Building service and building service energy modelling

In all new hydronic systems the circulation of water in the distribution and through the

terminals is supported by electric pumps. pre-existing systems may exist in which

such circulation is supported by the earth's gravitational field, through what is called

the thermosiphon effect: hot water (heated by the generator) is lighter than cold water

(cooled in the room terminals) and therefore, in a closed circuit, the latter tends to

move downwards, pushing upwards l warmer water. These systems have now been

abandoned due to the evident difficulty in varying and controlling the flow of water

circulating in the system and therefore the power supplied.). The term water flow,

symbol Q, refers to the quantity of water (expressed in this case as the volume of

water) which passes through the section of a pipe in a unit of time and is expressed in

m3/h (cubic meters per hour).

The term power supplied, symbol Φ, instead refers to the thermal power (i.e. the

amount of heat per unit of time) that the generic plant terminal provides to the

environment in which it is located; the unit of measurement for power is W (watts). In

general, hydronic systems have advantages over other types of system for relatively

low initial installation costs, fluid availability, low operating costs and practically silent

operation. The main disadvantage of hydronic systems is that they are obviously

unable to address and resolve the ventilation requirements of the building occupants.

Humidity control is absent or poor: they are basically only systems for heating or

cooling the environment. These disadvantages can however be overcome by

combining hydronic systems with aeraulic systems, to obtain the best from each of the

different types. The hydronic systems for environmental air conditioning can be

classified by operating temperature (flow temperature of the hydronic distribution

circuit), and can be divided into: -low temperature hot water (BT) <40 ° C; medium

temperature hot water (MT) from 40 ° C to 60 ° C; high temperature hot water (AT)>

60 ° C; chilled water (AR) from 2 ° C to 16 ° C, usually 7 ° C; dual temperature water

(ADT) MT and AR (38-65 ° C, 4-7 ° C). Most of the hydronic systems present in

commercial buildings are medium temperature systems (MT), although today the

trend is to have low temperature systems already used in residential buildings, chilled

water systems (AR) also in these buildings. o dual temperature systems (ADT).

Objectives of balancing

The design and installation of balanced circuits is needed to guarantee the correct flow

rate inside the emitters and inside branches of the hydraulic circuit, in order to: ensure

the correct performance of the emitters; provide the correct amount of thermal power

to the heated/cooled area; prevent excessive water velocities which can cause noise

and abrasion; prevent circulating pumps from operating under low efficiency

conditions, leading to overheating and limit the value of the differential pressures

acting on the regulating valves, thus, preventing damaging.

Example of hydronic circuit

circuit A and circuit B are two independent circuits which

flow into node 1. In this node, the circuits have the same

differential pressure. Circuit A: higher pressure drop

branch Circuit B: lower pressure drop branch Usually,

to size the circulation pump, the design differential pressure

is chosen according to the circuit with the highest pressure

drop. We guarantee that even the most critical emitter is

able to fulfill the thermal energy need of the room. But we

risk to have higher water flow rate in the other emitters with

lower pressure drop: overheating of rooms and excessive

velocity inside circuits.

Network extension

The degree of imbalance of flow rates through circuits depends on the number of

branches served:

if the number of branches is small, the differences between the required flow

 rates and those obtained are generally within acceptable margins;

if the number of branches is large, the imbalances in the flow rates can be

 considerable.

For medium and small systems with a constant flow rate, proper sizing of the pipework

will normally be adequate to ensure that the circuits are balanced. For systems with

extensive networks, or with variable flow rates, it is necessary to include equipment

capable of regulating water flow. Hydraulic network can be:

Direct return circuit: Elementary hydronic system installed by (1) heat

 generator, in this case a boiler for hot water, (2) a circulation pump, (3)

terminals for heating in the spaces served (e.g. heating emitters if system high

temperature or radiant panels if low temperature system or fan coil units if

medium temperature

system), (4) the pipes that

connect the various devices

to each other and (5) an

expansion vessel. The

purpose of the expansion

tank is to allow the

expansion and volumetric contraction of the water due to temperature

variations in the system and to maintain the pressure of the hydraulic circuit at

the required value. It is necessary to cross the continuum from the water, it is

generally insulated at the same level as the pipes to minimize the losses or

thermal gains of the system. Different lengths of circuits of each terminal can

cause high risk of unbalanced pressure drop on different branches. Distance

between pump and emitter E much higher than distance pump-emitter A

→higher resistance to water flow → higher flow rate in emitter A, lower flow rate

in emitter E. Especially for extended network, they need to install balancing

valves to guarantee the nominal flow rate to each terminal.

