Technical Information

This is a page of a few extra facts, figures and technical information that you may find of help if you are using Roundhouse parts in your home builder projects or just of general interest.


Scale & Gauges Explained

The multitude of scales and gauges that are in use for model and miniature railways can be quite confusing, particularly with the smaller sizes, so here is a list of the more common one's.
We start our list with '0' gauge as this is really the smallest that is normally used for live steam. Although working steam models are built for scales/gauges smaller than '0', these are not common so do not appear on this list.
  • Gauge '0' (UK)
    7mm:1foot scale (1:43.5) running on 32mm gauge track. Models are of standard gauge railways.
  • Gauge '0' (USA)
    1/4":1foot scale (1:48) running on 32mm gauge track. Models are of standard gauge railways.
  • Gauge '1' (USA)
    3/8":1 foot (1:32) running on 45mm gauge track. Models are of standard gauge railways.
  • Gauge '1'(UK)
    10mm:1foot scale (1:30.5) running on 45mm gauge track. Models are of standard gauge railways.

    NOTE although certain scales are marked as either USA or UK, this is not a strict split of usage and in practice, both versions are found all over the world. I have marked them in this way to indicate where they are more commonly used. There are also a number of different sub divisions within the gauge field, i.e. 'finescale' 'standard' 'prototen' etc. but these relate mainly to track and wheel standards.

  • 1/2" scale (USA)
    1/2":1 foot (1:24) running on 45mm gauge track. Models are of narrow gauge railways.
  • 'G' scale
    13.5mm:1 foot (1:22.5) running on 45mm gauge track. Models are of narrow gauge railways. NOTE, Some models described simply as 'G' scale are actually built to 1:20.3 to represent 3 foot gauge equipment, LGB Porter, Bachman and other makes.
    Getting complicated already aint it!
  • 5/8" scale (USA)
    5/8":1 foot (1:19.2) running on 45mm gauge track. Models are of narrow gauge railways.
  • SM32
    16mm:1foot scale (1:19) running on 32mm gauge track. Models are of narrow gauge railways.
  • SM45
    16mm:1foot scale (1:19) running on 45mm gauge track. Models are of narrow gauge railways.
  • 7/8N2 (USA)
    7/8":1 foot scale (1:13.7) running on 45mm gauge track. Models are of narrow gauge railways and represent 2 foot gauge.

    Garden railway scales and gauges
    A little extra information is added for what are generally termed the Garden railway scales as there are several which are normally used together and can cause confusion. Although any railway laid round a garden can be called a garden railway, today the term is normally applied to Gauge 1, 'G' scale, SM32 and SM45.
    'G' scale, SM32 and SM45 are normally grouped together as size compatible, despite the scales being slightly different. Though we have two different gauges, 32 and 45mm, all three represent narrow gauge models and have similar overall sizes of locomotives and stock.
    Many locomotives from ROUNDHOUSE have adjustable wheels to allow them to run on either gauge or are available in two versions, one for either gauge.
    Because they represent narrow gauge stock, they are capable of operating round quite tight radius curves as small as 2 foot (600mm) radius. This means that a railway can be laid in quite a small area. In the USA, the situation is a little more confusing, as they also have 1/2" and 5/8" scale narrow gauge using 45mm.
    At Roundhouse, we put far more emphasis on looks rather than an exact scale in an attempt to keep everything looking right. In the case of our 'freelance' designs such as Lady Anne and Millie, no one can say just what scale they are as there are no prototypes for these and they could actually be said to be full size locomotives in their own right, built to operate in a miniature environment.
    Generally speaking, most of our models are built to fit in with a railway built to 16mm to 1foot scale (1:19), though we do design US and European models (e.g SRRL#24) to a smaller scale (as generally, the protoypes are larger than UK locomotives) to be size compatible with 'G' scale.
    If you compare the loading gauge of all our models, you will find them roughly similar, regardless of any scale that they may be built to. This is done so that you can run them with any 'G' scale, SM32 or SM45 equipment anywhere in the world, and they will look right.
    We always make a point of stating that our models are for 'G' scale, SM32 or SM45 rather than choosing one particular scale which, though it would produce a scale model, may well not suite everyone's requirements.

