Heat Network Calculator

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An online tool for calculating the efficiency of a heat network.


The aim of a heat network is to deliver hot water and central heating into properties, in a more efficient manner than using individual heating systems in each property.

If energy costs to the end user are to be comparable to other forms of heat, then the savings made from a central plant must outweigh the additional heat losses introduced by a heat network.

There are very few figures ever published on how efficient a networks actually are in practice, and as a result very little accountability. There has been a lack of design tools available to model and analyse network performance, a lack of data fed back from sites, and to top it all a complete lack of standards or published performance figures for different HIUs - Heat Interface Units.

The Heat Network Calculator has been built to provide as accurate model as we can of how a heat networks performs, using recognised data to work out where energy goes, how much is wasted, and how the situation can be improved.

Click here to open the Heat Network Calculator.

Design Conditions

Any calculator is only as good as the data fed into it, so it is critical to the accuracy of the results that we understand what data sources are using, and for what purpose.

In designing systems there are two separate calculations to be made:

  1. The maximum loads on the system
  2. The typical loads on the system

Maximum loads are what everyone tends to focus on - making sure the system will always cope with demand. There are penalties on designers if a system does not deliver specified loads so it is standard practice to design accordingly, with over-sizing systems been the standard way to be certain. The data sets used are those that tell us the maximum volume of water that could ever be drawn, or what the coldest days have been historically in a location.

The thing is, systems may rarely see these conditions, and the chances of a system experiencing peak water loads for a full day, on the same day that one experiences -5C conditions all day, is extremely remote. That is however what systems are normally setup to deliver all day every day, and as a result end up been an inefficient as they are oversized for normal use.

The data used in this calculator comes from extensive field studies carried out by The Energy Saving Trust, and some other sources listed below, and provides realistic data of what loads can be expected, as well as a feel for the exceptional loads that can be experienced. These data sets provide:

  • Volumes of DHW used per property
  • Seasonal variations in DHW volumes, and supply temperatures
  • Seasonal variations in cold mains supplies
  • Seasonal central heating loads
  • Hourly DHW usage figures
  • DHW and CH Diversity
  • HIU performance data for 5 manufacturers against a common standard

The calculator allows you to set peak conditions, and it is these upon which pipes, boilers, and buffer stores are sized, however it uses the real world data to calculate the operational efficiency of the peak-sized network.

And here lies the real value in the calculator - it enables a designer to understand the impact of different design approaches, so one can maximise seasonal efficiency while still been able to meet peak (and freak) loads.

Operational Analysis and Losses

Efficiency is improved by first understanding inefficiencies, and then trying out tactics to reduce them. The calculator separates energy use into loads and losses, totaling them up over a year to arrive at an annual efficiency.


Energy use is divided into the following:

  • Central heating loads
  • Hot water loads
  • Plantroom heat loss
  • Pipework losses at load
  • Pipework heat loss when in standby keep warm mode
  • HIU losses in standby

The big energy savings that can be made are in standby losses, both from pipework and HIUs, and are made by understanding the relationship between standby losses and the need to keep DHW delivery times reasonable.

Data Sources

To ensure that the calculations are as accurate to real life as possible we have taken data from the following sources:


The point of the calculator is to be able to model district heating risers as accurately to the real world as one can.

To achieve this requires a good deal of information to be gathered, and it also requires some assumptions. Therefore the calculator provides inputs that to gather all the necessary variables and allow assumptions to be altered.

It is the aim that no assumptions are fixed by the tool, other that is than the known laws of physics. This includes all the EST data input used in daily and seasonal calculations, which can be adjusted to look at special cases.

One thing you will quickly learn from using the tool is the relative impact inputs have on the results. For example, altering the keep-warm strategy can have a huge impact on efficiency, as can a fairly small change in pump pressures, whereas insulation levels and peak boiler temperature can have little impact in certain situations.

The calculator therefore lets one cut through generalised statements about what makes an efficient system, examining their validity under different circumstances. It shows how in one instance a particular approach has great results, but in another can prove disastrous.


