Do solders melt at the operating temperature of a heater?

Good question. There are a few simple answers. Most solders begin to become "plastic" at around 350 degrees Fahrenheit and melt at slightly higher temperatures than even that.

If you are asking about a hot water heater: If you are operating your water heating system at that temperature the water will have already boiled off and most likely the heater will either have exploded, the P/T relief valve dumped or both.

If you are asking about an electric heater, do not try to melt solder on it. The heating element is electrically "hot" and could present an extreme shock hazard.

The best way to melt solder is with with a soldering iron or some kind of torch.

Hope this helps