Domestic Hot Water

At various points throughout this post, where it was relevant to the matter in hand, we have mentioned domestic hot water. This is the hot water which is piped to bath and shower, to wash basins and to kitchen sink; very likely to washing machine and dishwasher too.

If the domestic hot water comes from an instantaneous gas fired water heater, or indeed from any existing plant such as a storage water heater which is in good condition, the rest of this post is unlikely to be of interest to the lucky owner. But for the rest we must bring under one heading what people should know about domestic hot water before choosing other equipment which might have a bearing upon it.

We could begin by asking how much water you need, and the answer lies of course with how many people use it, and for what purposes. It is less concerned with the size of the establishment, for we may find a castle holding only perhaps four people, while in a three-bedroom semi there might be seven or eight. Babies seem to need a lot of water. So do people who bath generously, while those who shower instead use far less. The main purpose in thinking about this is to make sure that you do not depart far from the average, and if you do you must do something about it. Do not neglect to include uses other than washing. A lot of people could mean a lot of laundry, and by hand or machine this uses water. A dishwater is also somewhat extravagant in its demand for hot water.

This is represented by a hot water cylinder of 170 litres capacity, with a heat supply of 3 kW. Obviously if we think about 4 and 5 bedroom properties with appropriate numbers in residence then we move to a 214 litre cylinder and up to 6 kW make-up rate. For present purposes it is sufficient to think of 3 kW, and we would not recommend dropping below this level even if the premises is only a one-bedroom flat. With plenty of insulation on the cylinder the supply will last longer.

Ignoring some losses, 3 kW will raise 45 litres of water through 55 deg C in one hour. If we are starting with cold water, which is by no means to be taken for granted, then the recovery rate in the cylinder is 45 litres an hour. People may well ask why we do not aim for a better rate, to avoid so long a wait between baths. But first it would not just be a case of putting more heat in, for there are times when a central heating boiler of say 15 kW rating is working only for the cylinder, and the rate does not improve. The limiting factor is the rate of heat exchange possible in the cylinder. Secondly, how much are you prepared to let hot water encroach upon central heating? Or in a slightly different way, how much extra would you be prepared to pay for a bigger boiler ? For in the first case, when both heating and hot water systems are at work, to take another 3 kW for hot water is to lose it from heating. It is this which gives us that hardy standby, the electric immersion heater. With very few exceptions this is rated 3 kW. The 3 kW/3kW is a very useful form of heater, suited only to cylinders which have the boss at the top, and it is made of two heater elements, one long and one short. These are separately wired to a changeover switch so that only one is at work at any one time. They are usually labelled Bath and Basin or something similar. When only small amounts of hot water are needed, as for washing up for instance, the short or Basin heater is switched on, and it heats only the upper part of the cylinder. For baths, or larger demands, the longer or Bath element is switched on and it heats water over most of the cylinder’s height, as is usual with this type of heater.

Electric immersion heaters are the only form of heater which may be considered suitable for a direct cylinder, and indeed they would very often fail to enter fully a cylinder which contained an internal heat exchanger. Proprietary immersion heaters have a thermostat incorporated, and this is usually preset by the manufacturer. Some provide a knob for temperature adjustment.

There is little to say about the installation of this simple device, except that all wiring and switching must conform to IEE Regulations and these will be allied to local Building Regulations. But there is a feature of operation which crops up year after year, and with no hope of finality we deal with it now. The question is, is it more economical to leave an immersion heateron all thetime, or to switch it on only when required? The answer is the latter. It may well be more convenient to leave it on all the time, but convenience is not economy. The reason is simple. Heat loss from one body to another proceeds at a rate proportional to the difference in temperature between them, in this case the cylinder and the surrounding air. Loss is also proportional to time: a cylinder which is always running is always at maximum temperature and therefore at maximum heat loss. A cylinder is left to go cold loses no heat at that time, and proportionately less than maximum when cooling and warming.

