This may be slightly off topic, but I think I have a good idea that could allow you to reuse a considerable amount of the waste water from your heating system. Depending upon how much space you have to spare, you could route the open loop into a large holding tank with a livestock water tank float which would divert the flow back down the drain whenever the tank filled up. When it comes time to do water changes of top offs you could condition this water with prime and use it again.