Twice a year. Don't let it sit too long. But wait, you are fresh water. I would not think you would have any build up. It is far more likely that your exhaust elbow is clogged. I'd start there. Not something that will work long time, but try this: take the HE outflow hose off the elbow. Then take the fitting off the elbow. Get a 6" spring maybe 3/16 to 1/4 diameter. Put it in a drill. Spray come carburetor cleaner in it the elbow. Run the spring in and out of the elbow while spraying some more carburetor cleaner in from time to time. Reinstall everything and see if you still have a heating issue. If you don't, you ID'ed the problem and you should add to your winter work a pull out & replace or clean. At least, that's my take.