GWR Standard Boilers - A Beginner's Guide

"The modern Locomotive Question is principally a matter of boiler". G.J. Churchward, 1906

This article principally uses the GWR classification system for boilers, originated under Churchward, which divided boilers into classes based on the physical size of the boiler and thus which locomotives it will fit. This system included a two letter code, which the GWR called a prefix. The first letter indicated the boiler class, and thus the principle dimensions of the boiler. The second letter can be regarded as a subclass, and in some cases there were twenty or more of these. Differences between subclasses would include superheat (presence and degree), water feed type and location and firebox type, as well as subtler things like minor variations in dimensions and the presence or absence of brackets to locate the boiler or tanks, which would have an influence on exactly which locos a boiler could be fitted to. The classes were sometimes referred to by the first letter of the prefix, but they also had names, usually either a standard number, or else that of a class associated with the boiler.

Another system which will be encountered is that used in the RCTS volumes. This complex system enables classification of the appearance of boilers, and is usually one or more letters followed by a number, which describe firebox type and dome location (or absence). The RCTS volumes further allocate boilers to over 100 "Groups", which are boilers of substantially similar type. Note that the two systems have very little overlap. RCTS describes boilers of several different types as being R3 or D4 and within their 100 odd groups some groups contain boilers of more than one GWR prefix and vice versa.

General Development

It's worth noting that Broad gauge and Narrow (=Standard) gauge locomotives were, in the middle of the 19thC, designed by different designers and built at different workshops. There seems to have been little interchange between the two.


When Gooch was in charge at Paddington/Swindon he seems to have largely delegated responsibility for standard gauge affairs to Joseph Armstrong at Wolverhampton, producing only a few designs for the narrow gauge. These had quite standardised boilers, typically domeless with raised round topped or (earlier) dome shaped fireboxes.

The Armstrongs and Dean.

Joseph Armstrong moved to Swindon when he was appointed to replace Gooch, and his brother George Armstrong took over at Wolverhampton and maintained his own line of development. Boilers with domes and without raised fireboxes appeared with Joseph Armstrong's 360 class goods type of 1866. These were followed by the numerous 0-6-0 Standard Goods and the Sir Daniel 2-2-2s, which had, as it turned out, the first two of what were to become standard boiler classes - boilers based on those for these classes were in use 70 years later. Under the Armstrongs and William Dean one sees these parallel boilers without raised fireboxes as being the dominant type. Domes moved forward and back along the boilers and pressures increased, but the external style was generally reasonably consistent. Other boilers introduced in this era that became widely used were on the Dean Goods and Metro classes.


Churchward transformed GWR boilers. His work started under Dean, but during that era one can see a definite progression, worked out it would seem mainly with the 4-4-0s like the Badminton class, and then the early 4-6-0s. What became the standard tapered GWR boilers started out with parallel barrels with a considerably raised belpaire firebox - perhaps 8 inches above the boiler. Some of these had domes and some did not. The taper came later, strongly influenced, it seems, by the Brooks Locomotive Works of the USA. Having noted the Wolverhampton ancestry of narrow gauge design its perhaps appropriate to comment that significantly raised round-top fireboxes were very much a broad gauge feature.

At the same time there was also much development of the earlier parallel boiler types. The development of these is much less obvious to the uneducated eye, but it is clear that there was a great deal of work with all sorts of variations in tube layout and the like. Although various domeless types were tried, in the end it appears it was decided that, with room to make them of generous size as there was with the smaller locomotives, the increase in steam space and freedom from priming provided by a large dome at a moderate cost in weight was justified.

Weight was a significant design factor. The Standard 2 and Standard 4 were variations on a theme to keep the weight, and thus axle loading and route availability within bounds on some classes, but in general Churchward locomotives were within the available limits. However post WW1 schemes were drafted for uprating 2800, 2900 and 4000 classes with the Std 7 boiler. These would most likely have been very potent locomotives, but the enlarged Saint and Star designs could not be managed at all within the required weight limits, and the 2800 would have had reduced route availability.

