Few WH are designed for continuous use. Getting one big enough for both heating and WH use will not be inexpensive, either. If by open, you mean that your potable water goes through the heating loops, don't do that. The cost of bronze pumps and special fittings to keep heavy metals from entering your potential drinking water are both costly, and may not be allowed in your area; not counting the potential health issues. Constantly adding oxygenated water to a heating system means much more wear; best to isolate the potable from heating.
A modulating boiler and indirect will likely be more efficient and last longer.