The Phoenix type kilns that I’ve been involved with have all proved their worth, a design I can recommend as efficient and straight forward to build.
My first experience with this style of kiln was in 1998 at the Auckland Studio Potters. We were a group of members who were interested in wood-firing. We built the kiln at the ASP as we wanted to encourage other members and students to get involved. So an account was started, bricks purchased, location found and construction started.
The basic plan was for a large firebox directly underneath the kiln and a chamber size of around 30 cuft - large enough for the 18 or so members to put work into for the group firings, but not too large that we'd never fill it. The grate was originally metal fire-bars but we ended up going through a number of various grate options as we learnt how to fire the kiln. The most successful version was a Dutch oven style grate made of brick which is slightly higher than halfway up the firebox. This solved most of the problems of ember build up and choking the firebox and lasted very well.
Other tweaking we did with the kiln after the first firing was to build the chimney higher by about 3 feet. We also tried different door systems in-front of the firebox opening - it is a hot kiln to fire and a series of hinged flaps or iron shields were experimented with before realising that the most successful firings were ones that just left the firebox open and piled in the wood. Most of the firings were either soda or soda/salt firings.
The only repair work done on the kiln is to re-build the throat of the kiln. This was necessary as the part of the firebox arch nearest the throat was slowly creeping into the gap, which slowly lengthened each firing. The kiln was demolished in 2015 after well over a 100 firings - it was looking distinctly shabby at the end but still fired ok.
In August 2007 while taking part in a wood firing festival in France I had the opportunity to build another Phoenix kiln, using an odd assortment of bricks at the pottery I was staying at. This kiln came together fast as I only had a few days left on my visa, which also meant I didn’t get a chance to fire it, but reports from the pottery are that it worked well after the usual learning curve firings.
Back in NZ and working for the Waikato Society of Potters I used their Phoenix kiln which was built by the members in their central city studios. It is a beautiful kiln to fire and reaches temperature in about 9 hours with pallet timber. You have to be very careful of the smoke produced as the WSP is only two blocks from downtown Hamilton. It has a taller firebox than the ASP kiln and the front of the firebox is bricked up with a stoking port, making it considerably more pleasant to fire - the chamber is also a little taller. Again soda is usually used in the kiln - only 1.5kg dissolved in hot water and sprayed in.