Top Feed

The location of the water feed into the boiler has a significant effect on the life of the boiler. Boilers tended to pit in the area of the water feed, and dissolved solids would be precipitated as scale on the barrel, affecting the heat transfer.

In the mid Victorian period many locations for the boiler feed were tried, but clack boxes (valves) on the side of the barrel were a common option. Towards the end of the Dean era a location one each side of the back plate of the boiler became preferred. The arrangement is obvious on photographs of Steam Museum's Dean Goods. This arrangement persisted on some classes as late as the 1930s.

After a brief and unsuccessful flirtation with feed on the underside of the boiler Churchward decided on his own arrangement of top feed, inspired by American practice. In the GWR implementation top feed meant that boiler water was introduced through the top of the boiler into the steam space, and delivered onto trays. These trays sloped down to the front of the boiler barrel where they had notched ends so that the water fell onto the main water in the barrel in a series of fine streams. The trays tended to catch the scale and it could be washed out from there.


In the 20thC the major development was superheating. A "saturated" boiler normally has all the fire tubes of around the same size, on the GWR typically between 1⅝" and 2". In a superheated boiler a number of rows of the small tubes are replaced by fewer rows of fewer large tubes, on the GWR typically a bit over 5". Into these large tubes are inserted superheater "elements" (Units in GWR terminology). These are usually tubes in a U shaped arrangement, about the length of the barrel, and depending on the design, on the GWR there were usually 4 or 6 of them in each large tube. After the steam leaves the main boiler space it passes through the regulator and then these tubes on the way to the cylinders. Steam in the main boiler will not heat to above the boiling point of water because of the presence of liquid water, but there is no liquid water in the superheater and the temperature of the steam may be considerably increased, which increases both the power and efficiency of the locomotive.

The primary drawback to the increased steam temperature is lubrication. The high temperatures carbonise the oil, more oil is required to maintain adequate lubrication and the steam passages, valves and pistons become coated with carbon. After early experiments with a high degree of superheat Churchward opted to use only moderate superheat. Superheat, although much tested on various pannier tank classes, was eventually rejected for the smaller classes used extensively for shunting. I understand that it appreciably reduces the speed of response to the regulator which makes shunting more difficult.


Collett immediately ran into the problems with weight that had prevented Churchward from upgrading Stars and Saints with Standard 7 boilers. The Castle's Standard 8 boiler was effectively a Standard 7 with a smaller diameter barrel, intermediate between the Standard 1 and the Standard 7, and brought the locomotive right up to the weight limit. The King's Standard 12 was an enlarged Standard 7 up to a new weight limit for a limited number of routes. The Manor brought another problem since it required a higher capacity boiler (and especially a larger firebox and grate area) than the Standard 4 of the 4300 it was replacing. However the locos needed to be much lighter their sisters, the Granges with Standard 1 boilers, so the all new Standard 14 had to be designed. It came between a Standard 2 and a Standard 1 in all the main dimensions. Collett also introduced high tensile strength nickel alloy steel to build some boilers, mainly Standard 2 for 2-6-2 tanks, of higher pressure but no extra weight.

Collett's other problem was several hundred absorbed locomotives with what must have seemed like several hundred designs of boiler. GWR maintenance practice required a stock of spare boilers, and clearly this would be impractical. So three of the standard boiler designs were modified, mainly with different fireboxes, and these were widely used on rebuilds of absorbed locomotives that could not be readily adapted for existing standard boiler designs. The Standard 9 and 11 were based on Dean era parallel boilers, but with longer fireboxes, and the Standard 10, which was later used on new GWR 0-6-0 and 0-6-0T designs, was a short taper boiler based on the Standard 2/3.