Reverse return circuit: The advantage of a reverse return system is that the

 resistance to water flow for each of the individual paths is approximately equal,

so the system is essentially self-balancing. However, the reverse return piping

system is more expensive due to the additional length of return pipes required,

and it may be more costly than the direct return system with balancing valves.

Control system

An hydronic system shall be designed and controlled in order to properly fulfill energy

needs of the building, which vary along the heating/cooling season according to

external conditions (e.g. external temperature, solar radiation) and internal gains. The

control system is composed of all devices (e.g. 3 way

valves, autoflow regulators, BMS) which allow to adapt

the thermal energy provided by the system to the

actual conditions, in order to reach indoor comfort

(e.g. avoiding under-heating or over-heating). In Figure

12, you can see one of the most common and

widespread hydronic heating systems: a residential

boiler that feeds radiators placed in different rooms. In

this case, the control system consists of thermostatic valves that, based on the preset

temperature (position of the valve's rotating head) and the detected ambient

temperature, open and close, increasing or decreasing the quantity of water (flow)

passing through the individual radiator per unit of time. Figure 13 shows the typical

trend of the power emitted by a radiator based on the percentage of water flow for an

assigned supply temperature (60 °C) and ambient temperature (20°C). The almost

proportional relationship is due to the fact that a reduction in flow also corresponds to

a decrease in the water's outlet temperature from the radiator (the water stays longer

inside it and, therefore, has more time to transfer heat to the surrounding environment

and cool down more). This lower temperature at the outlet causes an increase in the

temperature difference between the inlet and outlet water, and therefore a greater

temperature rise that partially counteracts the flow reduction. The thermal power

transferred from water to the radiator, and thus to the environment, is indeed given by

the product of the mass flow by the temperature difference between the inlet and

outlet: Φ = ��(��� − ����).

Balancing of hydronic system

The control of the system is something different from balancing, but they can

influence each other. Also for balancing, the aim is to control the proper amount of

thermal energy given by each emitter... Emitters are designed to fulfill the nominal

thermal power in design conditions, with specific operating temperatures and water

flow rate. When the flow rate is different from the nominal value, the thermal energy

provided by emitters will be lower or higher than actual needs. For this reason

balancing is a key point for the design of an hydronic system. Aim of balancing →

supplying emitters with the flow rate required by design specification.

In practice, balancing flow rates in the two branch circuits is not as straightforward as

this simplified discussion suggests. In the presence of a constant-speed pump, varying

the resistance of one circuit also affects the available pressure and, consequently, the

flow rate in the other circuit. In fact, the operating point of the entire system (i.e., the

flow rate provided with the pressure difference) is determined by the intersection of

the pump's characteristic curve with the hydronic circuit's characteristic curve, as

shown in Figure 16. The pump's characteristic curve is a property of the specific

pump employed (Water flow rate provided by pump is inversely proportional (non-

linear) to pressure drop) , while the circuit's characteristic curve depends on the

hydraulic resistance of the circuit, which, in turn, varies with the flow rate �(�): the

higher the resistance, the steeper the curve, and this steepness increases with the

flow rate. The characteristic curve of the circuit is defined as a parabolic curve

which passes from the orgin

and the theoretical point

(Q-∆p) of the circuit. The

hydronic resistance is not a

characteristic quantity of

the constant circuit, but

depends on the flow rate

circulating in it. The bond

between resistance and

flow rate can be considered as a linear approximation, that is R (Q) ≅Z Q, with the

characteristic constant Z of the circuit, called pressure drop coefficient.

Changing of characteristic curve of the circuit

The slope of the characteristic curve of the circuit increases with higher ∆p, which, in

turn, depends on the water flow rate Q → ∆p(Q). When, in order to balance a branch of

the circuit, a balancing valve is introduced, it increases the overall resistance of the

hydronic circuit, which passes from R to R+ΔR, and there will be a reduction in the

overall flow rate delivered: in the next picture,

the operating point moves from P to P1 and the

total volume flow changes from Q to Q1 <Q.

Consequently, the available pressure increases

and while the plant terminal on which the

balancing valve has been introduced has the

desired flow rate, the others see the pressure

increase and therefore the supply flow rate,

delivering more thermal power than required.

When a balancing valve is introduced, the total

hydronic