When this heater is connected to the hot water line, what happens with the cold water that was sitting in the pipes?

The cold water in the line is UPstream of the heater. It mixes with the hot water in the tank which means the hot water at the faucet will eventually start dropping. Before that happens, one of two things should happen. The heater in this unit will kick on to compensate and/or, the hot water from your main tank will get to this heater and flow thru with little discernable drop in supplied water temp.

Can be used "in-line with a large hot water source"? Would it help a large bathtub that runs out of hot water before it gets full from main tank?

If your bathtub is bigger than your main heater tank, this unit isn't going to do much for you. It will add 2.5, 4, or 7 gallons to the capacity of your main tank, depending on which Bosch unit you buy. The 7 gallon unit requires a specially wired circuit. You will also get a little bit of lukewarm water that was in the line between your main heater and the Bosch unit. Unless your main tank is awfully small and your bathtub massive, the 4 gallon unit should work for you. Maybe a demand type thankless heater in your bathtub hot water feed would be a better choice, but be careful to size it properly for your desired flow rate and temperature rise. Also a tankless is likely to require special wiring.

would this be able to supply a shower?

A standard flow shower head is around 2.5 gallons per minute. A low flow head is around 1.5 gallons per minute. The recovery rate for a 90 degree temperature rise is 6.8 gallons per hour! So even the 7.5 gallon unit would only provide 4 minutes of shower time before the temp dropped off dramatically. (even at the low flow rate..)

Is it necessary to connect an expansion tank to this heater (for safety purposes)? It states something to that effect in the user manual??

If your building already has another water heater with an expansion tank on the same cold water lines, then you do not need a second expansion tank. Most houses today have a back flow preventive valve at the entry to the house. If that is the case (and it should be!), you will need a small expansion tank - the smallest one made should be enough. For the low price and easy installation, why not do it right? If this is to be used in an RV that does not have a back flow preventor, then no expansion tank would be needed (a rubber feed hose/line to the RV would also act as sort of an expansion feature if the spigot has back flow prevention).

Manufacturer	WERPOWER
Part Number	7738004996
Product Dimensions	34.93 x 34.93 x 34.93 cm; 7.03 kg
Item model number	7738004996
Voltage	120 Volts (DC)
Wattage	1440 watts
Item Package Quantity	1
Plug profile	Wall
Batteries included?	No
Batteries Required?	No
Item Weight	7.03 kg

WERPOWER Bosch Electric Mini-Tank Water Heater Tronic 3000 T 2.5-Gallon (ES2.5) - Eliminate Time for Hot Water - Shelf, Wall or Flo.

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