    Gauge '1', though using the same (45mm) track gauge as 'G' scale etc. is quite a different animal. Built to the smaller scale of 10mm to 1 foot (3/8" to 1 foot USA) it represents standard gauge equipment and really is at its best with long sweeping curves.

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    Service Sheets

    We are slowly building up a database of service sheets that will be of help to Roundhouse locomotive owners in the repair, maintenance and troubleshooting of any problems that may occur in their locomotives.

    SS01 Forney valve gear checking and re-setting.
    SS02 Walschaerts valve gear setting.
    SS03 Care of gas system.
    SS04 Slip-eccentric valve gear setting (early internal versions).
    SS05 Servo smoothers.
    SS06 R/c steam regulator adjustment and service.
    SS07 Water top up valve overhaul.
    SS08 Loco driving wheel change.
    SS09 Hackworth valve gear setting.
    SS10 Forney flexible steam pipe replacement.
    SS11 Rheidol whistle valve.
    SS12 Silver Lady smoke box front change.
    SS13 Russell pony truck adjustment, including gauge conversion.
    SS14 Fitting instructions for slide bars, crossheads and dummy combination levers.
    SS15 Flexible water pipe connection on Mk1 S.R&R.L #24.

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    All Roundhouse models are fitted as standard with centre buffers, except for Tom Rolt, which has the option of a centre hook and outside buffers as used on the Tallylyn Railway.
    This type of coupling has a hook on top of the buffer shank, just behind the head, and uses a three link chain which sits over the hook and over the top of the buffer head as shown in the photographs below. Most models are fitted with cast metal centre buffers however, some later engines have them formed from a stainless steel laser cut part. In all cases, the method of coupling is the same and a three link chain sits over the top of the buffer head and links the two hooks.
    Please note that with the laser cut stainless steel couplings, although the buffer head has a similar profile to a chopper coupling (though upside down with the cutout to the bottom), it is not intended to work as such and will not couple to other chopper couplings. The cutout is purposely placed at the bottom to prevent the chain from becoming wedged in it. Below are pictures of the laser cut centre buffers in use and are shown coupled to a Silver Lady fitted with the cast centre buffer.

    There are many other types of coupling used by other manufacturers or available from after market suppliers. The fitting of these alternative couplings is left to the customer and some drilling or other modification is often required to the locomotive buffer beam in order to achieve this.

    Taliesin and Silver Lady
    Silver Lady (cast) coupling to a Taliesin (laser cut).

    Darjeeling B with Silver Lady
    Silver Lady (cast) coupling to a Darjeeling B (laser cut) .

    Leek & Manifold with  Silver Lady.
    Silver Lady (cast) coupling to a Leek & Manifold (laser cut).


    Classic series slide valve cylinderPiston valve cylinder
    Slide valve cylinder on the left and piston valve cylinder on the right.

    All current Roundhouse locomotives are fitted with a pair of 9/16" bore x 5/8" stroke double acting cylinders, which are operated by slide valves housed in a valve chest on top of the cylinder block.
    Piston valve cylinders were used on the Forney model produced between 2000 and 2004. There are several variations of the slide valve cylinders and though they look slightly different externally, bore and stroke are the same.
    The cylinder block is manufactured from brass, all glands and the piston ring are silicone '0' rings and gaskets or '0' ring seals are used for all covers and joints. They are designed for easy maintenence and will give a lifetime of excellent performance and service, requiring only the occasional replacement of seals.

    Slide valve cylinder overall dimensions
    Slide valve cylinder general arrangement. All dimensions in millimetres.

    Currently, only the slide valve cylinders are available to home builders. They are mounted to the frames by two M3 screws, and come complete with dummy crossheads, inlet and exhaust pipes attached and instructions for installation and maintenance.
    The valve gear can be anything you care to make, or, if building valve gears is not your thing, the Roundhouse Walschaerts or Hackworth sets are detailed elsewhere on this page. Full valve travel is .140" ( 3.56mm).

    You can download the instructions for this item as a PDF file from our 'DOWNLOADS' section.

    Slide valve cylinder exploded diagram
    CLICK HERE for an exploded view of our standard slide valve cylinder.

