Is there any merit in a shelf of that refractory board on top of the walls below the boiler tubes to make the flow of hot gasses move to the front and up creating a longer path before exiting. I had a shelf in mine for a couple of years before it disintegrated and haven't found a source for a replacement. I'm not sure if it really helped or not. Sometimes I think the heat just stays up on top longer without, then I wonder if the shelf creates more flow over the top and exits quicker and extracts less heat. The shelf idea came from Adobe. Just trying to get the most out of this boiler.