From 1925 a significant design change was introduced on non-superheated belpaire boilers. A pair of 5⅛in. flue tubes were installed in the top corners of the firebox tube plate. This concept, which appears to have been unique to the GWR, was intended to reduce a tendency for the tube plate to crack at this point. This would have brought a slight reduction in boiler efficiency, but an improvement in boiler life. This development was introduced on replacement boilers for 517 class 0-4-2Ts in 1924, extended to 850 and 2021 boiler replacements in 1925, and then included in the boilers for subsequent new classes with parallel saturated boilers, notably 57xx pannier tanks and 48xx 0-4-2Ts. The design wasn't used on unsaturated Standard 10 taper boilers.


Hawksworth must have had similar weight problems when designing the County Class 4-6-0s. A Standard 8 Castle boiler would presumably have been a possibility, especially if Hawkesworth could have accepted the lower boiler pressure, but tooling was available from building LMS 8F (hence W.A. Stanier designed) boilers for the MOD, and this was used to build a notably shorter and stumpier boiler than the GWR designs.

Collett had followed Churchward's lead in using only moderate superheat even when improved heat resistant lubricants became available with advances in refining technology, particularly in the 1930s. While its an exaggeration to say that GWR locomotives would only steam on best steam welsh coal - the Castles on the exchanges with the LNER and LMS used good quality local coal - they were arguably more vulnerable than some other designs to poor quality coal. While good quality coal was available to the GWR moderate superheat brought no steaming problems. However during and after WW2 the quality of coal available diminished significantly and so higher degrees of superheat were introduced, rather later than on other lines. This came at a cost, as consumption of lubricants increased and high superheat locomotives were normally fitted with mechanically driven lubricators and increased oil flow.

