I agree with your statement about the tank losses, whether it'd be either an indirect storage tank or a tank-style water heater: you'd usually enter them in the WaterHeater:Mixed using the On or Off Cycle Loss Coefficient to Ambient Temperature. As stated in the Input/Output reference guide here:
Field: On-Cycle Loss Coefficient to Ambient Temperature
The loss coefficient [W/K] to the ambient air temperature. Often this coefficient is identical to the “UA” for skin losses. If the loss effects of the flue are being modeled in the Off-Cycle Loss Coefficient, than this field would have a different value accounting only for the skin losses.
Field: Off-Cycle Loss Coefficient to Ambient Temperature
The loss coefficient [W/K] to the ambient air temperature. Often this coefficient is identical to the “UA” for skin losses. However, it can also be used to model the loss effects of the flue in a combustion water heater, in addition to the skin losses.
For skin losses, you'll likely be already familiar with the UA coefficient if you've used other softwares such as eQuest before.
You can also implement some ambient temperature losses in Pipe:Indoor, instead of using a Pipe:Adiabatic. You'll have to set the Environment Type to Zone, specify which zone the pipe is in, create a pipe construction object, specify the inside diameter and the length of the pipe. Point is, it's surely not easier than the above. Make sure you save yourself some trouble and only model uninsulated piping.