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    Spirit Firing System

    From 1982 to 1994, Roundhouse produced spirit fired locomotives.
    These were externally fired and featured a firebox round the boiler to contain the heat and keep out the weather. Spirit was contained in a tank beneath the cab floor.
    The growing popularity of gas firing, which required no setting up by the customer and offered instant control of the fire, led to a decline in the popularity of spirit firing. Also, 'methylated spirit' was fast becoming harder to get hold of and what you could get, was often of very poor quality. Consequently, the demand for spirit fired locomotives dropped to a level where it was no longer ecconomic to manufacture them.
    If a good burner/firebox/boiler combination is used then, in performance, it is the equal of gas firing and, as a method of producing steam, it does it's job very well. It does however require far more input from the operator in terms of maintenance and setting up.
    Wicks require regular cleaning and re-packing or replacement and this in itself is something of an art which many failed to grasp.

    Spirit firing arrangement

    For a closer look at spirit firing and how to get the best out of it, go to our 'An Introduction to Spirit Firing' section

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    Gas System

    First, a little background -
    Roundhouse haven't always produced gas fired locomotives, in fact, untill 1988, all models were spirit fired, but it was introduced at that time to allow greater flexibility in design.
    A single closed flue system was adopted as components were readily available from specialist manufacturers, and it was simple and compact. After some years of using bought in parts, and finding the problems and limitations of the system, Roundhouse decided it could produce a better system 'in house'. During 1994/5, a great deal of time and money was spent on developing a new gas system that would do just what we wanted when we wanted. With the help of a number of specialist's in the gas field, including time in the Department of Fuel and Energy laboratory at Leeds University, the 'FG' gas burner was eventually developed which was easy to manufacture and is efficient, quiet and weather proof.
    The recent introduction of our 'Budget' range, meant another lengthy development stage to produce a similarly good system for use with externally fired boilers. The 'FX' burner was the result and once again is quiet, efficient and pretty weather proof.
    Gas is carried on the locomotive in a purpose built storage tank which is filled from a gas cannister as used for filling cigarette lighters or larger types used for small blowlamps or camping stoves etc.

    Brief technical notes -

    How it works 1
    Single closed flue gas system as used on internaly fired Roundhouse models.

    Gas, stored in the tank on the locomotive, passes through a needle valve regulator, then on to the gas burner. The burner has the job of mixing precisely the gas and air, so allowing it to burn correctly in the closed flue. The hot gasses pass along the flue, transferring heat into the surrounding water, and exit via the chimney along with exhaust steam from the cylinders. A superheater, carrying steam to the cylinders, also passes down the flue tube to ensure that the steam reaching the cylinders is as dry as possible.

    'FX' Gas system
    Externally fired system using 'FX' type burner

    With this system, air passes up through the bottom of a firebox which surrounds the boiler. The gas burner is mounted beneath the boiler and hot gasses from this pass up, around the outside of the boiler and exit at the top. The firebox controls the passage of the hot gasses to ensure that maximum heat is transfered into the water inside the boiler and keeps out any unwanted cold air and wind.
    The main steam pipe passes through the firebox in large loop around the inside. This gives a certain amount of re-heating to the steam on it's way to the cylinders, but it does not afford the same level of re-heating as with the closed flue internally fired system.

    The most important thing to realise is that gas firing is not just down to the burner. We are dealing here with a complete system and many factors come together to make it work correctly. You can not just buy a Roundhouse burner, fit it in your own boiler and expect good results. Because of the requirements of a closed flue system of this size, every aspect of the entire system has to be 'just right' in order to get good performance and very small alterations or variations can throw the whole thing into dissarray. Every care is taken at the factory to ensure that ready to run locomotives are set up correctly and every aspect of it's operation is carefully checked, culminating in an exhaust gas analysis test to see just how efficiently it is working.

    Ordinary Butane or Iso-butane gas (as used in gas cigarette lighters) is the preferred fuel, though for economy, the larger canisters as used for blowlamps or camping stoves etc. are better. The larger canisters have an EN417 threaded self sealing valve on top and require a special adapter to couple up to the filler valve on the locomotive.
    Mixed gasses, i.e. Butane with a proportion of Propane mixed in, are available, and may be used on current models (see details below) if straight Butane is unavailable. These come in a variety of mixes ranging from 90/10 to 60/40 with one of the most common being 70/30. The figures refer to the proportions of the mix i.e. 70/30 contains 70% butane and 30% propane. If using mixed gasses, always choose the one with the largest proportion of butane. The addition of propane slightly alters the gasses properties. This can make the burner a little more difficult to light when cold or after filling the gas tank. Always open the regulator very slowly when lighting, and only just sufficient for ignition to take place. Opening too much too soon may extinguish the flame until the burner reaches normal operating temperature.
    Due to the higher storage tank pressure of mixed gasses, they should not be used in the following models.

