Preparation of hot water in autonomous engineering supply systems is made mainly in two ways: by heating water in instantaneous water heaters or tank water heaters.
An instantaneous water heater is a device in which the heated water flows on its contour; due to the large area of the surface of heat transfer, the intensive process of transferring warmth from the heating medium (electricity or burning gas) to the water.
Obvious disadvantages of this device are a lack of accumulative ability and direct dependence of productivity on the heat input, but the main thing is the probability of formation of scale in the channels of the heat exchanger, preventing the free flow of water, and significant worsening of the heat exchange process. The undoubted advantage is the compactness of the device and the simple installation scheme of hot water supply (see Figure 113) which includes the input of cold water into the device and output of hot water directly to the taps. An instantaneous water heater heats the water only at the moment of demand. Instantaneous water heaters have also another advantage, that the hot water appears at once at full volume and as long as it is needed without decreasing performance.
A tank water heater (boiler) differs from an instantaneous one, having a large volume of stored water inside it. Heating of water to a predetermined temperature in this case takes place in advance and, as a rule, using relatively low power. For example, an electrical tank water heater heats a certain volume of water to the temperature 55–85 degrees C and automatically supports the temperature at the set level. Since heating occurs slowly, such a device doesn’t need high electrical power and can often be connected to an ordinary wall outlet. Even boilers of 150 litres may consume not more than 1,5 KW on half power. Hot water is constantly available in the boiler and when the water is used, cold water comes into it and is heated to the required temperature. To prevent heat loss through the walls of the boiler, they are heat insulated. Thanks to the thick layer of heat insulation, the water inside the water heater cools down very slowly and therefore it is turned on quite rarely.
A working scheme of a water heater (boiler) requires a supply of cold water into the low point of the device and the supply of hot water comes from the upper level. The performance of the device depends on the quantity of heat input or the power of the electrical heater. Also there is a constant supply of already heated water in the boiler which covers the peak loads during periods of intensive water demands. To compensate the warmth expansion of water during heating, a membrane expansion tank is installed into the pipeline system.
Tank water heaters can be made in horizontal of vertical form and this allows optimal placement of the boiler in the boiler room. The main advantages of the device are its accumulative ability and its long life without maintenance or needing to be cleaned. Usually a tank water heater which works from the boiler (see Figure 114) is a «tank in the tank», which is a heat exchanger with functions of a heat accumulator, which is made of two concentric tanks: the inner tank is for sanitary hot water heated from the warmth carrier of the heating system which is contained in the outer tank and which circulates between the two walls.
The inner tank is the «heart» of the boiler: it works with flowing water which is actively corrosive, under high pressure and variable temperature. The tank is made of chrome-nickel stainless steel, welded in a protective atmosphere of argon. Before assembly the convective surfaces of the tank are strengthened and passivated to extend the life of the tank and to improve its practical resistance to corrosion. An undulating profile is given to the outer walls of the tank. This design provides resistance to pressure and limits the deposit of scale by elongation and contraction tank cycles. The outer tank which contains the warmth carrier of the heating system is made of carbon steel. High density polyurethane foam with a thickness of 50 mm is applied to it. On top, the boiler is covered with a polymer polypropylene, a plastic material which provides high resistance to shocks.
There is no universal opinion on calculation of the capacity of the boiler with inclusion of a water heater of indirect heating in its heating contour. Some people think that by inclusion of a water heater into the heating contour, there is no need to increase the capacity of the boiler because the boiler works to its full capacity not more than five days a year when the temperature outside reaches its minimum. Other people think the opposite and they recommend that the capacity of the boiler should be increased by approximately 20%. The reason they think this is that the boiler working most of the time at 50–70% of its load will last longer.
The volume of the water heater is selected according to the calculated need of hot water. According to the norms, the following amount of hot water is required: for one sink with mixer, 0,09; for one bathroom mixer, 0,18; for one shower, 0,09; for one bidet, 0,05; for one kitchen mixer, 0,09 litres/sec.
