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first firing- double venturi flue updraft/downdraft rebuild and a

updated fri 6 may 05


Ben on thu 5 may 05


For those who've followed my numerous questions about rebuilding and
converting our over large updraft to a downdraft, we finally fired it.

We took a 6'x10' Halverson updraft and cut it in half bricked up the
back wall and built the double venturi flue from Nils Lou's book with
minor modifications, (basically no soldier course.) 20' x10" culvert
chimney (height per 3x depth plus heigth I think I'll cut it down later
but I want to make sure I have plenty of draft, i'm kind of at the upper
limit of what this flue is rated and not overly confident)

Cut the cart in half, built the wedge into the floor.

The original burner ports were vertical which we maintained but

My last post on the subject was shelf configuration- we settled on 6"
on the burner sides and 5" front and back. I'll still have to do the
other half when I can afford some brick but for now we've got 64cu ft of
stacking space to work with.

So... long slow bisque, long candling, drying fire. I'm using a skutt
kiln controller, rewired, for a digital pyrometer but someone had given
me a thermocouple. Whoops. I'm getting color in the top fast, but it's
only reading 450 is 010 softening?. zinc melting off the culvert like
giant gobs of solder, I'm still only at 3" w.c. whats up? Turn down the
gas, change the thermocouple. 1400 already,probably pushing 16 hundred
in the top before I backed off. But this was 5hrs into it, no problem.
I'd been puzzling why it was reading so slow and I'm keeping a close
eye, back off the gas a little to even out top to bottom. Stalled at
1400 with the damper wide open 1.8"w.c. brought the damper in til it
started climbing. 1.8" good to 1800. Turned up to 2.5" finished in 40 min.

I was pleasantly surprised that I never went over 2.5" w.c. I only sized
it for 750,000 btu @ 11" So that was less than 200k btu/hr for a 64 cu
ft downdraft bisque. I didn't know if I had enough gas and we never got
up to medium :-) . I was very encouraged about the flue/ draft,
control, half cone differnce top to bottom really overall an excellent
firing..... The only problem....

The ware that face the burner channel was a little reduced on the flame
side. This kiln has ancient ceramic tip venturis which don't seem very
efficicent. My thinking is that I should cut down the orifice size to
closer to my actual btu needs (for bisque) to maximize the gas
velocity/primary air entrainment and make sure all my seals are are
tight.. It needs a little more work for that. It's not bad but it could
be better. I'm thinking that the long lazy flame of the low pressure
end of the scale was probably the cause. It seemed like overall the
firing had plenty of air. Any thoughts or suggestions greatly appreciated.
Take care,
Looking forward to the glaze firing