The Boiler Classes

Class Name
Mainly Fitted
to Classes
Boiler Barrel Firebox Superheater Total
Front Diameter Rear Diameter Length Heating Surface Length Heating Surface Heating Surface
V 1393 1393, 1361, 1366 3'9 3'10 8'2 816 3'11 75 0 891 10.7 flat
R 850 850,
16-18 Crane Tank
3'9 3'10 10'0 905 4' 0 76 0 981 11.1 flat Extinct by end of 1950s
S 517 517 3'9 3'10 10'0 4'6 0 14.4 flat Classes SA-SD. Built up smokebox. Extinct by 1950s
S 4800 1400/5800 3'9 3'10 10'0 870 4'6 83 0 953 12.8 flat drum Classes SS-SV. Modernised S class
U 2021 2021, 3571, some 517 3'9 3'10 10'0 905 5' 0 90 0 995 14.7 flat Extinct by end of 1950s
Q Standard 16 1600 3'9 3'10 10'1 877 5' 0 80 0 957 14.9 sloping drum Modernised U class
T Metro Metro,
some large
0-6-0T Classes
4'2 4'3 10'6 1125 5'1 101 0 1226 14.6 flat Extinct by 1950s
Q Sir Daniel Sir Daniel, River,
some early
0-6-0T classes
4'2 4'3 11'0 1059 5'4 99 0 1158 15.6 flat Somewhat interchangeable with O (Standard Goods). Extinct by 1940s.
L Standard 11 various absorbed 4'2 4'3 10'6 1008 5'6 82 0 1090 16.8 sloping Modified T class for absorbed locos
F Standard 21 54/64/74 4'2 4'3 10'6 1004 5'6 82 0 1086 16.8 sloping drum Modernised Std 11
M Bogie Single 30xx, 7,8,9,10 4'2 4'3 11'6 1170 6'4 115 0 1285 20.8 flat Extinct by 1915
E Standard 5 4400, 4500, 3900, 4600 small 4-4-2 4'2 4'9 10'6 993 5'10 94 78 1165 16.6 sloping drum Early variants were not superheated.
N Duke Duke, Barnum,
Queen, Cobham,
Dukedog 90xx
4'4 4'5 11'0 1029 5'10 114 81 1224 17.2 flat Earlier variants were not superheated.
P 2301 2301,3521,
later large
0-6-0T, 57xx, 97xx
4'4 4'5 10'3 1013 5'4 103 0 1116 15.5 flat There were very many variants of the P class boiler, with and without superheaters.
O Standard Goods Armstrong Goods,
some Sir Daniel,
some large 0-6-0T
4'4 4'5 11'0 1180 5'4 106 0 1286 15.5 flat Extinct by 1940s. Somewhat interchangeable with Q. There were a few superheated Class O boilers.
W Badminton Badminton 4'4 4'5 11'0 1175 6'4 122 0 1297 18.3 flat Extinct by 1913
J Standard 9 various absorbed 4'4 4'5 10'3 961 6'6 104 75 1140 19.7 sloping Modified P class. Ten were constructed without superheaters.
K Standard 10 2251, 9400, 1500,
numerous absorbed
4'5 5'0 10'3 1070 6'0 102 76 1248 17.4 sloping drum Shortened Std 2, originally for absorbed classes. There was an unsuperheated version used on the 1500s and most 9400s.
B Standard 2 33/3400 (Bulldogs)
(Badminton / Atbara / Armstrong),
some 26xx,
2221, 3101, 30xx
,56xx, 41/51/8100,
numerous absorbed
4'5 5'0 11'0 1145 7'0 122 82 1349 20.4 sloping drum Early variants were not superheated.
C Standard 3 36xx, 3521, 3514,
some absorbed,
some 33xx
4'5 5'0 10'3 1069 7'0 122 76 1267 20.4 sloping drum Early variants were not superheated.
Z Standard 14 Manor 4'8 5'3 12'6 1286 8'8 140 190 1616 22.1 sloping drum
D Standard 4 some 26xx, Cities,
Some Badminton,
3150, 3800 County,
4200, 4300,
7200, Collett 3100,
Barry 0-6-4
4'11 5'6 11'0 1349 7' 0 129 192 1670 20.6 sloping drum Early variants were not superheated.
A Standard 1 2800, 2900 (Saint),
4000 (Star),
4900 (Hall),
6800 (Grange),
6959 (Mod. Hall)
4'11 5'6 14'10 1687 9'0 155 263 2105 27 part sloping drum Early variants were not superheated.
O Standard 15 County 4-6-0 5' 5'8 12'7 1545 9'9 169 265 1979 28.8 part sloping drum
HA Standard 8 4073 (Castle) 5'2 5'9 14'10 1886 10' 164 263 2313 30.1 part sloping drum 14 large tubes, 201 small tubes, 84 s/h elements in 2 rows (6 elements per large tube)
HB Standard 8 4073 (Castle) 5'2 5'9 14'10 1856 10' 163 263 2282 29.4 part sloping drum 14 large tubes, 201 small tubes, 84 s/h elements in 2 rows
HC Standard 8 4073 (Castle) 5'2 5'9 14'10 1800 10' 163 313 2276 29.4 part sloping drum 21 large tubes, 170 small tubes, 84 s/h elements in 3 rows (4 elements per large tube)
HD Standard 8 4073 (Castle) 5'2 5'9 14'10 1670 10' 163 393 2226 29.4 part sloping drum 28 large tubes, 138 small tubes, 112 elements (4 elements per large tube)
G Standard 7 4700 5'6 6'0 14'10 2062 10' 170 288 2519 30.3 part sloping drum Used only on the 4700 class of 9 locomotives, but considered for use in several design proposals.
W Standard 12 King 5'6 6'0 16'0 2008 11'6 194 313 2515 34.3 part sloping drum
F Standard 6 Great Bear 5'6 6'0 23'0 2597 8'0 159 506 3262 41.8 part sloping drum Extinct 1924
This table needs to be read with considerable caution. In particular the heating surfaces varied considerably over the years. Just because boilers were outwardly of the same dimensions and interchangeable doesn't mean that they were the same inside. There were very great differences. Superheated boilers would have much lower heating surface area in the tubes than saturated boilers because the superheater elements were installed in large tubes. Looking at the dimensions over the years there was a definite tendency for the heating surface in the tubes to be reduced, even without changes to the degree of superheating. There seems also to have been a tendency for the size of superheaters to reduce: later versions of the 4 row superheater in Castles had less surface area then earlier ones, but of course still far more than the three row ones.

