Follow Inta’s experts guide to Heat Networks and find out about how they are designed, built and optimised

Schematics, drawings and example figures do not constitute an actual design, and each project should be reviewed individually as a unique design process.

Note 1 (Ref G in the drawing).
The reverse acting differential pressure controller (RADPCV) is sensing the pressure differential between flow and return riser pipes as the HIU control valves close down, the head pressure will increase causing the increase in the pressure difference between flow and return pipes. When there is no demand from the HIUs, then this valve provides the alternative flow path. The valve opens to compensate, and bypasses the flow into the return bringing the pressure difference back to the set value. The valve will continually modulate the pressure differential. In most cases, the valve is set to control a value about 10% above the operating point of the system at the design conditions.

Schematics, drawings and example figures do not constitute an actual design, and each project should be reviewed individually as a unique design process.

Pipework terminology - District Heating

As described in the CIBSE CP1 Heat networks code of practice.

  1. Primary pipework (or primary heat network).
  2. Secondary pipework (or secondary heat network).
  3. Tertiary pipework (or tertiary heat network, heating and DHWS internal to the apartment and separated from the primary network).
  4. Sub Station (2)

Note 2 (Ref D in the drawing).
The Substation separates the buildings from the district heating pipe network or used as a pressure break in very high building to prevent extreme pressure differentials. Where the temperatures and pressures of the building’s heating systems differ from those of the district heating network, it is usually necessary to utilise a Substation. A Substation consists of a plate heat exchanger which creates pressure break between the district network and consumer system. The district heating supplier will provide information on the actual minimum and maximum differential pressures, as measured at the service connection valves. This data should be used for determining the sizes and capacities of control valves and heat exchangers.

Pipework terminology - Communal Heating System

As described in the CIBSE CP1 Heat networks code of practice.

  1. Boiler plant room
  2. Primary pipework
  3. Secondary pipework

District heating networks in some cases can cover large areas and service multiple buildings and homes. They can be added to later and also linked to other schemes to form an even larger network. As such it is important that the M&E consultants and Developers understand where the key lines of responsibility lie.

Demarcation and ownership is important and should be clear to ensure that the responsibility for maintenance and metering understood by all involved. Demarcation is important for small community heating schemes as well as larger networks.

Centralised plant rooms in large buildings and separate substations for smaller buildings are recommended. The plate heat exchangers in these plant rooms and substations create clear demarcation lines that separate the main network from each individual building or estate, and providing a pressure break.

Pressure breaks in communal heating plant rooms and substations ensure that taller buildings do not affect the pressure in
other parts of the network.

Heat Network System

Heat Source

The design engineer will be looking to achieve the most efficient source of heat that is practical for each individual project. Gas condensing boilers, multi stage boilers, heat pumps, and to a lesser degree combined heat and power (CHP) or biomass boilers. Other low carbon methods such as solar thermal are also to be considered as secondary heat sources. Larger systems can tap into waste heat recovery projects.

Shunt Pump

Usually a twin pump set for circulating water and heat from the heat source to the buffer tank at constant speed / flow.
The pump will be moving heat from the heat source and into the buffer as required.

Buffer Vessel

The purpose of the buffer vessel is to provide heat to meet peaks of maximum demand which occur over short periods,
storing heat for later use and supplementing the heat source when demand is high. Stored heat is immediately available
without the heat source needing to get up to temperature. The buffer should be sized correctly to match the load demand
with consideration to the heat source, building construction, and even the number of people that possibly can create a
demand during peak demand times, usually morning and evening.

Primary distribution pump (or secondary pump in the case of a district heating network)

Variable speed circulating pump capable of efficiently operating at operating at part load. The pump should be sized to meet
speed and flow design requirements and be controlled so that there is always sufficient pressure and flow available to feed all
the HIUs in the network equally.

Riser end loop with Automatic air venting and differential bypass control * also see note 1

Provision for venting air from the Risers, Provision for maintaining circulation when all control valves in the HIUs are closed.

Design Principles

  • 30 apartments each with its own HIU.
  • Each apartment is the same.
  • DHW peak load 38.6 kW for this demonstration.
  • Heating load is 5kW for this demonstration.

Schematics, drawings and example figures do not constitute an actual design, and each project should be reviewed individually as a unique design process.

  • Exercise 1
    Calculate each apartment’s heating peak load
  • Exercise 2
    Estimate each apartment’s hot water peak load. The temperature lift or delta T.
  • Exercise 3
    Coincidence factors and diversity. Sizing the heat source.
  • Exercise 4
    Calculate the flow rates to size each section of pipe in the distribution network. Calculating flow rates and buffer sizing.
Building systems are designed to meet peak heating demand and ignore working at part load. Designing a system to peak outputs and factoring in a margin for additional capacity to guarantee meeting performance capacity is mostly the chosen method. Peak demand may only occur for a few hours and would differ from year to year. While heating systems consume energy for nearly half the year, and the system works at peak load for only week or so, then the rest of the year it is over-sized. Using a diversity factor very much depends on judging the building on its 1) occupants 2) geographical location of the building and climate 3) the building fabric. So coincidence factors can be used, up to 80%. It needs to be evaluated from project to project. For Heating design, refer to – BS EN 12831-1:2017 Energy performance of buildings. Method for calculation of the design heat load. Space heating load, Module M3-3 Types of heating in apartments;
  • Panel radiators, inc LST
  • Underfloor heating wet systems
For dwellings over 150m2 Part L as good practice recommends two space heating circuits each with independent time and temperature control, and thermostatic radiator valves. The use of pre-settable radiator valves is recommended for the correct balancing of radiators.