The layout of a building is most important as it determines what strategies will work best. It is also one of the only 'fixed' inputs, only variable at the early architectural design stage.

Currently the tool allows a good degree of versatility in designing a layout, and if future functionality will be added to customise further.

  • Floors, Branches, Feeds - The layout, with a number of branches (or laterals) off a riser, each feeding a number of properties (feeds).
  • Run up to Riser - The length of pipe that feeds a riser from the plantroom, (or main network).
  • Height per Floor
  • Run to first Property - The length of pipe from the riser to the first property on a branch.
  • Feed to each Property - The length of pipe feeding a property off a branch.
  • Space between Properties - The length of pipe between properties. If properties are in pairs or oddly spaced, then the average should be used, i.e. the total length of pipes between properties, divided by the number of properties.

As a rule of thumb, longer branches reduce efficiency. Once a branch becomes long enough, the time it takes for an HIU to clear cold legs from a riser becomes unworkable. One choice of HIU makes big difference in its ability to rapidly clear these cold legs, so an HIU that boost the flow to 20 litres per minute if a tap is opened on a cold leg will clear the pipes in seconds, where an HIU with a proportional control valve will take considerably longer.

If the branches are too long, or the wrong HIUs selected, then a permanent keep-warm needs to be deployed, and a significant amount of constant heat loss is introduced to the system.

The following images give a feel for the variety of layouts possible.

Single BranchTwo BranchesMultiple BranchesSpread OutTower Risers

HIU Functionality

This is the section of inputs that we feel is most important for building services engineers.

The functions that an HIU provides can have a huge difference. It is the reason we created the calculator, in an effort to educate the industry at large just how important decent control features are to delivering an efficient system, where energy costs are comparable to other forms of heat.

  • HIU Keep Warm Temperature - Where an HIU is kept warm to a specified temperature, this is where the temperature is set.
  • HIU Flow - When a fixed bypass is used at the HIU, this is where that flow rate is entered.
  • Riser Flow - Where no keep-warm is used on the HIU, a riser bypass is the fall-back. The bypass flow rate is entered here. If a fixed temperature bypass is used, then you will need to take a guess and adjust flow until the predicted temperature matches your set temperature. This option is always the most efficient selection, but can only be used where the last HIU on a branch can clear an entire cold branch in time.
  • HIU Boost Flow - This is the flow rate that an HIU pulls from the network when a tap is opened, and the network supply is too cold. It is possibly the most important detail of an HIU to be looked at, as it determines if keep-warm modes are required. Quite simply, if an HIU can draw water from a network quick enough to clear cold legs in the network, when needed, then it means a keep warm is not required, and the majority of heat loss from a network can be eliminated in a single move (i.e. use an HIU with a decent boost flow). Figures for the Boost Flow come from independent testing at the Technical Institute of Sweden (SP) under the new UK test regime for HIUs, and are read from the DHW response tests after overnight standby.
  • HIU Trickle Heat Loss - This figure is important is you intend to keep HIUs at elevated temperatures at all times using fixed flow rates or set keep warm temperatures. It is an estimate of the heat losses form an HIU (in Watts) for every degree above ambient temperatures. Data on this is difficult to find, but can be drawn from the SP test data. In general however, efficient systems do not need to keep HIUs hot all the time, so this figure can have little or no impact under the correct keep-warm strategy. Heat loss from an HIU during heating season is also arguable a positive contribution to the heating load.
  • HIU Standing Heat Loss - This is to cover the option of using hot water cylinders or thermal stores in properties in place of HIUs. Such 'Storage HIUs' will give of heat all the time they are hot. This figure should be set to the average heat loss from the storage system (including from any pipes maintained at temperature). Modern A-Rated cylinders are far lower than older cylinders with poor insulation, and one should factor in an allowance for how long cylinders are turned off. This input should be set to zero when using instantaneous HIUs.
  • Peak CH Return Temperature - This is the temperature of the network return from a HIU when running central heating at the stated full load. Where radiators are to be used we have used VWART figure for central heating from the SP test data. It is however a function more of the choice of heat emitters, and should be quite low for underfloor heating. This input also allows one to examine the impact of a poorly balanced heating system where return temperatures are higher on average from the design criteria, and for HIUs without a return temperature protection it should arguably be set higher. The figure is used in conjunction with the peak load figures for primary temperatures and heating loads to determine temperature drops and flow rates required from the network. As such it will impact on return heat losses, network pipe sizes, and therefore on keep warm strategy.
  • Peak DHW Return Temperature - This is the temperature of the network return from a HIU when running domestic hot water. We have used VWART figure for central heating from the SP test data. The figure is used in conjunction with the peak load figures for primary temperatures and heating loads to determine temperature drops and flow rates required from the network. As such it will impact on network pipe sizes, and therefore on keep warm strategy.
  • DHW Priority - A feature on an HIU where the central heating is shot down when a tap is run. This is a feature common to gas combi-boilers, and some HIUs. It enables the HIU to hit higher DHW loads with a given limit on network supply, and as such impacts on pipe sizing for peak loads. The input includes an option for networked hot water priority, a function under patent pending, whereby all HIUs in a network can reduce central heating load in response to demand from the network. This option has a big effect on pipe sizing as one only uses the larger of either hot water or central heating, rather than the sum of both, but requires specific HIUs as well as a communications network to work with.
  • CH Control - Nearly all HIUs target a fixed central heating temperature, often in the region of 75C in order to meet peak load winter conditions. This temperature is fixed throughout the year, and prevents one from lowering network supply temperatures as they must remain higher that the heating setpoint to avoid valves opening. Some HIUs however, provide functionality to compensate central heating target temperatures based on the available network temperatures. When this option is selected, the seasonal calculation will weather compensate the network primary flow temperatures to reduce heat loss from the network. It is a future feature for the calculator to calculate the effect on heat generator efficiencies for a selection of generator types (boilers, heat pumps, waste heat etc.).
  • DP required into property - Most HIUs come with a requirement for a differential pressure to be maintained for peak load performance, typically around 50kPa. This figure can also cover additional pressure losses such as commissioning sets, or elbows in pipework, if needed.

