The boilers used on Garrett wagons were cross water tube boilers.
The first wagon built (34143) had a boiler that was a direct copy of that used on a Sentinel waggon. It is suspected that this was built solely for the purposes of developing the wagon with a known reliable boiler, however it led Sentinel to protest the infringement of their patent.
By the time the second wagon, 34181, was constructed in December 1922, the new Garrett Patent boiler had been developed; this patent was GB196173, applied for on April 4th 1922. The layout chosen is shown above. The boiler fitted to this wagon, 16496, survives, although modified from its original form.
Principal dimensions of the boiler were 4'-4½" tall from foundation ring to top of the boiler, 2'-8¼" overall diameter, with 62 1½" watertubes, giving a total heating area of 63.598 square feet. The grate area was 4.27 square feet. Working pressure was 250 PSI.
The outer shell consisted of a welded tube fitted with a rivetted internal butt strap and an ogee shaped ring rivetted in place at the top. The firebox was pressed in two pieces (with a split line down the centreline of the wagon in the middle of the tube banks) and electric welded together, transitioning from the circular cross section at the bottom, to a square cross section in the middle, and back to a circular cross section at the top. This was rivetted into the ogee ring at the top and the foundation ring at the bottom. In the square cross section, the sides of the firebox had horizontal corrugations pressed in them to provide stiffness and the tubes ran front to back, slightly inclined. The patent was not followed entirely, as it showed the boiler with large doors on the front and back giving access to both ends of the tubes. In practice, the door was fitted on the front giving access to the tubes, and a smaller elliptical manhole was fitted on the back. There was a firing chute at the back, below the level of the tubes and offset 25° to the nearside from the centreline, in the form of an elliptical tube that passed through the outer part of the shell and was welded into a hole cut into the firebox. There was also a clinker door, for cleaning and lighting the fire, mounted on the front centreline of the boiler, and a spacing ring was rivetted in place to seal the joint through the shell. The steam take-off was on the front of the shell above the access door, with an internal perforated tube as a steam collector. A single gauge glass was fitted on the nearside of the boiler, but a second gauge could be ordered for mounting on the offside ahead of the driver. All fittings were mounted on pads electric welded to the boiler (with studs passing through to the boiler shell). There were three screwed bronze washout plugs mounted above the foundation ring - one at the front offside, one at the nearside and one at the rear. There was a screwed water filler plug of the same pattern as the washout plugs mounted on the offside near the top of the shell.
There were developments in the details of the boilers; by the time 34415 was sold in January 1924, the washout plugs had moved to being equally spaced 25° on either side of the clinker door.
The next major design change was around April 1924, when the clinker door was moved around 67° towards the nearside of the wagon. It is believed that this was done to simplify production of the boilers, as it allowed the clinker door hole to be pressed into the firebox, eliminating the intermediate plate. The joint was changed from being rivetted to being welded. When this change was made, the washout plugs were also moved around with the door, and the blow down cock was moved to the offside.
In June 1928, with the introduction of the later design of cab, pushing the driver forwards, the single gauge glass was moved from the nearside of the wagon to the offside, just in front of the driver.
A significant revision came with drawing number 24410, which was for the 6 wheeled Suffolk Punch tractor. Aside from revisions to mount the boiler on the tractor, and rearrangement of fittings, the thickness of the firebox was increased from ½" to 17⁄32", and the boiler pressure increased from 250 PSI to 275 PSI. The screwed washout plugs were replaced with mudholes with doubler plates rivetted over the shell. The firebox pressings were altered such that the welded seams were in the corners of the firebox (allowing the clinker door to be pressed in the front position, unlike the earlier design). These revisions were brought over to the wagon boiler on drawing 24497, albeit leaving the clinker door offset to the nearside, and the first wagon to have that boiler was 35363, sold in April 1930. A second water gauge (fitted in the same position as the sole gauge on early wagons) was made standard in April 1930.
19480 7/16" plate (shell) 1/2" plate (firebox) 250 PSI screwed plugs
23886 7/16" plate (shell) 1/2" plate (firebox) 250 PSI screwed plugs
24410 7/16" plate (shell) 17/32" plate (firebox) 275 PSI mudholes
24497 7/16" plate (shell) 17/32" plate (firebox) 275 PSI mudholes