Introduction

Can hot water heaters also be used for hydronic house heating?

Sure, if they are designed for that. It is not clear how well an on-demand hot water heater would work for powering a hydronic system.

Speculation: Since on-demand heaters are optimized for on-demand use, they would work but be less efficient than dedicated gas hydronic heaters. Further, a normal hot water heater would probably do well for hydronics.