Water Treatment in Central Heating Systems

Water in this context is rarely treated for acidity, mainly because acidity under the gentler conditions is rarely a great problem. Water softening however is often practised for a number of reasons, if nothing more than the supposition that hard water is bad for washing hair. There are really two levels of softening treatment. There is that which employs a softener, a piece of apparatus interposed in the line. Permutit is a long established example of this. The other method consists in treating the water in the feed/storage cistern, by suspending in it a container of crystals which dissolve slowly in the water and inhibit any scale material from depositing in crystalline form. Calgon or Micro-met is obtainable, in suitable containers, from builder’s merchants.

Let us pause and think what we know so far about a preferred type of system. It is very likely small bore, and a two-pipe system. It is indirect, perhaps with water softening applied to the secondary water supply. There is something else, which has become possible only in quite recent times. It may be open or sealed. The system which we all know in the UK, which has one or sometimes two cisterns in the roof, is an open system. It is precisely at these cisterns that it is open – to atmosphere. This means that in the ordinary way it cannot build up more pressure than that which is due to the ‘head’ of water measured up to the cistern level. If there were a surge of pressure it would vent off safely at the cistern.

The open type of system, though it proves satisfactory to most people, has been an irritant to some technical workers for a long time, partly because its supporters have claimed for it blessings which, apparently, the rest of the world cannot see. For it is almost exclusive to us. Not least of the complaints is that it forces us to store a large amount of water in our attics, and, some might say, makes us nationally dependent upon the ball valve.

The alternative to an open system is a closed or sealed system, and this may be accompanied by either cistern fed or mains fed domestic hot water. It must first be understood that sealed systems are capable of building up pressures beyond those of an open system. This means that most of the apparatus – boilers, cylinders etc, supplied for open systems may be neither suitable nor safe. It means too that the general standard, even of making joints, must be higher; and that certain extra items must be fitted, which low pressure systems do not require. Thus in cost terms any economies due to saving a cistern and pipework are wiped out.

If we couple that with a warning, that sealed systems are not yet through the wood in terms of total acceptance by authorities; that high pressure appliances are not yet easy to come by; that the installation work is not for amateurs and not yet for a lot of professionals; we hope that readers will accept that in ordinary circumstances there is a lot of useful life left in open systems.

A sealed primary system is one in which only the primary system is sealed, the secondary continuing to be supplied from a cistern. This vessel, in which a rubber diaphragm separates the water in the system from a cushion of air or nitrogen, takes the place of the expansion cistern in absorbing any expansion taking place during heating of the water.

The advantages claimed for a sealed primary system are: (1) No cold feed/expansion cistern and consequently no freezing problems. (2) No need for the head of a header tank, and the whole system can be accommodated on one floor level. Note that this is advantageous only in a system solely for heating. If domestic hot water is added, a loft cistern is required. (3) It follows from that if convenient the entire system may be accommodated in the loft to save useful space, subject to the same condition as. (4) Sealing eliminates oxygen pick-up, thus reducing one source of corrosion. (5) A sealed system can operate at higher temperatures since the boiling point of water is raised according to the amount of pressure imposed upon the system. This means that smaller heat emitters may be used for a given duty. It will eliminate any tendency to boiling noises which afflict some installations.

It is a fact, but not an advantage, that if higher flow temperatures are used, conventional radiators cannot be used since they will develop unsafe surface temperatures. Convectors must be used instead. A distinct disadvantage of these systems is that, at the present time, they may not be connected to the mains water supply. They must be inspected regularly to see whether there is a loss of water, and if necessary topped up by manual means.

This, it is fair to predict, will be the common pattern of the future, and not only on technical grounds. But to look at the technical factors first, this system begins by claiming those advantages listed for the sealed primary system. It goes on to claim, for the pressurised secondary or domestic hot water side, that: it overcomes complaints about low pressure systems not giving sufficient flow or pressure at appliances. The outstanding example of this is the shower, which in the UK is on average so much worse than continental practice; (2) it makes possible a high performance total water installation on one floor, without need of gaining height ; (3) it clears the way for much smaller and neater water terminal fittings, taps and mixers, and for interconnections of smaller pipe diameter.

The third point is an important one, with economic as well as technical issues involved. If UK water fittings can conform to continental standards in size and performance, which is to say an E.K.C. standard, we can use imported fittings and our manufacturers can make for continental markets without the extra cost of making ‘specials’.

The sealed primary system is being installed, to a limited extent, and if a gas boiler is involved the gas industry is able to set out the limiting conditions for a safe installation.

At the time of writing a lot of high level activity is taking place in an effort to spell out the basic requirements of a wholly pressurised system. This involves the National Water Council and the regional Water Authorities, the Department of the Environment, and interested bodies such as the Institute of Plumbing. The impetus and the ingenuity will come from industry, who will design and make apparatus capable of performing right up to – and sometimes beyond – the limits to be laid down by the legislative bodies.

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