Stabicad for AutoCAD and Revit offers a powerful tool for calculating sanitary networks, and in particular hot water loop diagrams. It's important, however, to be fully aware of the importance of this hydraulic set-up, and of the fundamental principles that are integrated into the software and that contribute to the quality of the design.
It is compulsory to install a DHW loop in all multi-family housing and commercial, sports, leisure and healthcare buildings. While this is now widely accepted, it is important to remember the objectives of this permanent circulation of DHW and its temperature maintenance.
First and foremost, the DHW loop system guarantees water hygiene by preventing the proliferation of micro-organisms that are dangerous to human health, while the hot water arrives. Legionella comes to mind, and the respiratory illness caused by these bacteria is known as legionellosis. Always at over 50°C, hot water stops the growth of bacteria, making it easier to treat this public health problem.
Secondly, collective DHW looping ensures comfortable use of the sanitary system by reducing the waiting time for hot water at the tap. Bear in mind that user expectations are high: a 90% satisfaction rate is achieved with a maximum waiting time at the tap of five seconds. At 10 seconds, this rate falls to 60%, and at 11 seconds, it plummets to 40%.
The other side of the coin: the looping of domestic hot water systems consumes energy. That's why, through its design and choice of components, the hydraulic system must be able to control heat loss in the distribution circuit and overall consumption for DHW production. Trimble's Stabicad software enables fluid and energy design engineers to meet this challenge head on.
Sizing the DHW loop diagram
Circulating water in a closed circuit is crucial to the efficiency of the system. Sizing the DHW loop diagram is therefore essential.
Users of Stabicad for Revit take a step-by-step approach to their looped systems. First, they design the hot water production system - a boiler, a hot water tank, an electric or thermodynamic water heater, a plate heat exchanger, etc. - then position the tapping points - sinks, washbasins, showers, bathtubs, WCs, etc. - and draw the hot and cold water supply networks, as well as the hot water loop circuit.
Then, with a single click to launch the diagram analysis, Stabicad performs a complete calculation of the installation; the software complies with the NF DTU 60.11 standard on calculation rules for cold and hot water supply networks, and on the design and sizing of looped networks.
At the end of this process, the engineer has a note describing his fully dimensioned and equipped network. This includes pipe diameters per section, insulation adapted to losses (based on standard NF 12828, which defines the six insulation classes available), positioning and adjustment of balancing valves, charging pump characteristics, indication of the thermal power required for DHW supply and return circuit, loop heater power, etc.
Designing the collective DHW loop supply network
Stabicad performs these calculations automatically. However, given the importance of the initial design of the DHW loop for the correct operation of the structure, it is important to go back over the phases of this precise engineering work and its fundamentals to fully understand all its aspects.
Sizing starts with calculating the supply circuit. This "outgoing network" runs from the domestic hot water production point to the tapping points of the various risers or horizontal loops.
The designer of the hydraulic diagram follows a few rules to produce a system that is controllable in terms of energy, health and comfort of use:
1. architecture that reduces network length and heat loss;
2. use pipe diameters strictly adapted to the flow rates (maximum 2 m/s, preferably 1.5 m/s) and pressures (minimum 1 bar) at the taps;
3. insulate all pipes, equipment and special points (circulators, valves, exchangers, clamps, etc.) using effective, thick materials.
Make a realistic calculation
This calculation of the supply network to the tapping points is based on the division of the vertical columns or horizontal loops into elementary sections. This is clearly shown in the final calculation note produced by Stabicad.
In concrete terms, this operation includes :
calculate total flow rates by adding those for each section - they take into account installed appliances (sinks, WCs, washbasins, bathtubs, etc.), pressure losses produced by the installation (linear losses linked to pipes, elbows, valves, meters, exchangers, etc.);
the application of a simultaneity coefficient on the section to determine the probable maximum flow rates. This simultaneity calculation ensures that opening several taps in a dwelling simultaneously will have no noticeable effect on comfort of use - whether in terms of temperature, flow rate or pressure. It should be noted that the simultaneity coefficient differs according to the use of the building: housing, health care, sports locker rooms, schools... ;
the choice of pipe diameters per section to take into account the calculated flow rates and a circulation speed of 1.5 to 2 m/s ;
checking pressure at tapping points - NF DTU 60.11 calls for the most unfavorably placed tap to be taken into account, i.e. the furthest away in the network; in reality, it is preferable to check the entire network.
Domestic hot water loop diagram: sizing the "return" circuit
The design of the return circuit of a looped DHW system must meet equally stringent requirements. At the very least, the temperature at all points must be above 50°C, and heat losses as low as possible. Hydraulic engineers know just how to achieve this:
limit the temperature drop to a maximum of 5°C between the hot water generator outlet and the most unfavorable point in the "return" circuit;
maintain flow velocity between 0.2 m/s - to avoid quasi-stagnation - and 0.5 m/s - to reduce network noise;
produce the lowest possible flow rates;
open balancing valves by at least 1 mm, both to comply with the operating ranges supplied by the manufacturers, and to maintain minimum flow in the event of scaling;
minimum internal diameter of pipes - 12 mm for copper, 12.4 mm for PVC-C, 13 mm for extruded polyethylene, multilayer or polybutylene, 12 mm for other materials (stainless steel, etc.).
On this basis, for the domestic hot water loop diagram, the sizing of the return takes place in several stages:
1. Calculate the flow rates and diameters of the return pipes for each riser or loop, bearing in mind that the internal diameter of these pipes must be smaller than that of the outgoing manifold;
2. Choose insulation (material, thickness, etc.) based on heat loss calculations and taking into account standard NF EN 12828 ;
3. Calculate the balancing, i.e. the settings (kv) of the balancing valves at the foot of each column and the one on the general return;
4. Choose the charging pump for the DHW loop; note that suppliers on the market (Grundfos, Wilo...) - offer electronically-controlled, self-adapting, high-efficiency versions that help control loop energy consumption.
Precise knowledge of the system's total heat loss - supply circuit and return circuit - and of the loop flow rate enables calculation of the DHW production system's capacity to meet occupant demand.
Given the complexity of the installations in certain structures - a large hospital, for example - it's easy to see why it's important to automate these calculations in order to avoid data entry errors and reduce design times.
Stabicad also helps designers to propose alternative scenarios, such as :
the installation of specific equipment - a Charot hot water loop heater, an Atlantic collective thermodynamic DHW production system;
the inclusion, in the sanitary water distribution diagram and in the calculation, of circuits that are not looped for various reasons - their distance from the generator, their low draw-offs, their high energy costs for looping sanitary hot water;
treatment of tapping points located in unheated rooms: pipe insulation thicknesses are automatically calculated on the basis of the ambient temperature selected by the designer.
Updated on a monthly basis and available in the Stabibase, Stabicad for Revit is a genuine quality production tool capable of providing the wealth of information essential to installers and operators alike.