Exercise 1

For the purpose of this exercise, we assume heat losses are calculated and there is a heating load for each apartment type of 5kW. For each apartment type, the demand flow rate has to be calculated. This is determined by the number of outlets (taps and showers). It is unlikely that all will open at the same time, but again a peak load has to be estimated. First calculate the flow rate the apartment users will expect, and the calculate in kW the power needed to deliver the design flow rate and the temperature for the hot water. A key influence is the cold water in temperature, 10C is set as the base line, so to achieve 55C DHWS then the temperature ‘lift’ is 45C. In some HIUs data sheets the temperature for HIU performance tables is 50C, so then ‘lift’ value is less, and the power rating for the HIU is greater. Here are four possible examples. Example 1 Hot water table, where the total flow though the outlet is the maximum and then mixed 70% hot with 30% cold, with the exception of the kitchen sink tap which we assume the user is using maximum temperature. Hot water temperature is 55C, and the cold inlet 10C. We are not saying this is what always happens, it’s just one way of looking at DHW use.
Shower 7.75
L/min (70% HW)
Shower 9
L/min (70% HW)
Basin 6 L/min
(70% HW)
Basin 4 L/min
(70% HW)
Kitchen Sink 6 L/
min (100% HW)
Bath 12 L/min
(70% HW)
Bath 12 L/min
(70% HW)
Power
Ltr/hrLtr/hrLtr/hrLtr/hrLtr/hrLtr/hrLtr/hrkW
325.5378216168360504
outlets in use simultaneously – flow rate in ltr/hr
OFF378OFFOFFOFFOFF37819.8 kW
325.5OFFOFFOFF360OFF685.535.9 kW
OFFOFFOFF360OFF738*38.6 kW
OFFOFFOFFOFF36050486445.2 kW
Note, figures are examples of possible flow rates and should not be used as actual design parameters. Each project should be calculated by the design engineer. Example 2 Now we change one thing, we now carry out the same exercise but with the cold inlet at 15C. Less power is now required as the temperature lift is less. An example in deciding which is the most realistic profile for sizing.
Shower 7.75 L/min (70%
HW)
Ensuite shower
Shower 9 L/min (70% HW)
Main shower
Basin 6 L/min
(70% HW)
Basin 4 L/min
(70% HW)
Kitchen Sink 6 L/
min (100% HW)
Bath 12 L/min
(70% HW)
Total flow in required for
heating to hot water
Power
325.5378216168360504l/hkW
outlets in use simultaneously – flow rate in L/h
OFF378OFFOFFOFF37817.6 kW
325.5OFFOFFOFF360OFF685.531.9 kW
OFF378OFFOFF360OFF738*34.3 kW
OFFOFFOFFOFF36050486440.1 kW
OFF378OFF168360OFF90642.1 kW
Note, figures are examples of possible flow rates and should not be used as actual design parameters. Each project should be calculated by the design engineer. * For example purposes only, we will select this value as our peak DHW flow design rate, 738 l/hr converted to 0.205 l/s

Exercise 2 – Peak Load for DHWS calculations

Example 3 NHBC standards 2020 – DHW flow rates (supplied by NHBC reprinted with their permission) Table 3: Flow rate and temperature requirements
OutletDesign flow rate 1Minimum flow rate 2Supply temperature °C 3
L/sec(L/min)L/sec(L/min)
Bath (from storage)0.30(18)0.15(9)48
Bath (from combi)0.20(12)0.15(9)40
Shower (non-electric)0.20(12)0.10(6)40
Wash basin0.15(9)0.10(6)40
Sink0.20(12)0.10(6)55
Notes: 1 The design flow rate should be used to establish the hot and cold pipe sizes to provide the flow rate quoted at each outlet when that outlet is used on its own. 2 The minimum flow rate should be available at each fitting is used simultaneously with one or more other fitting(s) as shown in Table 4. 3 The supply temperature is the temperature at the outlet. In accordance with BS 8558 the water temperature at an outlet or thermostatic mixing valve should be at least 50°C within 1 minute of running the water. Recalculate using NHBC Table 3 for design hot water use. Take into account that using these guidelines the hot tap is the total flow, and these are figures used for stored water systems (bath figures used are from combi figures). DHW set temperature 55C, Cold water supply temperature 10C. Table 3
Shower 9 L/min
100% hot water
Basin 6 L/min
100% hot water
Kitchen Sink 12 L/min
100% hot water
Bath 12 l/min
100% hot flow
Total flow in ltr/hr hot water requirementPower kW
540 Ltr/hr360 Ltr/hr720 Ltr/hr720 Ltr/hr
54054028.25 kW
72072037.67 kW
540720126065.9 kW
720720144075.34 kW

Example 4 – what can we consider the most realistic DHW peak load flow rates?