The electric immersion heater owes its continuing existence to its great convenience. It has a low first cost, takes up next to no room, and is operated merely by switching on. Those factors count against the running cost, which is unquestionably high. But if we have a heating system of the electric storage type, which is charged at off-peak tariff rate, then it is advisable to ask that hot water be put on the same tariff, being another facet of storing. This will entail a change of method of operation, for off-peak current is available only during off-peak hours, say 11—7. All the hot water for next day must be made overnight, if the cheap tariff is to be enjoyed, and in most cases this calls for greater storage capacity, and a very determined effort to retain by insulation the heat which is received. These matters can be arranged by discussion with the local Electricity Board. It is not usual for the user to be left entirely at the mercy of his own operational calculation, though. The cylinder may have a second immersion heater fitted, connected to standard tariff current in the usual way, which may be switched on in case the hot water stock becomes depleted. Regular use of that device deserves to be looked upon as an admission of failure, and will certainly cut into any savings made on running costs after the expense of buying a larger cylinder.

This is an appropriate time to kill off another bogey concerning hot water cylinders. It is quite usual, and sensible, to build a cupboard around the cylinder, which has no visual attractions, and to make it into an airing cupboard. It is then not uncommon to assume that since it is now an airing cupboard some provision must be made to introduce warmth. This often results in part or all of the insulation around the cylinder being removed or omitted. There was never a more blatant case of taking a steam roller to crack a nut. Three simple facts must be appreciated. (1) The cupboard is for airing, not for drying. (2) With the best quality insulation it is likely to get, the cylinder will still lose enough heat to keep a mild atmosphere inside the cupboard. (3) What most airing cupboards need most and lack most often is ventilation. This means, foremost, openings at top and bottom, so that warmed moist air can escape and be replaced by fresh air. It also means giving those items which are supposed to be ‘airing’ a chance to breathe, as they have when on the clothes line – not folded flat and bundled deep in a solid heap.

So do not skimp on the insulation for well intentioned but wrong reasons. This applies with equal force to any hot water cylinder, irrespective of the method by which it is heated.

The electric immersion heater with direct hot water cylinder is appropriate either to the special case of electric heating, preferably by off-peak storage, or to the very unspecialised area in which no other issue such as heating is involved. That is, the area in which a direct cylinder with immersion heater giving hot water is the only amenity.

When we come to consider the provision of hot water as a byproduct of a wet central heating system with boiler, taking our own advice and using an indirect type of cylinder, it is unlikely that an immersion heater could be used even if it were wanted. If it is desired as a standby it could be attempted, if the cylinder has a top entry boss. But you may ask why should it be wanted? Surely the boiler solves the problem of hot water quite satisfactorily?

It solves it, certainly, but not with entire satisfaction, as we shall show in a moment. Hot water cylinders in this context may be indirect, or semi-indirect or self priming. If you already have a cylinder, the way to find out which of those two you have is to count the cold water cisterns which are connected to it. A fully indirect cylinder requires two cisterns, one for the primary circuit, which is continuous circulation of the same water, through boiler to cylinder and heating system and back to boiler; the other is the supply cistern for the domestic hot water, and in most houses cold water too.

A self-priming cylinder has only one cistern and one water supply, which sorts itself out inside the cylinder. It goes primarily to the secondary or hot water side, of course, but contributes make-up water to the primary circuit if this is wanted.

There is a structural difference which accounts for this difference in intakes. An indirect cylinder has full separation of the primary and secondary circuits, the former running through a coil or annulus which acts as a heat exchanger, the secondary water being on the outside. In a self-priming cylinder separation of primary and secondary is undertaken by a buffer of air, which is quite stable and adequate to keep the two apart over the whole range of normal operating temperatures and pressures. By the nature of the device however it must be recognised that a physical disturbance well outside the normal range could have the effect of breaking the seal, with temporary mixing of the two water supplies, primary and secondary. This would be very undesirable, since it would in effect return the system to all the drawbacks of a direct system. A disturbance which could bring about such a breakdown would be boiling, perhaps through thermostat failure on a boiler. Another possible cause could be a circulating pump which is far too powerful for the installation, creating great turbulence. It must be stressed however that such troublesome factors are rare, and that self-priming cylinders correctly sized for the job they have to do are in very wide use and quite satisfactory. They are for open topped systems only, and cannot be used with sealed systems such as are described in a preceding post.