A lot of options to be sure, but each one can make the difference. Ignore any of them at your peril.

System Parameters

  • DP across system - This represents the pump head provided from the central plant. A higher DP enables smaller pipework, and as such can have an impact on the type of keep-warm strategy. When everything else is fixed, increasing the pump head can reduce pipe sizes to get you across the line than enables keep warm to be turned off. In other words, higher pressure result in smaller pipes that can be cleared faster. The maximum DP would typically be 2 to 2.5 bar, as one must allow for the increased static pressure on pipework and components. Peak DP should be checked against HIU limits to check if options for additional DP limiting valves are required.
  • Time at Load Temperatures - We need more real life data to put a more certain figure on this. It represents the percentage of the time that a heat network sits at full load temperatures relative to keep-warm temperatures. It will depend on the frequency and location of tap use, as well as pipe losses, and the percentage of time spent with central heating on - although one can argue pipework heat loss during heating season is not all a loss. We have estimated the figure as best we can from the EST data on tap frequencies. The only way to get this figure lower is to use managed hot water storage where cylinders are reheated at set times, ensuring the network can go cold (or weather compensated) at other times. It is a future improvement of the calculator to draw data from existing sites that can provide such data to more accurately determine this figure, however the input is provided so the user can run calculations at their own estimated figure.
  • Peak primary temperature - The temperature the heat network is run to hit peak load. In the seasonal calculations, and where a compensating HIU is selected, the figure is reduced to seasonal and DHW requirements. It is important to know that a higher peak temperature typically results in lower efficiencies die to higher heat loss, but it does reduce pipe sizing. When used in conjunction with a compensating HIU, higher peak network temperatures will in fact result in significantly improved efficiencies, as pipe sizes can be reduced to hit peak load conditions (mid winter, high DHW use) but there is no additional heat loss at all other times as temperatures are reduced.

