable water flow stops the return water temp rises fairly rapidly toward the average tank temp due to mixing currents (leftover momentum from the flow) within the tank- it doesn't stay very stratified for long after the flow stops.

Thanks a bunch.