For anyone who has a direct cylinder and wishes to change it there is a conversion set made, a coil type heat exchanger which is fitted into the cylinder by way of a boss intended for an immersion heater. Such an arrangement is a regular part of a microbore system.

A fully indirect cylinder has four pipe connections: the flow and return from the boiler, the cold feed and the hot water offtake and vent pipe. In most cases the primaries connected to the boiler work by gravity. It is necessary therefore to see that their vertical progress is maintained. A pipe must never rise, then fall again before rising, for that way lie air locks which will soon shut the installation down. Even horizontal runs are to be avoided, partly because they can so easily lean a little the wrong way, partly because they absorb energy from the flow system. If some horizontal run is unavoidable it should be offset by at least three times the length of vertical run.

Another place to be on guard against an air lock is on the secondary flow pipe. The vent pipe must go upward as straight as possible, with no horizontal run at all. It must never be fitted with a valve. This is wrong, and will encourage air locking at that point. The correct fitting is either a straight tee or a reversed swept tee.

Now we will examine why domestic hot water as a byproduct of central heating is not the ideal system. We know that the present value of the rating of an average domestic boiler is in the range of 13 to 15 kW. This excludes the open fire with back boiler, referring to independent boilers mainly. Its work pattern on an average winter day may be 45 minutes on during every hour that it is at work.

The outstanding difference between the heating and hot water loads is that the latter is almost unchanged across the whole year. Whether the boiler is left for long periods in a potential on position, or whether it is switched on only for an hour or two each day, the fact remains that it is unable to deliver at a faster rate than 3 kW, or less than 25% of its rated output. It will therefore have long periods of inactivity in which to cool off, losing heat through dissipation and having then to reheat the structure of the boiler before coming into fully effective work.

The overall efficiency under these conditions is appalling. The reason why we put up with it is the common one, that it is convenient to do so. Although the logical answer is to separate the water heating function, and give it its own heater rated at 3 kW or thereabouts, this would cost more initially; take up more space, which is perhaps not available; require another connection into the flue; so we do not do it.

Yet the objections are answerable, at least in the case of gas. The cost would be offset by the lower cost of a smaller heating boiler. A circulator, as a small gas boiler is called, may be wall mounted, and it may be obtained in balanced flue form.

A variant of that method, and again the one which belongs to gas, is as before to keep the main boiler for space heating only: but to give water heating to an instantaneous heater, so doing away with the need for a hot water storage cylinder. It should perhaps be noted that this type of heater is forever heating and cooling off, but it is of very light weight construction and so not responsible for heavy losses as is a cast iron boiler.

An instantaneous heater is piped into a boiler hot water circuit so as to be in parallel with the regular boiler. Heater and cylinder have valves fitted on one of the connections, so as to be able to stop any flow in one of the circuits. Thus valve A stops the inlet to the heater, valve B the outlet from the cylinder, and in each case the one valve is sufficient, and easiest to understand. This last is an important qualification, since there must always be reservations about handing out responsibility for valve operation to an unknown and generally inexpert public.

The method of connecting the instantaneous heater is preferably as shown by the broken lines, the water supply coming from the cold supply from the cistern. But there are bound to be cases in which the pressure, or vertical head, from such an arrangement is inadequate to work the heater, which has automatic valves and safeguards built in. In such a case the heater, being suitably constructed, may be connected direct to mains. But if that happens it cannot any longer be piped back into the existing hot water system. This would constitute making a cross connection between a high pressure and a low pressure supply, which is very properly contrary to the bylaws of the water authority. The existence of an isolating valve does not affect the matter. The requirement, that from the outlet of the heater a separate supply must be run to all outlet pipes, with their own taps, tends to make the thing so cumbersome as to be unacceptable. It would be better to go back to the scheme proposed, of removing hot water from the boiler altogether and giving it to an instantaneous heater.