Flow rate reference is BREEAM Wat 01 Water consumption performance level 3. Factor in a more realistic boosted cold water supply to the apartment at 17C Factor in TMV2 mixing on showers to maintain Basin and shower temperature of 41C, and bath at 44C (mix ratio 8.5 : 1.5) DHW set temperature 55C, so now the temperature lift is 38C.
Shower 9 L/min
85% mixed
hot water
Basin 6 L/min
85% mixed
hot water
Kitchen Sink 12 L/min
85% mixed
hot water
Bath 12 l/min
85% mixed
hot water
Total flow in
ltr/hr mixed
hot water
Power kW
459 Ltr/hr306 Ltr/hr612 Ltr/hr612 Ltr/hr
459
45920.28 kW
61261227.04 kW
459
612107147.32 kW
459306612137760.84 kW
612612124454.08 kW
DHWS demand is difficult to predict as it’s down to multiple factors involving peoples’ lifestyles, numbers of occupants per apartment, seasonal conditions, and work patterns. For this reason, a ‘factor’ is applied to attempt to replicate the situation where not everyone will be using hot water at the same time. In basic terms, the more apartments or homes the less likely it becomes that they are all running simultaneously, so we can reduce the peak design load. The design standards in Scandinavian countries have often been held as the best example for this factor, and the Danish Standard DS439 is often used by UK design engineers. There is much debate as to whether this is suitably applicable to the UK, and other variations on this have been discussed but as yet, there is no available UK standard. The DS439 standard identifies 37.6 kW as the peak load for a standard apartment. The coincidence factor simulates how unlikely it is for all the individual apartments to be peaking at the same time, to prevent oversizing of the overall system. The diversity factor is the reciprocal of the coincidence factor. For larger apartments or homes, then a common practice is to scale up the factor proportionately. In fact, this is not always true, because the larger apartment would not mimic the DHW requirements of two smaller households, though may have larger peak load. So, each project must be judged on its own requirements, and the coincidence and diversity factors are at best a guide.

Exercise 3 – determine the co-incidence factor for 30 apartments

Table 5 – Coincidence Factor
Qty HIUDHW Load (kW)Factored loadper HIUMax Power per HIUCoincidence Factor
137.637.637.59100%
246.623.375.1861.94%
353.717.9112.7747.65%
46015150.3639.88%
565.613.1187.9534.90%
670.811.8225.5431.39%
775.710.8263.1328.76%
880.310300.7226.70%
984.79.4338.3125.04%
10898.9375.923.66%
11938.5413.4922.50%
12978.1451.0821.51%
13100.97.8488.6720.64%
14104.67.5526.2619.88%
15108.37.2563.8519.20%
16111.87601.4418.60%
17115.36.8639.0318.05%
18118.86.6676.6217.56%
19122.26.4714.2117.10%
20125.56.3751.816.69%
25141.45.7939.7515.04%
30156.35.21127.713.86%
35170.54.91315.6512.96%
40184.14.61503.612.24%
45197.34.41691.5511.66%
502104.21879.511.18%
60234.63.92255.410.40%
70258.23.72631.39.81%
85292.13.43195.159.14%
90303.13.43383.18.96%
100324.63.237598.64%
110345.73.14134.98.36%
120366.33.14510.88.12%
130386.734886.77.91%
147420.52.95525.737.61%
150426.42.85638.57.56%
175474.62.76578.257.21%
200521.52.675186.94%
250612.42.49397.56.52%
337763.82.312667.836.03%
400869.62.2150365.78%
50010332.1187955.50%
10001802.11.8375904.79%
1000013797.61.43759003.67%
Based on Danish standard DS439

Diversity and Coincidence Factors

The likelihood of everybody opening all their taps and using their showers at the same time is extremely remote, in fact, it would never happen. So as mentioned before, system designs incorporate this into their pipework sizing. Over-sizing has obvious disadvantages in increased capital costs, as well as increased network heat loss. For Risers, the effect of over oversizing vertical pipes is less critical, air and dirt can be easily eliminated, and low temperatures on the return riser pipework limits heat loss issues. In general, sizing with a ‘safety’ factor to allow for unknowns reduces thermal efficiency.

Applying the co-incidence factor allows each section of pipework to be sized in accordance to its position in the network, and the peak loads it will have to carry. For this example we choose the 37.6 kW as in the Danish DS439 table.

Schematics, drawings and example figures do not constitute an actual design, and each project should be reviewed individually as a unique design process.