Calculate a two-pipe installation with direct return

In the installation of the house in the figure, the radiators are placed according to the power required in each of the rooms to be heated and the connection of these to the pipe network is made, listing the sections through the most remote radiator, as It is shown in the plane of the figure below.

As previously mentioned, for the design conditions a maximum water velocity of 1m / s and a pressure drop per meter of 30 mm.w.c. are set.

The flow rates of the sections are calculated and the corresponding diameters are assigned, verifying that the water velocity and the pressure drop R, comply with the adopted design condition. Thus, for example, for section 1-3, which transfers 883 Kcal / h, it is read that for the Multilayer AIS pipe of 16 * 2 mm, a pressure drop of 2.02 mm.w. and 0.11 m / s speed, parameters that are perfectly acceptable.

Table 6.1.2.3.1.1. Shows the distribution of calorific powers required in each section. As it is the direct return installation, the dimensions of the flow and return pipes by sections are identical, since the flows in both coincide. When designing an installation with an inverted return, two tables of the sections must be made, one for the impulsion and the other for the return, since the flows in this case will be inverse.

**Figure 6.1.2.3.1.1.** Two-pipe distribution with direct return

To obtain these values of speed and loss of load, it is enough to look at the corresponding tables of the thermal jump and the average temperature of the water, thermal jump of 20ºC and average water temperature of 70ºC, with which the installation is working, and search the flow rate that circulates and the diameter chosen (see AIS multilayer pipe pressure loss tables attached in the annexes of the AIS technical manual).

**Table 6.1.2.3.1.1.** Selection of AIS Multitube pipe diameters

Multiplying the unit head loss or head loss per meter (R) by the length (L) of the section, the head loss (ΔP CT) corresponding to the pipe in this section is obtained.

Head loss due to accessories is calculated using any of the above methods. For this case, the assignment of a 20% percentage to the pressure drop due to the pipe is used. Adding both, the pressure drop of the pipe ΔP CT and that of the fitting ΔPCAC, the total pressure drop ΔPC of the section is obtained.

To obtain the most unfavorable circuit in the pipe network, we start from each radiator and add the pressure drop of the sections that lead us to the boiler or production equipment. This pressure drop is called ΔP ORG (mm.c.a)

The pressure losses are multiplied by two, since in the direct twin-pipe installation, the flow and return pipes have the same flow and the same pressure losses, so that the diameters of one and the other are exactly the same.

The development of the calculation that has been commented, together with the diameters chosen in each of the sections of the installation, are detailed in table 6.1.2.3.1.2.

As shown in it, the most unfavorable circuit is the one that goes from the boiler to the radiator in bedroom 3.

Knowing the power and the total flow of the installation, the circulation pump can be calculated if it is not incorporated in the boiler itself. This pump must be capable of supplying flow to the entire installation and capable of overcoming the pressure drops of the most unfavorable circuit. The pressure losses of the most unfavorable circuit will be the sum of losses in valves, radiators, boilers, etc., in addition to the losses inherent to the pipeline distribution network.

**Table 6.1.2.3.1.2.** Determination of pressure losses in the installation

It follows that the head losses due to friction in the impulsion and return pipes of the boiler-radiator circuit (bedroom 3), together with those due to the accessories of that circuit, are:

ΔP ORG = 2 x (ΔP C15-16 + ΔP C13-15 + ΔP C11-13 + ΔP C9-11 + ΔP C8-9 + ΔP C6-8) = 2 x 278.6 = 557.2 mm.c.a.

The pressure drop that the pump must overcome will be:

ΔP PUMP = ΔP ORG + ΔP CAL, where ΔP CAL is defined by the manufacturer of the pump. So,

ΔP PUMP = 557.2 mm.c.a. (without considering ΔP CAL).

Finally, we need to know the flow that the pump must deliver. Knowing the power of the installed boiler, the necessary flow for the installation can be calculated using the following expression:

Q = P CAL / 3600 x ΔT CIRCUIT

where:

Q: flow rate (l / s).

P CAL: boiler power (Kcal / h).

ΔT CIRCUIT: thermal jump of the heating circuit (ºC).

For a boiler with the necessary power to satisfy the total thermal demand for heating the home (5665 Kcal / h), and considering between 12-15% more (800 Kcal / h approximately), to compensate for the heat losses that may be in the pipe distribution network, if a thermal jump of the circuit of 20ºC is considered, it must be:

Q = P CAL / 3600 x ΔT CIRCUIT = (5665+ 800) / 3600x20 = 0.09 l / s

The required pump must have the characteristics described, being capable of supplying a flow rate of 0.09 liters per second with an overpressure of 0.56 meters of water column.

In the following pages, the different pieces of material are detailed with the different Multitube systems that are necessary for the design and execution of the installation.

Parts breakdown with MM Multilayer System (multimordaza) in brass or PPSU:

a) PERT / AL / PERT multilayer pipe with pre-insulated: 16 * 2.00mm and 20 * 2.25mm.

b) 20 * 2.25-3 / 4 ”mobile multi-jaw fitting.

c) Radiator connection curve 16 * 2.00 or radiator connection elbow 16 * 2.00.

d) Reduced multi-jaw tee in brass or PPSU 20 * 2.25-16 * 2.00-16 * 2.00.

e) Equal multi-jaw tee in brass or PPSU 16 * 2.00.

f) Central heating / cooling 6 zones, 230 V.

g) Heating room thermostat, 230V.

As in the case of sanitary installations, if the installer is concerned with the speed of assembly, the PROtec Multitube system can be used, which is the fastest and safest in connection with Multitube multilayer pipes, does not require tools and is also perfectly recessed due to its very compact design.

The list of materials for the direct return heating installation with the PROtec Multitube System (accessories with brass body or PPSU) is:

a) PERT / AL / PERT multilayer pipe with pre-insulated: 16 * 2.00mm and 20 * 2.25mm.

b) Mobile fitting 20 * 2.25-3 / 4 ”.

c) Radiator connection curve 16 * 2.00 or radiator connection elbow 16 * 2.00.

d) Reduced tee in brass or PPSU 20 * 2.25-16 * 2.00-16 * 2.00.

e) Equal tee in brass or PPSU 16 * 2.00.

f) Central heating / cooling 6 zones, 230 V.

g) Heating room thermostat, 230V.