Out of the box, water heaters have more safety controls than the typical boiler. Horror stories where water heaters used as boilers blow up or blast off through the roof are almost always the result of improper installation or of someone doing something nutty like plugging the relief pipe. Properly installed, storage water heaters are as safe as any other heating appliance in my opinion.
There are several ways to hook up water heaters to supply a heating system. The method you use will depend on balancing initial cost with system life and operating efficiency. If all you need to do is add a warm floor to a bathroom or some baseboard convectors in a small den and you are willing to live with lower efficiency, the least expensive...
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