    (1) All Roundhouse Beck locomotives.
    (2) Early 'Lady Anne', 'Dylan', 'Old Colonial' and 'Charles Pooter' models with external gas firing,
    (3) Early 'Lady Anne' and 'Dylan' models with rectangular gas tank in the right hand side tank.
    (4) Any models with 1" square, vertically mounted gas tank in the cab.
    (5) Early 'S.R. & R.L #24' and 'Fowler' with one piece rectangular gas tank.

    Replacement gas tanks suitable for mixed gasses are available for models in catagories (5) & (6), enquire for further details.

    Basically, we have an excellent system that works well but if you want to incorporate our parts into your own locomotive, ask our advice. We do offer certain parts in our range of 'Home Builder Parts', but it is not always a simple matter to achieve the correct results.
    A range of shapes and sizes of tank are available, CLICK HERE for full datails

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    Valve Gear

    Outside Walschaerts

    Simplified Walschaerts valve-gear
    The simplified Walschaerts type valve-gear used as standard on many Roundhouse models.

    We use the description 'simplified' because it does not include a functioning combination lever/union link/drop link, it does offer forward and reverse running but not variable cut off. Used in conjunction with the Roundhouse cylinders, very slow steady running is easily achieved.
    Quality materials are used throughout - rods and links are laser cut in stainless steel - return cranks and lifting arms in cast nickel silver - stainless and silver steel also used in other parts where appropriate.
    Available as a complete set - not including coupling or connecting rods - for the home builder and includes full instructons for assembly and setting.

    CLICK HERE for adjustment information on Roundhouse Walschaerts valve gear.

    Simplified Walschaerts valve-gear

    General arrangement of valve gear - coupling, connecting rods and outside cranks shown for referance only but are not included in the valve gear set. All dimensions in millimetres.

    Simplified Walschaerts valve-gear dimensions
    Dimensions for Roundhouse simplified Walschaerts valve gear.

    You can download the instructions for this item as a PDF file from our 'DOWNLOADS' section.


    Hackworth valve gear on Carrie
    Hackworth valve gear.
    Hackworth valve gear was used extensively by a number of British locomotive manufacturers on their smaller industrial engines as it is quite simple and compact. It is available as a complete set - not including coupling or connecting rods - for the home builder and includes full instructons for assembly and setting.
    Please note that the special inclined cylinders with horizontal valve chests are required for this set, it is not suitable for use with our standard cylinder set.

    CLICK HERE for adjustment information on Roundhouse Hackworth valve gear.

    Hackworth valve-gear

    General arrangement of mounting holes required for Hackworth valve gear set. All dimensions in millimetres.

    Hackworth valve-gear dimensions

    Dimensions for Roundhouse Hackworth valve gear.

    You can download the instructions for this item as a PDF file from our 'DOWNLOADS' section.

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    Displacement Lubricator

    Displacement lubricator
    This simple but effective device, takes care of lubricating the inner working parts of the cylinders. It is called a displacement lubricator because of the way the condensed steam displaces the oil in the chamber.
    It comprises a hollow chamber with the steam pipe passing through and has a filler cap at the top and drain at the bottom, both of which seal against the internal pressure.
    The steam pipe has a small hole in it, within the chamber, to allow the transfer of steam and oil.
    Before use, the lubricator is filled with a special type of oil that is formulated to mix with water and withstand high temperatures - this is known as 'steam oil'.
    As steam passes along the steam pipe and through the lubricator, some of it enters the chamber via the small hole in the pipe. Because the lubricator is cooler than the boiler, the steam condenses into water which then sinks to the bottom. This in turn displaces the oil upwards and a small amount passes out through the same hole and is carried along with the steam to the cylinders.
    This means that while ever the locomotive is running, a constant oil mist is being transported to the inner working parts of the valve chest and cylinders.
    Periodically, the water must be emptied out of the bottom via the drain screw, and the chamber re-filled with more oil.
    There are other slight variations to this design, some which are placed on the end of a seperate pipe rather than have the main steam pipe passing through it, and some which also have an integral needle valve to adjust the oil flow, however all work on the same basic principle.
    All Roundhouse locomotives have a lubricator similar to the one illustrated though the drain screw position may vary.

