Do you ever have to worry about freezing? A boiler where it can freeze is a pain. You can run antifreeze in it, but it decreases the efficiency, and you still have problems with an autofill valve or some means of filling it up...that can't be full of antifreeze. A boiler can be quite small - mine hangs on the wall. An indirect WH can normally be smaller than a gas-fired waterheater, since the boiler may be larger than the typical burner on a tank. If you have access to underneath the floor, you could do staple up hydronic verses radiators, and not take up any floor or wall space in the house. In fact, many of the new mod-con boilers can be put in a closet in the living space. They are essentially silent. If I'm more than 3' or so from mine, I can't hear anything, and even standing next to it, I have to listen hard and I can hear it only because I know what to listen to. A mod-con would let you match your limited heating needs better than something that can only run full-bore (which you may need to reheat the water in the indirect WH).