If you got to this article through a search, you probably already heard something about the mixing pipe fittings used in heating systems of private houses and apartments. So, without lengthy preface, we suggest discussing 3 questions: how the thermostatic three-way valve works, where it needs to be installed and how to choose the right one so as not to waste extra money.
The principle of operation and types of valves
The task of any 3-way valve is to supply water of the required temperature to the line by mixing or separating 2 streams. Accordingly, the element is equipped with three outputs, one of which is always open, and the other two completely or partially overlap during operation. Hence the name of the crane - three-way (sometimes they also say "three-way", which is not right).
Note. The heating circulation pump is always installed precisely from the side of the open output, otherwise the circuit will not function correctly, as we wrote earlier in another instruction.
According to the method of preparation of the coolant of the desired temperature, thermostatic valves are divided into 2 groups, shown in the photo:
- Mixing. They are supplied with 2 streams of water - hot and chilled (entrances are marked with the letters "A" and "B"), and from the third pipe (marking "AB") there is a mixture of the set temperature. On the brass case there is a mark in the form of an arrow converging from two directions.
- Separation or distribution. The incoming coolant is divided into 2 flows of adjustable size. Marking on the body - 2 diverging arrows or the letters "A", "B" on the outlet pipes and "AB" at the entrance.
For reference. There is a third kind of 3-way thermal valves - flip-flops. They are placed in wall-mounted gas boilers with a water circuit and, with the help of an electric drive, switch the flow movement between the main heating heat exchanger and the secondary one providing DHW. Outside heat generators, such elements are used extremely rarely.
According to the principle of operation, three-way cranes are also divided into two types - saddle and ball. The device of the first is similar to ordinary water valves, but instead of a threaded rod, a push is used. A plate is fixed on it, moving between two saddles and overlapping 2 passes in turn. Pressing the stem is carried out in three ways:
- integrated thermocouple
- thermal head with remote temperature sensor
- servo drive.
How this happens is shown in the diagram and described in detail in our other publication.
Ball heat mixing valves operate on the principle of the same valves, with only three outlets. They are controlled manually or by an electric drive rotating the rod at the command of automation. The elements are full bore and have a high throughput, and therefore less hydraulic resistance. The disadvantage is the dependence on the voltage in the mains and the need to install an uninterruptible power supply (UPS).
Where to put the 3-way valve and when it is not needed
Before choosing a three-way valve, it is advisable to make sure that it is really necessary. Indeed, on the Internet and in real life there are enough advisers who poorly understand the essence of the issue. So, we list the situations when this valve is really needed:
- To protect the solid fuel boiler from the supply of cold coolant and condensation on the inner walls of the furnace.
- To regulate the temperature of the water in the heating circuits.
- To limit the heating of the coolant in the contours of the warm floor.
A lot has been said about the condensate that provokes the formation of sticky growths on the walls of the chamber of the TT boiler, including on our resource. It appears during the heating process, when the temperature in the furnace is already high, and the water from the heating system is cold. To avoid this, the supply and return line is connected bypass, where a 3-way valve is installed. It makes the coolant from the boiler tank flow in a small circle, and only when heated to 50-60 ° C begins to mix water from the system.
Important notice. The valve serves as a safety element for a cast-iron heat exchanger, if installed in your heat generator. Imagine that the house turned off the electricity for 1-2 hours, during which the radiator network manages to cool. Without a mixer, cold water will abruptly go into a preheated boiler when power is restored. From such a difference, cast iron will experience a temperature shock and may crack.
Temperature control in heating circuits using a mixing unit is necessary in such cases:
- in complex heating systems, when several lines with different temperature conditions must be connected to a common comb, for example, a radiator network, underfloor heating and an indirect heating boiler;
- when connecting the same consumers to the buffer tank - to the thermal battery;
- when supplying heated water to a heat exchanger of a ventilation air handling unit used for air heating of a country cottage.
Since the heat carrier with a temperature of not more than 50 ° C is directed to the heating circuits of the warm floors, and 85 ° C can also come from the boiler, it should be limited. Usually (but not always!) The issue is solved by installing a mixing unit with a 3-way valve on the distribution manifold. The latter mixes chilled water from the floor circuits with an “external” coolant coming from the boiler.
Now we will designate situations when the purchase and installation of the mixer (or splitter) is not required:
- If the length of each loop of the warm water floor does not exceed 50-60 m, which is quite possible to achieve, then regulation is done without a mixing unit. Instead, RTL heads are placed on the return manifold, restricting the flow by the amount of coolant.
- When 2-3 heating units alternately operate to heat a private house, maintaining a constant network temperature of at least 40 ° C, then you do not need to install a three-way valve for a solid fuel boiler.
- In heating systems with natural water circulation. The reason is the pressure drop across the valve, which prevents the coolant from moving. The same applies to heat accumulators involved by gravity flow.