The best working temperature for stored hot water, and the use of a cylinder thermostat to control that temperature, are matters dealt with in a preceding post. Here we will repeat the warning that any means of totally isolating the primary circulation through the cylinder must not be used unless the boiler is capable of instantaneous automatic response to its own thermostat.

While so far we have shown how a gas fired system can be used to achieve better than average efficiency, we must come back to the fact that we live with the average, and that oil and solid fuel do not offer that degree of adaptability. Looking at solid fuel first, there are two levels of appliance. Numerically the most important is the fire back boiler, and there are undoubtedly many people who are forced to light a fire in midsummer because that is their only means of getting hot water. The free standing and more controllable boiler is more adaptable. If the load is sufficient to keep it alight it may be left to slumber with occasional bursts of energy which keep the fire going. It is not in any better case than the fully automatic boiler in terms of working efficiency. It does not dissipate heat by intermittent cooling, but on the other hand it is always consuming fuel at some rate, however small. Although it would be difficult to obtain anything resembling an accurate costing, there is nothing here which justifies us in dismissing the electric immersion heater as a uniquely costly device to run. It does after all operate at 100% efficiency, near enough. A solid fuel boiler mainly slumbering, or a gas or oil boiler spending a lot of time shut off on its own thermostat, may well do no better than 50% efficiency. If we take a rough guess that electricity costs twice as much as any of those other fuels, the net cost comes out the same. The net cost ought perhaps to take account of extra wear and tear on the boiler, which is greater particularly under on/off conditions.

Although the purpose of this section is to consider hot water only from the heating point of view, leaving the plumbing work for Section 3, it is relevant to mention one aspect of hot water plumbing. This is the size of pipe to be used to convey hot water to taps. When the tap is open this pipe carries hot water; when the tap is shut the pipe is full of hot water, which slowly cools. When the tap is reopened the first stream of water is cold. This is called ‘dead leg.’

All the rules of fluid flow indicate that we should use pipes of fairly generous proportions, so that friction is reduced and the tap is able to give a rapid discharge rate. But the more generous we are, the greater the volume of ‘dead leg’, with its accompanying inconvenience and sheer waste. Consequently hot water pipes are always calculated to be big enough but in no way generous in diameter. The desirability of planning to cut down the length of runs will also be obvious.

All the comments made about boilers apply to those mainly full scale back boilers built into combination units for gas or oil firing, and to the larger capacity hearth units which are the closed or closeable solid fuel room heaters with back boiler.

Instantaneous heaters are available for bottled gas, and are much in demand for mini-domestic conditions, in caravans and yachts. The only practical application for a domestic electric instantaneous water heater is in a shower, in which a very small quantity of water can be fashioned into a useful spray by a well designed rose. The limitation upon this type of heater is the electric loading which would be needed for say basin or bath filling – well beyond the capacity of a normal domestic meter.

Though we are considering domestic hot water as a service based upon a central heat source and a pipe network, this image does not have to be pursued regardless of common sense. It can often happen that a house or other type of dwelling is so constructed, or modified, that most of it lends itself to a compact pipe system: but that there is one part, perhaps a wing, an extension or the new cloakroom which used to be the laundry room, which stays obstinately outside any neat solution. If the system has to be extended to run hot water to a basin or shower it will mean a long pipe run, hot and cold, with nothing else on the way. So there is the cost of pipe and labour, the inconvenience, the continuing heat loss, the long dead leg, and particularly due to the last, a not very satisfactory service. This is a clear case for not extending the system, but for treating this outlet on its own, with saving of material, perpetual higher efficiency through less heat losses, and high user satisfaction.

