How many BTU does the thing draw? What's its rated gallons/minute? I assume you have a heat exchanger for the radiant heat, as you wouldn't want to run potable water through all those lines.
What is the heat load on the house? In other words, how much heat do you need in your boiler (heater)? Does the system operate with priority zones? If the priority scheme is not set up properly, (i.e., when you call for hot water for washing, showering, etc.), if it is still providing heat to the heat exchangers for space heating, the potable water will never get hot enough. It needs to bypass the heat exchangers. If the zone valve or pump is still running, you won't get any hot water for the shower. Both units need to restrict flow to the heat exchanger(s). A diagram of what you have might be useful.
It could be as simple as a stuck or improperly positioned check valve, zone valve, or pump.
I'm not a fan of using a potable water heater (tankless or conventional) for space heating.