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    Steam Oil

    'Steam oil', to use it's common name, comes in many forms and weights and often under some other name. There seems to be a commonly held belief on this subject, that 'thicker is better', but this is just one more 'old wives tale'. Also, to say that a certain viscosity of oil is correct for all, without any other qualification (eg. at certain steam pressures, temperatures or pipe size etc) is a little misleading. It may work fine in one design of engine, but be less suitable for another. By choosing the correct weight and composition of oil, you can obtain a more efficient lubrication.
    There are a number of things to be taken into consideration when deciding which oil to use such as, temperature of steam, size of piping, point of entry into stsyem, type of lubricator etc. etc.

    The current oil supplied by Roundhouse is, to use it's correct title, 'Compounded Bearing Oil 220' (220 being the weight or viscosity) and was chosen after consultation with the oil's manufacturer and discussion as to its precise application and working environment. The change of oil was brought about a few years after Roundhouse adopted internal gas firing and a small number of engines were experiencing blocked super heater pipes after prolonged use. The blockages were caused by carbonised oil, though this had never been a problem with the earlier externally fired models. The key points that dictated what oil to use were as follows.

    1/ Working on a (relatively) low steam pressure of 40psi.
    2/ Displacement type lubricator positioned in cab, so 'wet steam' pickup.
    3/ Long pipe run (again, relatively) between lubricator and cylinders.
    4/ Small diameter of piping.
    5/ High degree of superheating between lubricator and cylinder as superheater heated directly by gas burner and will often be glowing red during operation.

    What was needed in this case was an oil that would be picked up and carried by the (relatively) low pressure wet steam, then travel through all the small diameter piping, through the high temperature of the superheater (without being carbonised), and arrive in the correct proportion in the cylinders where it cools again before doing its job.
    All of this pointed to a medium weight oil so that it would pick up and pass quickly along the narrow pipes and one with a low 'solids' content so that carbonisation in the superheater was kept to a minimum. The 'solids' reffered to, are additives such as tallow and other fats, which are used as they are a good lubricator in wet conditions.
    To deliver the correct amount of 220 weight oil, the feed hole in the lubricator had to be reduced in size as the now thinner oil picked up and flowed far more freely.

    We offer the above case both to illustrate the fact that no one oil is 100% suitable for all needs, and to explain why the current Roundhouse oil is so different from it's predecessor.
    Please note that using the current 220 oil in an older Roundhouse loco that was originally supplied with the thicker oil will not cause any problems other than an oily engine. Using thicker oil in the current models should not be a problem as far as lubrication is concerned (slightly less getting to the cylinders), but may lead to long term carbonisation in the superheater.

    When choosing an oil, you should look at the particular requirements of the model in question. A low pressure oscilating cylindered loco with displacement lubricator will need quite a different type to say a coal fired engine running at 90 psi with oil fed by pump directly to the cylinders. Also, if you are playing about with or changing between different weights of oil, a lubricator with adjustable feed rate is desirable as the this can vary considerably.

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    Safety Valve

    Saftey valve diagram

    Typical safety valve
    Safety valve on a Silver Lady. Position does vary on other models.

    This device controls the maximum pressure allowed to build up inside the boiler and is an important safety device. All Roundhouse locomotive boilers have a single safety valve set to prevent pressure from raising above 40psi (pounds per square inch). The unit screws into a bush in the boiler with a 5/16" x 32 ME thread.
    As can be seen from the diagram above, it is a simple device comprising a rubber seal mounted on a stainless steel or brass spindle which is held in contact with it's seat by a stainless steel spring. The spring is compressed by a screw-in ring in the top of the valve body and is set so that a steam pressure of 35 to 40psi will lift the seal and spindle from the seat. This allows the excess steam to pass through and exit the top of the valve via four slots which are machined up the sides of the screw-in ring.
    These are a slow release type valve which means that it will start to open the valve before the maximum pressure is reached. The nearer the maximum, the more the valve opens so that by the time 40 psi is reached, it is fully open. Note that there will be a small amount of steam escaping from about 10 psi below the maximum level and that will slowly increase as pressure rises.
    The valve can be adjusted to compensate for the '0' ring and spring 'bedding in' and, even on a new locomotive this is sometimes necessary.
    The center, screwed ring, has four slots around the edge into which fit the two prongs of the safety valve adjuster tip of our 'Multitool' or, the tips of a pair of thin nosed pliers. Adjustment of the valve should be carried out with the locomotive in steam but take great care as very hot steam is blown upwards out of the valve during the operation. The Roundhouse 'Multitool' has a specially designed adjuster that channels the steam out sideways but it is advisable to wear safety gloves to protect your hand.