It may be done in one of the following ways: 1. By single point instantaneous gas water heater, possibly balanced flue type since the presence of a convenient flue would be a coincidence. This would involve running one cold water pipe, from main or from cistern, and a gas pipe; or in a non-gas situation having a bottle of gas nearby. This type of installation is adequate for a sink or wash basin, or for a shower so long as the shower equipment is carefully chosen to complement the heater. 2. By electric storage heater of the smaller size, usually not more than 20 litres capacity. This may be of the free outlet type, to be situated above the outlet point; or it may be a pressurised heater which would stand under the floor, most usefully under the basin so that dead leg is almost nil.

Either of these heaters may be fed directly from mains or from the cold cistern supply. The free outlet or displacement type of heater is open topped and not itself under pressure. By opening a tap the user allows cold water to enter the heater, and this pushes out hot water. It must not have a control fitted to the outlet. The pressurised type of heater must first of all be bought for high pressure use. It must then be fitted either with an expansion pipe or with a pressure relief valve, and before undertaking to of these the user should consult the local Electricity Board for detailed advice.

Up to the useful capacity, say 20 litres, either of these heaters may be used for a variety of purposes at any reasonable rate of flow. The free flow heater cannot be used for a shower since it must not have a mixing valve, a form of restriction, imposed upon its outlet.

The only services to be supplied are cold water and electricity, and perhaps in one case a vent pipe. 3. The third possibility, useful for little but a shower or a basin spray tap, is the instantaneous electric heater which we have discussed already. It would usually need to be 6 kW loading. Its requirements are a cold water supply and electricity.


The domestic hot water supply is inescapably at the mercy of whatever the local water supply has in store for it. We have shown how to protect the primary circuits by the use of an indirect system, excluding contact with continuous raw water. The domestic system has to suffer it, whatever it is. Things are not generally as bad as that may suggest. Troubles due to soft acid waters are rare since copper tube came into general use. The effect upon lead was of course potentially dangerous. But hard waters are different, since they do not attack their surroundings but simply deposit their hardness as solid matter when heated. If by the use of a chemical additive such as Calgon these deposits can be kept as a sludge to be washed away, not allowed to adhere as hard scale, then trouble with heat exchangers and pipes is long deferred. If not, the time must come when those items have to be defurred, and if this can be done by chemical descaler it saves dismantling the system. Signs of trouble are that the hot water gets less hot, and/or that the flow rate decreases.

The least harmful but most annoying symptom is probably the slight stain or encrustation which develops, like a junior stalactite, on bath or basin if the tap has a tendency to drip, perhaps when being closed. If the bath or basin is wiped down after dripping stops this may be kept off indefinitely. But if it does occur, it will usually be removed by being wiped with a cloth on which is a little proprietary scale solvent solution, then rinsed with clean water. But ask the bath manufacturer before applying this treatment, if you have a new and perhaps plastic bath.

To attempt to sum up this post: 1. Wet heating systems usually make provision in the size of heater to supply hot water as well as heating. 2. In terms of logic and efficiency this is not the best arrangement but is cheaper in first cost and takes less space than 3. The separation of hot water from heating by giving hot water its own heater. 4. A separate water heater cost is partly offset by needing a smaller heating boiler. 5. Means of providing hot water at about 3 kW rating include electric immersion heater, gas circulator; instantaneous gas water heater. 6. Electric immersion heater goes with storage radiator system at off-peak rate. 7. Electric immersion heater may cost no more to run than a large boiler running at 3 kW load. 8. Hot water cylinder should always be well insulated. 9. The self-priming cylinder is a form of indirect cylinder. 10. Instantaneous multipoint heater may be fitted into a wet system for summer use on hot water service. 11. The use of a cylinder thermostat. 12. Avoidance of excessive ‘dead leg’. 13. Domestic hot water for the remote part of the house. 14. Maintenance of the hot water side.