Schemes of attaching water heaters to the heating contours are almost all of the same type regardless of the type of the connection to the device, top or bottom (see Figure 115). Cold water from the water supply or the pumping station (from connection to a well) goes to the lower part of the water heater. In this case, the entering water must pass the filter-sump or filters for purifying water which destroy lime salt and prevent them from being deposited on the walls of the water heater as scale. Next on the path of the water with pressure in the water supply system of more than 6 bar, a pressure reducer «after itself» is installed in order to that pressure of the entering water doesn’t exceed the permissible norms. After that a check valve must be installed which lets water go to the tank and prevents it from going back. Everything is quite clear with these devices in general. Further consideration of the pipeline is more interesting.
The safety valve and expansion tank are installed on the pipeline of the entering cold water. Why are they here? Consumption of hot water in a house is a cyclical process. Sometimes it is used and sometimes not. For example, at night it is heated in the water heater and expands in volume. And where should the water expand if all the taps are closed? For this purpose, we put an expansion tank on the pipeline of the entering water supply. At this point, the replenishment of the system with hot water is not happening anyway; then let, in the meantime, a part of the water supply work for the expansion tank. For relieving pressure, we’ll put also a check valve nearby. It will work in case of excessive pressure in the water heater and also in the case of excessive pressure from the water supply input if an accident happens and an abrupt pressure jump occurs.
Then cold water is heated in the water heater and is supplied to the taps by the pump. Here it is possible to install a three-way mixer and connect it with the cold water pipeline; then the mixer can be adjusted to a certain temperature of hot water, so it will not be too hot. But another problem occurs during the installation of the pump. When the taps are turned off, and this is most of the time, the pump will work with the valves closed and increase the local pressure in the pipeline but the water will stay in its place and get cooled. In order for the pump not to work idly, it is necessary to include a recirculational pipeline in the system which sends unused hot water back to the water heater. Now when the hot water is not needed, it will circulate in the pipeline ring from the water heater and back to it, forced by the pump. The pump is working, using electricity, but not doing anything useful. Let’s include heated towel racks into the scheme and put them onto the recirculational pipeline. The pump is working anyway, the hot water circulates, so let it do something useful. Let this water dry towels and heat the bathrooms and with proper calculation, you can attach a small system of «warm floors» to the recirculational pipeline.
Now trace the pipelines supply hot water from the boiler to the water heater. If you simply connect the return and supply of the boiler with the water heater then the warmth carrier will constantly circulate along the contour of the water heater and heat the sanitary water in it regardless of whether it is needed or not. In order for the water from the boiler to circulate in the water heater only when it is needed, let’s equip the pipeline of the supply with a two-way thermostat and connect its servodrive with a temperature sensor installed in the water heater. As soon as the temperature of the sanitary water in the water heater reaches the value which we have predetermined, the sensor will send a command to the thermostat and it will stop the flow of warmth carrier from the boiler. Likewise, instead of a two-way thermostatic valve, install a three-way valve in this place which will either open the water to the boiler or send it through a bypass back to the boiler contour. If a separate pump is installed for the supply of hot water to the water heater, then the temperature sensor will send commands to the pump through the controller, either turning it on or turning it off. The liquid circulates around the inside tank and heats the sanitary water. When the desired temperature is reached, the thermostat stops the circulational pump.
Nowadays combined water heaters exist which combine the functions of storing hot water and instantaneous heat exchanger of quick heating. When the supply of warm water is too great and the water heater tank is emptied quickly then the instantaneous heat exchanger starts to heat the water. Usually it is installed on the water heater and is sold along with it (see Figure 116).
The preparation of hot water is also done with the help of a two-contour heating boiler. Usually it has a built-in water heater with a volume between 50 and 200 litres, although many boilers prepare hot water also by instantaneous means like water heaters. Combined boilers which have two systems of heating are used more often. Two contours are working in winter time: the heating of water is done by the water from the heating system and in summertime one contour is working only for the hot water supply.