I moved into a new house and the inspector didn't explain the heating system, and I can't find any manual. We have demand hot water and I am told that we have radiant heat (but it's not in-floor heat.)

There is a thermostat downstairs and each room has a wall vent with an individual fan control. When you turn on the fan the room warms up - but even if you don't turn it on, a little bit of warmth comes out as long as the thermostat is on.

So do we save any heating energy by keeping some of the fans off? Or are they just blowing heat that's being produced anyway? What is behind the vents in the rooms? And what is the most overall efficient way to run this system? It's a 2-story 1800 sqft townhouse and the living room has 20' ceilings.

And is it the same water heater providing both heat and hot water to the house? It's a Rinnai R94LSi which I looked up on Amazon and no one mentions using it for heating a house. There are a ton of lines coming off it and I don't see anything else - but I haven't been in the crawl space.