We do not recommend open loop heating systems as they can be dangerous and more and more local codes are not allowing them. So be careful if you decide to use an open loop heating system. For a few extra dollars you can add a brazed plate heat exchanger to the system and get a good heat system and a safe potable water system with one heat source. The Example below illustrates this and you will need to use a brazed plate heat exchanger with 10 to 50 flat plates. You may not need all the components illustrated.
In the picture below we show a tank - but this could be any heat source that you want to isolate from the heating system. For example - a wood or gas boiler. (If you use non-oxygen barrier PEX with a boiler - then you would need to use something like this.)
Picture of all the parts needed to separate drinking water from your heating system with a Brazed Plate Heat Exchanger and more.