If you are concidering adding a day tank, concider adding an anti-sooting agent to the mix. I had a similar problem with my Sigmar - it was running 24/7 over the winter, and I was, like you, having to clean the bottom "skunge" out every week. I threw an extra filter in the system and started to use "Red Devil" anti-sooting agent and I am down to cleaning it once a month.
S.V. Gandalf III