    Multitool adjustment. Adjusting with thin nosed pliers.
    Adjustment of safety valve blow off pressure with 'Multitool' and thin nosed pliers.

    If the blow off pressure is too low, slowly screw the center adjusting ring clockwise to increase pressure on the internal spring. If the blow off pressure is to high, turn the ring anti-clockwise to release pressure on the spring. Make small adjustments at a time and wait for the boiler pressure to settle between movements. When assembled and set at the factory, a small dimple is pressed into the body of the valve near the top. This is to prevent the adjuster ring from moving on it's own, but the friction it produces is easily overcome during adjustment.
    The procedure above assumes that the pressure gauge on the locomotive is reading correctly. With older models, it is advisable to have the gauge checked against a known, correctly reading gauge before attempting adjustment. Most valves only require a small adjustment to obtain correct operation, often only 1/2 to 1 full turn so, if you have to screw the adjuster ring in more that two or three turns, or it is not possible to adjust the valve correctly, then it either requires a new spring and internal seal or replacing completely.

    Remember, this is a safety valve and is fitted to protect the boiler against over pressurizing and must function correctly. Do not adjust it to allow the boiler pressure to rise above 40 psi. If in doubt about the above procedure, please contact the factory and if there is any doubt about the functioning of the valve, then replace it.

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    Well, water is just water isn't it?.
    Actually no, it's not quite that simple.
    Our miniature steam boilers are made mostly of copper, with some bronze and silver solder and there are also brass steam fittings for the steam to negotiate before it exits to atmosphere. Although all these items are made from non-ferrous metals and therefore do not rust, they are subject to chemical attack in other ways. Here is a quick run down of the common sources of water with their pros and conn s.

    Tap water - in soft water areas, this is fine but in hard water areas it can quickly lead to a build up of 'lime scale' on the inner surfaces of the boiler, fittings and pipe work. The easiest way of establishing what your water is like is to look inside your kettle. If it's thick with white 'lime scale' deposits, avoid using it.

    Rain water - free of charge and quite good if you can get sufficient quantity. It must be filtered to get all the dirt, grit and other crap out of it and the filters used by home brewers and wine makers does an excellent job.

    De-ionized water - this is often sold for use in steam irons and the general opinion amongst the small scale live steam community is that it should not be used. Because of the way it is 'purified', it can cause long term problems by slowly removing zinc from the brass fittings - commonly called de-zincification.

    Purified water - tricky one this, as it is not always clear how purification has taken place. Shops that sell it will variously tell you it is de-ionized, or distilled. If you can't be sure that it is distilled, don't use it.

    De-humidifier water - another good source. A de-humidifier is a bit like a fridge in reverse, and the water that collects in the tank is quite safe to use.

    Refrigerator/deep freezer ice - good. It is basically moisture in the air that has frozen and once thawed out is good to use if you can get sufficient quantity.

    Distilled water - the best water you can use. Unfortunately, it is difficult to get hold of. If buying distilled water, be very sure that it actually is. Some retail outlets will sell 'purified' or 'de-ionized' water (for steam irons or car batteries) and tell you it is distilled but it is actually quite different.

    You will probably find that a combination of these will supply all your needs but if you store or stock pile collected water, be sure to filter it well before use and change the containers from time to time. If not, you will find algae and other deposits forming in the water which will cause steaming problems. Dirt, algae and other debris can cause the water to foam as it boils and this will cause priming to the cylinders and syphoning at the safety valve.

    If you arrived at this page directly from a search engine or a link from another web site, please CLICK HERE to go to the full ROUNDHOUSE web site.

    © 2002 Roundhouse Eng. Co. Ltd.