Note. In gravity systems, pipes of increased diameter DN40 — DN50 are used. This means that they will have to buy not a conventional coupling mixer, but a bulky flange-type valve at a decent price. Such a decision is not reasonable.
If you are interested in why it is better to choose RTL heads and how they control the floor heating circuits, watch the video from an experienced master and our expert Vladimir Sukhorukov:
Bandwidth calculation
Just pick up a three-way valve according to the diameter of the pipe of the heating unit or the supply pipe will not work. The fact is that in the process of automatic control, the element creates a variable hydraulic resistance, which the circulation pump must overcome in order to provide the required coolant flow rate. By calculation, the valve is selected in such a way as to allow the desired volume of water to pass through at different positions of the stem.
The main structural characteristic of any 3-way valve is the conditional throughput, denoted by the letter Kvs and expressed in m³ / h. This value, indicated in the product passport, reflects the amount of cold coolant passing through a fully open valve in 1 hour. In this case, the pressure drop in the area before and after the regulator is 1 Bar.
Example.If through a three-way valve with Kvs = 1.6 m³ / h, just such a volume of water is passed for an hour, then the pressure drop (hydraulic resistance) will be 1 Bar or 10 m water column. This is too much for the heating system of a private house, therefore, in the calculations, a real pressure drop is taken - 0.15–0.2 Bar (1.5–2 m water column).
To select a control valve by capacity, you first need to determine the flow rate of the coolant going along the regulated line. The following formula is used:
Here:
- G is the required water flow, m³ / h;
- Q - thermal load on the heating branch, kW;
- ∆t is the temperature difference between the supply and return water, usually taken equal to 20 ° С, and in warm floors - 10 ° С.
Example. The house with an area of 100 m² is planned to be heated with floor contours, which should provide a heat transfer of 10 kW. Then G = 0.86 x 10/10 = 0.86 m³ / h of coolant must be supplied to the distribution manifold.
The next step is to calculate the real coefficient K of the throughput of the mixing valve, taking into account the differential pressure on it 0.2 Bar according to the formula:
For the same example, the value of K will be equal to 0.86 / √0.2 = 0.86 / 0.45 = 1.9 m³ / h. Next, open the catalog of the selected valve manufacturer and select from the line a three-way valve whose Kvs is equal to or greater than the obtained value. Take the famous Danfoss brand (Danfoss) and choose a valve with nozzles DN15 and Kvs = 2.5 m³ / h from the VRB3 product series. The previous nominal in the row is 1.6 m³ / h, which is clearly not enough in our case.
Selection tips
An ignorant homeowner who decides to look up a catalog of any well-known company in search of a three-way valve may be confused by the number and variety of products offered. To help you choose the right valve from a wide range, we will give some recommendations and start with a list of brands whose catalogs are generally worth opening. Here is a list of well-known companies whose products are trustworthy:
- Danfoss (Denmark);
- Herz Armaturen (Austria);
- Honeywell (USA);
- Icma (Italy);
- Esbe (Sweden);
- Caleffi (Italy).
For reference. The listed companies sell a huge variety of fittings for heating systems, including two-way, safety and four-way valves, electromagnetic gates and thermostats. From manufacturers of countries of the former Soviet Union, we can recommend products of the Valtec brand (Valtek).
Now the main block of recommendations:
- To protect the solid fuel boiler from condensation, you can choose 2 types of three-way valves - with a fixed setting and a thermal head with an external sensor. The second option will cost 20-30% more and is not always justified, since a change in return temperature is unnecessary here. Purchase a regulator with an internal thermostat set to a temperature of 50 or 55 ° C.
- To control the heating of individual branches and circuits of underfloor heating, a 3-way valve with an external sensor and a thermostatic head is definitely needed. The sensor flask is installed on the collector or pipeline whose temperature must be controlled.
- Ball (they are also rotary) regulators are paired with an electric drive or set manually. If you do not want to complicate the circuit and depend on electricity, choose a product that is suitable for the characteristics among the seat valves that work from thermal heads.
- The most “running” case material is brass or bronze. Corrosion-proof elements are more expensive, and cast iron is afraid of temperature shock and has a decent mass.
- In schemes with the same success, both mixing and dividing three-way cranes are used. But if you are not a specialist in the field of heating and assemble the system yourself, it is better to take a valve - mixer. It’s easier to figure it out and put it right, as the expert will tell you in detail in his video:
Two final recommendations
Since we presented a simplified method for calculating and selecting a 3-way valve for capacity, we strongly advise you to consult with knowledgeable people about this. If this is not possible, buy a valve with a margin, regardless of the price. There is another option: agree with the seller on a possible replacement of the product in case it does not fit.
If you need to install water heating in a large cottage heated by a radiator network and underfloor heating, and it is planned to provide hot water from an indirect heating boiler, then without the help of experienced specialists you can not do. You will have to make 4 to 10 adjustable branches, for each of which you need to calculate and select a three-way valve, and then balance their work in the complex.