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Water for your steam loco

in which Andy discusses various types of water available, and which ones may be the best for your steam locomotive.

Distilled water from the Chemists is getting harder to find, and is starting to get a bit expensive. What other options are there for water for my model steam locomotive?

Photo of water dripping from a tap
Photo courtesy of
'Randy Son of Robert'

Tap Water

Probably the simplest option as it doesn't need any setting up, but the most obvious disadvantage of using tap water is that it's full of all kinds of chemicals to make it safe to drink, and in most parts of the UK, tap water is also full of calcium where it trickled through the rocks before reaching the water company. We call this calcium "limescale" when it leaves stains on your shiny kitchen sink, and is just as likely to fur up the inside of your loco as it is to fur up the inside of your kettle.

Consensus: NO. Tap water is for drinking, not for putting in locomotives. Use it to make a cup of tea whilst waiting for your loco to raise steam.

De-ionised water

This is the stuff you can get from Halfords and other "car" shops, and is intended for use in car batteries and steam irons. The de-ionisation process removes mineral ions, such as the calcium we don't need, using a chemical process which makes these mineral ions want to attach themselves to the "ion-exchange-resin" instead of the water, allowing them to be removed from the water. This process is (as far as I can work out) also known as reverse-osmosis.

De-ionised water can degrade brass by removing the zinc, therefore it's not good inside your loco. Apparently, it's also unsafe to drink.
Roundhouse Engineering have more on the subject here

Consensus: NO: De-ionsized water is for your car battery, not your steam loco. Leave it on the shelf in Halfords.


Photo of rainwater coming out of a downpipe
Photo courtesy of
Watt Dabney

Rainwater is a good bet. It's cheap, free from calcium and most of the other stuff found in tap water, and can be easily collected using a gutter and a rainwater butt.

It won't be as pure as distilled water from the Chemist's, but your steam loco doesn't need the levels of purity required for medical purposes. Why isn't rainwater as pure? Rain forms around tiny particles in the air, and picks up stuff on its way down. However, it's good enough for our purposes.

Now, the harvesting mechanism - the gutter - can easily get clogged up with leaves and stuff, which we don't want, so some form of filtration is needed. A leaf trap at the top of the downpipe seems like a good idea, and cleaning the green gunge out of the water butt at regular intervals is also wise.

A final-stage filtration using a domestic water filter (e.g. a "Brita" jug) is recommended by lots of people on the 16mm e-group. This will remove any small particles, and algae, which could otherwise clog up the small pipes in a model loco. A coffee paper can also be used to filter water, but a "Brita" filter will do a better job. (See below for some more technical information on "Brita" filters, and why they should be replaced every month).

If you want to store filtered rainwater, it's best to store it in a dark place, so that algae cannot grow in it; either in a dark container or away from sunlight (you might need to experiment a bit)

Consensus: YES. Use filtered rainwater.

Supplement: A Brita filter uses an ion-exchange-resin to remove some of the impurities chemically (which has no health benefits, by the way, but does help to keep your kettle de-scaled). As this process produces de-inonised water, there is a possibility that water cleaned with a Brita filter may result in degradation of brass fittings.


Like rainwater, snow is free from calcium, and pure enough to stick in a locomotive (once it's thawed, of course). Fresh snow, that hasn't got a layer of car exhaust all over it, is obviously the cleanest, and you can avoid the leaves-in-the-gutter problems.

the biggest problem with snow is having a large clean container to hand in which to thaw the stuff! I ran my snowmelt though a coffee filter paper to remove any last remaining bits.

Consensus: YES (if you have enough snow)

Dehumidifier water

Dehumidifer water is a good bet. As it takes water out of the atmosphere, it's guaranteed not to contain any limescale. If you need a dehumidifier to dry out your house, you'll generate more than enough locomotive water for your own needs.

If you don't need a dehumidifier to keep your house dry, you might find it's a good investment anyway, so that you can obtain plenty of loco water.

A condensing tumble drier also generates quite a lot of fairly pure water, and essentially the water comes from the same source as for a dehumidifier: moist air. In this case, a waste product from drying your clothes (so it's "free")

Water from defrosting the freezer is also clean enough to use. Again, the ice in the freezer should not contain any calcium or other mineral impurities. Catching it all is the entertaining bit, mind you!

Consensus: YES Water from the air is good.

Appendix - How a Kitchen Water Filter Works

Finally, it's interesting to know how a water filter works, and to know why you need to replace it after a month. This appendix comes from the G1MRA, via Jon Rowe.

These usually contain three active components and are designed to make tapwater more "drinkable". In fact they make it less healthy since we need calcium and magnesium ions for our nutrition. They do remove other "nasties" though. The first component in a Brita-type filter is activated charcoal. This removes organics from the water, removing and odour or nasty taste and clarifying the water in the process so it looks more "sparkly". Organics in water are of little concern to us in loco boilers as they don't react much the metal components or cause scaling (in very large amounts you'd get a slimy sludge). The second component in the filter is an ion exchange resin (see 1) which removes the "hard" calcium and magnesium ions and replaces them with sodium ions which are soluble and don't cause scale/scum. The third component is silver particles which inhibit bacterial growth and are there to keep the filter itself healthy to use for the month or so it is in the jug. There are two things to note: i) after sustained use the ion exchange resin will have exchanged all the sodium ions it has and will be exhausted. It is not rechargeable. This means that the water leaving the filter will be just as hard as it was before. ii) after sustained use the silver particles will be ineffective and bacterial growth in the filter may well have occurred, making the water leaving the filter unhealthy to drink. At this stage a Brita filter will simply be acting like a filter paper, removing specks of muck and not a lot else.

Credits: Photos on this page are licensed under a Creative Commons license.