It's a while since I've built an HT oven. It seemed like time to get off my backside and build another one, or two.
The last time I built one, I stepped the length up from 18" to 22.5", increasing the length by an extra brick. It was a bit slower to reach temperature than the 18" oven, but not excessively so. It seemed like there was a fair chance that an oven would still reach temperature if I built it with yet another brick for a 27" length. The only way to be sure was to build one and it seemed like a lot of effort with the risk that the resulting oven would not do the job. I tend to regard 1206 degC, 2200 degF as an acceptable design temperature, being a little over the maximum temperature requirement for any of the readily available blade steels I am aware of.
At Owen's hammerin earlier this year, I was chatting to a guy who had one of my earlier 18" ovens and wanted something longer. He already has the control box, which represents about half the cost and effort of a build. He also only works with Carbon steels. Carbon steels need lower temperatures than stainless steels, not usually more than 1000 degC, so there is minimal risk a 27" oven will not meet his needs, even if it cannot manage stainless temperatures.
The plan is to build a 27" oven and test it, for which I'll need a control box, then build either another 27" oven if it will reach 1200 degC, or build a 22.5" oven if it won't, to go with the new control box.
I've made a start on the oven.
My earlier ovens had fully-welded frames. These made element replacement a job for an angle grinder with a thin cutting disc and a welder. It also made them awkward to modify. This one has welded sub-frames which bolt together. A discussion with a well-known smith some time ago had me rethink the element connections. My early ovens had the element connections on the back: dead easy to do. Making the back removable entailed taking the elements out of the side and is much more hassle. The idea behind it is that it should be possible to bolt 2 ovens together and HT swords. I have already built one 42" sword oven which uses a modified control box to power 6 kW of elements from two 13A sockets, so I know there's no difficulty electrically.
The bits of box section between the oven and the hinges are there to space off the hinges and help to even up the pressure on the 1" blanket I use as a door seal.
I think the odd bits of faffing about with the elements coming out of the side add at least 50% to the build time, possibly more. Progress to date has certainly been slower than I'd like.