To give an idea of this I have included the dimensions of the 4 subtypes of Standard 8, fitted only to the Castle Class. Even within those there were variations: for all of them superheater size was later varied, generally reducing slightly with shorter elements.

In general, and with obvious exceptions, I have picked values for sub classes of boilers built in larger numbers during the 1930s, and definitely not those of the post war higher superheat developments.

Boiler Families

Taper boilers

Std 1-4 and 10.

The Standard 4 (D) is a short version of the Standard 1 (A), with both firebox and barrel shorter.

The Standard 2 (B) has very much the same size firebox as the Standard 4, but a smaller diameter barrel of the same length.

The Std 3 (C)was a standard 2 with a shorter barrel.

The Std 10 (K) is a Std 3 with a shorter firebox.

Std 6-8 and 12

The Std 7 (G) had the same diameters as the Std 6 (F - Great Bear), but with a conventional firebox and a much shorter tapered barrel.

The Std 8 (H) is a Std 7 with a smaller diameter barrel and a slightly smaller firebox.

The Std 12 (W - King Class) has the same diamters as a Std 7 with lengthened barrel and firebox.

Parallel boilers

T, Std 11 and Std 21

Std 11 (L) is a T class with a longer firebox, and a Std 21 (F - the code was reused) is a Std 11 with a drum style smokebox

P, N, Std 9

The N class has the same diameter as the P with a longer barrel and firebox

The Std 9 (J) was a P class with a longer firebox

U, V, R, S, Std 16

Std 16 (Q - reused code) is basically a U class with a drum style smokebox.

R,V and S are the same diameter as the U with different firebox and in the case of the V also different barrel lengths.

Boilers used on the 4800/1400 class (SS, ST, SU, SV) were basically an S class with a drum style smokebox.

Boiler Prefixes

This table has been put together from GWR documents, and represents how the GWR saw their boiler stock. It will be noted that the table is incomplete. This is where older boiler types had become extinct by the date the documents were prepared.

