Lovely, isn't it. It also has a better blower. The heater box really wasn't doing it, so I bought a bouncy castle blower off ebay. That does it - maybe too well. The only trouble is, if you look carefully you can see that the fumes aren't actually going up the chimney. When there's a lot of smoke coming from kindling it is unbearable. The first chimney was 1m of 5” flue. I tried upgrading to 1.5m of 6", and raised the hearth closer to the cowl; no better. Then I noticed that it was drawing OK before I turned the fan on. As soon as the fan came on, there was a massive downdraft pulling the smoke back out of the flue. Using a bouncy castle blower there is a lot of spare draft, and with the vent closed it all comes out of the bottom of the tuyere, through the ash plate.
So, we need to put that waste air in the right direction. A T piece in the air duct was easily fitted, allowing me to fit a length of dead exhaust pipe ducting up into the cowl. This didn't really help at all. It caused a lot of turbulence at the entry to the flue, and I could see the smoke rising up then turning around and coming back down. Extending the duct right into the flue did it, now there seems to be a nice laminar flow right up the chimney. I can now light the forge and breath at the same time. Here's the "after" view, with added lights as well.
