jackpott Posted November 28, 2016 Report Share Posted November 28, 2016 I am struggling to control the heat rise of my gas downdraft - it gets too hot too quickly! I'm test firing empty (I've put in all the kiln shelves) to learn how to control temperature. I'm trying to emulate a slow bisque as I will be firing large, thick sculptural pieces. I'd like to control heat rise to 20 - 30 deg. C per hour, particularly for the first 100 deg. The slowest I've managed is 200 deg. C in 30 minutes! This is with one burner lit, at it's very lowest setting (the flame is JUST clearing the port inside). The top of the kiln heats really quickly and after 30 mins., the top kiln shelves are too hot to touch, but the bottom shelf / false floor is just comfortably warm. I've tried different damper settings to no avail. I'm thinking that there is insufficient draw to pull heat down throughout the chamber. And maybe that my burners are just too powerful for a slow bisque? My next thought is to heat the stack somehow - maybe put a blowtorch in the damper slot to heat the flue air and create some draw. And/or look for a smaller burner that I can use for early stage heating (something like a domestic gas burner that can run at a lower flame/heat setting). I'd REALLY appreciate any feedback/suggestions anyone can offer. Thanks! P.S. It's a converted electric, approx. 8.5 cubic feet. Flue area is 6x6 inches narrowing to a 6 inch diam. pipe. The kiln is fired by 2 venturi burners, using LPG (propane), and the flue outlet matches the area of the burner inlets (as per Olsen). Link to comment Share on other sites More sharing options...
neilestrick Posted November 28, 2016 Report Share Posted November 28, 2016 It is very difficult to get gas kilns to heat slowly in the first few hundred degrees with the main burners, especially in a small kiln. You need a little pilot burner instead. Ideally, your kiln should already have a pilot burner as part of the safety system. If you leave the pilot burner on for several hours you'll get a nice slow preheat to dry out the pots and prevent steam explosions. Then you can turn on one of the main burners as low as possible, and eventually get up to both burners running. Link to comment Share on other sites More sharing options...
Mark C. Posted November 28, 2016 Report Share Posted November 28, 2016 You need a separate pilot light for slow heat rise at start as Neil says. Link to comment Share on other sites More sharing options...
jackpott Posted November 28, 2016 Author Report Share Posted November 28, 2016 That's great, thank you. It was one of the ideas I was going to explore. As the kiln is a conversion, there is no pilot - just the two venturi burners. Is there a recommended pilot light? Do they come with a thermocouple cut-out should the flame blow out? I'll have a google around, but if there is anything you'd recommend, I'm all ears! Thanks again! Link to comment Share on other sites More sharing options...
jackpott Posted November 28, 2016 Author Report Share Posted November 28, 2016 Also, is there anything else I should consider re: the uneven heating? Is it possible that I have insufficient draft at the low temps. I'm aiming for? Thanks again! Link to comment Share on other sites More sharing options...
neilestrick Posted November 28, 2016 Report Share Posted November 28, 2016 Even conversion kilns can and should have safety systems. What you need is a Baso valve. The main gas goes through it, and the pilot burner and a thermocouple are also connected to it. When the pilot is lit, the thermocouple senses if and opens the main valve. You can get them from Ward Burner. Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted November 28, 2016 Report Share Posted November 28, 2016 I was going to put this on ukpotters but noticed you posted here too. I wouldn't mess about with heating the chimney to make more draft in hope of a slower climb. I don't think it will work how you want it. If you have the typical venturi burners sold in the uk northern kilns should be able to supply valve/pilot. I am surprised it didnt come with a flame saftey valve. Saying that my home-made burner has no safety at all besides me watching to make sure it doesn't go out. Also if you are going to cone10 I would be aware that you may struggle with a venturi burner without the chimney being 2.5/3x bigger than the kiln height Link to comment Share on other sites More sharing options...
jackpott Posted November 29, 2016 Author Report Share Posted November 29, 2016 I've had a look at the Norther Kilns raku venturi burners - mine look very similar - everything appears the same include thermocouple and baso valve. What I can't see is a pilot - I light mine manually once the gas is flowing. Is there a source for suitable pilots in the UK? I've had a look at the Ward pilots - look ideal, but I'm in the UK - anyone know of suppliers of similar over here? I'm also looking at BBQ burners - ribbon or rod burners, but not sure how to connect them to the propane tanks... Thanks for all the help so far! Link to comment Share on other sites More sharing options...
neilestrick Posted November 29, 2016 Report Share Posted November 29, 2016 All the Ward parts are standard appliance valves and pilots. Look at gas water heater suppliers. Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted November 29, 2016 Report Share Posted November 29, 2016 I would give NK a ring and they can probably source or point you in the right direction. Link to comment Share on other sites More sharing options...
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