Water heaters have different efficiency rating conditions depending on the equipment size. Generally, cycling and other loses are factored into the efficiency for small water heaters, and calculated separately for large water heaters. See ASHRAE 90.1 Table 7.8 Performance Requirements for Water Heating Equipment. More detail on this is in ASHRAE 90.1 and the water heater section in the PNNL report Enhancements to ASHRAE
Standard 90.1 Prototype Building
Models.
As most gas water heaters are storage water heaters that operate by cycling, they shouldn't have a part-load curve. The are on or off. If you do have a curve, set it to one.
In the instance where you don't have a storage tank and you want to model a gas-fired instanenous water heater at a fine time resolution, the operation will be similar to a gas boiler. However, you'd need to explictly model water draws (rather than an average draw), and would want to include the ramp up/down conditions in the water heater in your part-load curve.
Pipe loses are a separate factor which matter quite a bit in service hot water distribution systems. You can model these explictly with pipe loss, or adjust the thermal loss from the water heater storage tank to account for pipe losses.