Boiler Prefix Description
Flue tube rows Superheater Tube Units
Blower etc Firebox Comments Fitment Pressure Feed
S/1 AH DR Superheater Old type 6 units B Top Feed (combined)
S/1 AK TR Superheater Melesco Units B Top Feed (combined)
S/1 AL DR Superheater Old type 6 units outside blower B Top Feed (combined)
S/1 AM DR Superheater New type 4 units B Top Feed (combined)
S/1 AN DR Superheater New type 4 units outside blower B Top Feed (combined)
S/2 BA SR Superheater Old type 6 units B Top Feed (combined)
S/2 BB SR Superheater Old type 6 units B 225 Top Feed (combined)
S/2 BC DR Superheater Melesco Units B Top Feed (combined)
S/2 BD SR Superheater New type 4 units B Top Feed (combined)
S/2 BE SR Superheater New type 4 units B Top Feed (combined)
S/3 CA SR Superheater B Top Feed (combined)
S/4 DG DR Superheater Old type 6 units B 200 Top Feed (combined)
S/4 DH DR Superheater Old type 6 units B 225 Top Feed (combined)
S/4 DJ DR Superheater New type 4 units B 200 Top Feed (combined)
S/4 DK DR Superheater New type 4 units B 225 Top Feed (combined)
S/4 DL TR Superheater Melesco Units B 225 Top Feed (combined)
S/5 EB SR Superheater Old type 6 units B Top Feed (combined)
S/5 EC SR Superheater New type 4 units B Top Feed (combined)
S/21 FA NSOO B Pannier tanks 165 Firebox Backplate
S/21 FB NS B Converted S/11 165 Firebox Backplate
S/21 FC NSOO B Pannier tanks 180 Firebox Backplate
S/21 FD NSOO B Pannier tanks 165 Top Feed (separate)
S/21 FE NS B Converted S/11 165 Top Feed (separate)
S/21 FF NSOO B Pannier tanks 180 Top Feed (separate)
S/7 GA DR Superheater Old type 6 units B Top Feed (combined)
S/7 GB DR Superheater New type 4 units B Top Feed (combined)
S/8 HA DR Superheater Old type 6 units B Top Feed (combined)
S/8 HB TR Superheater Melesco Units B Top Feed (combined)
S/8 HC QR Superheater Melesco Units B Top Feed (combined)
S/8 HD DR Superheater New type 4 units B Top Feed (combined)
S/9 JA SR Superheater B Side Tank Tdr
S/9 JE NSOO B Side Tank Tdr
S/10 KA SR Superheater Old type 6 units B Side Tank Tdr Pan Top Feed (combined)
S/10 KB NS B Pannier tanks Top Feed (combined)
S/10 KC SR Superheater New type 4 units B Top Feed (combined)
S/11 LA NS B Tender
S/11 LB NS B Pannier tanks
ROD MB DR Superheater Old type 6 units B Top Feed (combined)
113th NA SR Superheater Old type 6 units B Firebox Backplate
3200 NB SR Superheater Old type 6 units B Firebox Backplate
113th NC SR Superheater Old type 6 units B Top Feed (separate)
3200 ND SR Superheater Old type 6 units B Top Feed (separate)
113th NK NS B
S/15 OA TR Superheater Melesco Units B Top Feed (combined)
S/15 OB QR Superheater Melesco Units B Top Feed (combined)
2301 PA NS B Tender
2301 PD NS B Pannier tanks
2301 PE SR None B
2301 PH NSOO B Pannier tanks 200 Firebox Backplate
2301 PI SR Superheater B Tender
2301 PJ NSOO B Improved Pannier tanks 200 Firebox Backplate
2301 PK NSOO B Pannier tanks 200 Top Feed (separate)
2301 PL NSOO B Improved Pannier tanks 200 Top Feed (separate)
S/16 QA NSOO B Pannier tanks Top Feed (separate)
850 RA NS RT Saddle Tank
850 RB NSOO B Pannier tanks
850 RF NS B Pannier tanks
4800 SS NSOO B Side tank Firebox Backplate
4800 ST NSOO B Converted 517 Firebox Backplate
4800 SU NSOO B Side tank Top Feed (separate)
4800 SV NSOO B Converted 517 Top Feed (separate)
Metro TL NS B Side tank
2021 UF NS B Pannier tanks Firebox Backplate
2021 UG NS B Side tank
2021 UH NSOO B Pannier tanks Firebox Backplate
2021 UI NSOO B Side tank
1393 VA NS RT Saddle Tank 150
1366 VB NSOO B Pannier tanks 165
S/12 WA DR Superheater Old type 6 units B Top Feed (combined)
S/12 WB QR Superheater Melesco Units B Top Feed (combined)
1101 YC NS B Side tank 170
2194 YF NS RT Saddle Tank
781 YG NS RT Side tank
7 YL NS B Vale of Rheidol Side tank Top Feed
1331 YW NS RT Saddle Tank
942/1153 YX NS RT Saddle Tank Firebox Backplate
S/14 ZA DR Superheater Old type 6 units B Top Feed (combined)
S/14 ZB DR Superheater New type 4 units B Top Feed (combined)
NS: No Superheater. NSOO: No superheater, two 5⅛ tubes, SR: Single Row of superheater s, DR: Double Row, TR: Three Rows, QR: four rows.
B: belpaire firebox, RT: round top firebox

Return to the GWR Information Index Page.

The original articles on these pages are mainly contractions from my book, "An Introduction to Great Western Locomotive Development", a study of Great Western Railway locomotive classes, which is published by Pen and Sword Books. You may order it from here.

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© Jim Champ, last edit 20 Feb 2024

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