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I have six daughters and one son. They were all teenagers at the same time, so you can imagine what the hot water demand at my house was.

I installed an outdoor wood boiler just because of the domestic load. When we didn’t use the wood boiler during the warmer months, the entire family got by using a 50 gallon gas water heater. They learned to schedule their showers around mom and dad’s and each other.

I install a lot of boilers and indirects and an 80k btu boiler with a 40 gallon indirect is always more than enough for an average family. It’s only when there’s an unusual demand (like a car wash shower or a large hot tub) that more capacity is needed.

Yikes, those 3.5 gpm shower heads and the 6 gpm tub spout should be plugged into your math equation (time x flow volume) when figuring your dhw needs in a given use-scenario. Your boiler (and tank) is too small. Maybe…change shower heads? Co-ordinate showering and bathing habits? Ah, "first world" problems.

your indirect tank is too small for your dhw load. you could get somewhat more capacity buy keeping the tank at a higher temp and using a thermostatic mixing valve but to make for example 2.5 gpm of hot water with a 65 f rise you need 81,000 btu/hr. (i used about 35 f incoming water temp and 100 f final water temp). once you have mixed enough cold water in that indirect tank to male the water noticeably colder, you aren't replacing as fast as you are using it. your boiler is 80,000 btu/hr in and around 70,000 btu/hr out. there uis also some lag between when you start using water and the aquastat in the tank sees the tank is cooling, then some more lag for the boiler to pre-purge, light, and get hotter than the tank and start adding heat to the tank.

the tables in the manual for the indirect are for conditions of incoming cold water that is much warmer and a boiler that is much bigger than in typical northern climate installations.