Contemporary tendencies in the development of heating systems more and more incline to low-temperature floor and radiator systems in which the temperature of the supply of warmth carrier is much lower than the temperature given by the boiler. How can one achieve a flexible regulation of temperature of the warmth carriers if the outdoor temperature is constantly changing?
For low temperature heating systems and radiant heating systems, it is necessary to follow those technical decisions requiring cooled water from the return pipe to be mixed into the supply pipe. This process is called quality regulation of heating systems, i.e. there is regulation in which the supply of warmth carrier remains the same but the temperature of it changes to what we want, and at the same time we do not interfere with the work of the boiler and its circulational pump. Quantity regulation of heat system differs from the quality one in that in this case the temperature of the warmth carrier is not changed but the supply changes, i.e. a valve is installed on the pipe the closing of which increases hydraulic resistance and the circulation gets slower or stops completely, and so the supply of warmth carrier through the heating devices is decreased.
Quality regulation is achieved with the assistance of a three-way valve and a bypass, or a four-way valve, situated directly in front of the ring of low temperature heating (see Figure 26).
A turn of the handle of the three-way valve in a definite position opens the bypass and the circulational pump pulls the cooled water from the return pipe into the supply pipe where the mixing occurs with the hot water of the supply. Thus the temperature of the supply of warmth carrier can be regulated to the desired value. A three-way valve can work very flexibly, as it can close the bypass or the supply pipes or work as a mixer of return cooled water with hot supply water. In other words, if the three-way valve closes the bypass, then the hot supply goes completely into the heating ring; and if the valve closes the supply, then the heating ring works for itself. The warmth carrier will continue in circles until it gets cold; if the valve is open in an intermediate position, then the cooled water comes into the valve through the bypass and mixes with the supply water. After that, it comes to the heating contour with the needed temperature. A three-way valve which is installed for the regulation of the warmth carrier in this situation is called a «three-way mixer» (see Figure 27). The temperature of the supply of hot water in the heating system can be regulated manually using the scale on the mixer or with the assistance of a temperature sensor and an electric servomechanism.
Using four-way valves means that it is not necessary to use a bypass, but these valves work differently: one type, for example, with X-shaped shutter, can only close and open the supply and return flows: they cannot mix water; others, for example those with rotary shutters, can mix water. While using valves with X-shaped shutters, the hot water comes into the heating ring and the valve is closed and the pump forces the warmth carrier into the internal ring; as soon as the warmth carrier gets cooler, the valve opens and a new portion of hot water comes into the internal ring from the boiler and cooled water goes into the return. A four-way valve of such construction divides each contour into two parts. Its work is similar to the regulation of temperature of warmth carrier when the circulational pump turns on or off. But the difference between regulation by the four-way valve and that of the pump (turning the pump on and off), the regulation here occurs in a softer manner because the pump is not turned off and the circulation of the warmth carrier is not stopped. Of course, using a four-way valve with X-shaped shutters is possible only in automatic regimes because manual rotation of the valve with every cooling of the warmth carrier in the internal contour is simply impossible.
Four-way mixers with rotary shutters (and some others) provide constant and equal supply of hot and cooled warmth carrier and at the same time allow adjustment of the desired temperature of the warmth carrier both in manual and automatic modes (see Figure 28). Such a heating system doesn’t need to have a differential bypass: the mixer automatically provides the needed amount of water. In other words, the total amount of water which goes into the heating system and water which returns will be constant. The system of regulation presented is one of the simplest: according to the position of the shutter, the four-way mixer allows a definite quantity to go from the boiler into the primary contour and the same amount of warmth carrier is pulled into the return pipeline.
Usually low temperature heating systems are equipped with automatic controllers which measure the temperature of the warmth carrier or the temperature of the air in the heated room and send commands to the electric servomechanisms which rotate the handles of the three- or four-way mixers. Besides the mixers with «rotary shutters», other managing devices exist which are based on the stock three- and four-way valves (see Figure 29). Regulation (closing and opening the mixer channels) occurs because of lowering and raising of stock with a cone shutter. A sensor manages the mixer and this mixer is based on the thermal expansion of some materials, for example paraffin. A capsule with paraffin is put on the pipe of the heating system, and the paraffin expands because of the heat of the pipe and the contacts of the thermocouple meet or separate — i.e., the capsule works as a switch which sends an impulse to the servomechanism, which moves the three- or four-way mixer stock. Later, the temperature in the heating pipe goes down, the paraffin decreases in volume, the contacts separate and the mixer stock goes to its previous position.
Thus a heating system with low-temperature floor contour and high-temperature radiator contour can look as in Figure 30. The warmth carrier after getting hot in the boiler goes to the hot water collector from which it divides into two vertical pipelines — radiator heating and warm floors. The radiator vertical pipelines deliver water to the heating devices where it gets cool and goes to the collector of cooled water which is connected to the return pipe of the boiler. The warmth carrier which is forced by the circulational pump circulates constantly in this contour and through the boiler. In the warm-floor heating contour a somewhat different motion of warmth carrier happens. The circulational pump injects the warmth carrier from the supply collector not constantly, but periodically according to the opening of the supply by the three-way mixer. The rest of the time the pump «rotates» its own cooled water in the warm-floor ring. Here I must note that during manual regulation of the three-way mixers, the pump will constantly mix water from the supply collector and during automatic regulation of the mixer, two variants of working are possible: turning off the warm floors from the boiler completely; and hot water mixing. The fact of the matter is that manufacturers of three-way mixers make two variants of these valves and in the most common one, three-way mixers are adjusted in such a way that when the valve is closed manually, it shows «the supply of hot water is closed» on the scale of the device, even though it doesn’t shut off the hot water completely, but leaves it a little bit opened. This is called «protection from a fool». For example, if someone installed the radiator heating system with a mistake, and the user closes the supply to the floor heating system completely, the boiler will still heat the water and push it into the system. And where will this water go if the three-way valve is completely closed? Overpressure and overheating of the warmth carrier will be created in the system and it is possible that the warm-exchange pipe in the boiler or other pipeline can break. The three-way valve which has a small hole when the supply is fully closed will allow the circulation to continue and pass the warmth carrier through the low-temperature heating contour.