Fixed Parameters

These are inputs that do not fall into the above categories.

  • People per Property - used to determine DHW use.
  • Peak DHW Load per Property - The peak HIU output, however it should be understood that used diversity standards are based on 37.5kW per property. This effects pipe sizing.
  • Daily DHW Use per Person - In the first calculation run used to generate the pie chart, a fixed volume per person is used. In the season calculations EST data is used instead based on a fixed volume plus a volume per person.
  • Peak CH Load per Property - The peak central heating design load, and used in pipe sizing calculations.
  • Annual CH per Property - This is used in conjunction with the standardised figures used in the SP tests for VWART calculations to determine monthly energy use.
  • Pipe Insulation Levels (22mm) - A figure representing the heat losses from insulated 22mm pipework at 70C. This can typically be taken from literature on insulated pipework, and the calculator estimates the heat loss of larger pipework from this figure. It is a future feature to specify insulation levels by riser, branch and feed.
  • Plantroom Heat Loss - A fixed quantity of energy lost from the boiler plant over a year that is used in calculations of percentage total losses. In future a suggestion will be made from the load calculations based on kWh/year/kW capacity of plantroom drawn from real world data (not yet available).
  • Ambient Temperature - The average air temperature surrounding network pipework. In the season calculation this is varied.
  • Cost of Energy - A figure used for putting pounds to kWh. It is set initially to represent gas boiler prices. It is been considered to expand the calculator to include maintenance costs and energy pricing to work out 'profitability' of a network, as well as the options for standing charges vs direct energy charges.


As it says on the tin, this is a Calculator, and not a sales tool. In essence, we are taking published test data on a range of HIUs, and using this to calculate which ones meet the design criteria. But, given our HIUs usually provide the best results, how is it to be trusted?

The only way to be sure is for the code behind the calculator to be open-source and available for scrutiny, so it is. We invite all manufacturers listed in the Calculator to challenge the figures or methodology, and we will be happy to implement improvements or introduce additional data sources or further functionality.

It is also important that the Calculator provides detailed reports on the calculations it has performed, so they can be checked and confirmed as accurate by the user. To this end the Calculator generates a table of calculations that covers every single pipe in the network, listing all the calculations. Flow rates, temperatures, and pressures are all fully mapped and reported. All hourly and monthly calculations are tabled for scrutiny.

It must be stated however, that the final efficiency figure is not a guarantee of final performance. It provides the best possible estimates using standardised calculations, and drawing on data from recognised independent tests and trials.

In this part we describe in more detail the actual calculations performed by the Heat Network Calculator.

Network Map gives a visual overview and access to dataHover over pipework to see calculationsHIU Specifications to meet designHIU Selection Chart provides test data alongside design targetsChart of diversities used for DHW and heatingCalculations Chart provides detail on every pipeHourly Peak Load Analysis used for sizing boilers and storageHourly Load and Storage Analysis showing utilisation of central storage over 24hrsSeasonal Efficiency Analysis details monthly calculations to obtain an annual figureSeasonal Efficiency Analysis inputs and calculationsSummary CalculationsInputs

Pressure Mapping and Pipe Sizing

To obtain the figures used in the energy calculations requires a complete analysis of the heat network, down to every pipe.

Th first task, therefore, is to complete the design of the heat network by selecting the appropriate pipe sizes. The process can be best explained in the following steps:

  1. Calculate the peak DHW and central heating loads for HIUs, using known temperature drops and ratings.
  2. Calculate the peak DHW and central heating flow rates at every pipe, using diversity on downstream HIUs.
  3. Calculate distance from plant of index property.
  4. Work out maximum pressure loss per metre using supplied pressure and required residual head.
  5. Calculate each pipe size from peak flows and maximum pressure loss per metre.
  6. Calculate volume and heat loss coefficients of each pipe.

Following this we have a physical map of the full network for which we can go on to calculate heat losses under various load conditions.

Full Load Pipework Analysis

With known physical characteristics it is possible to now:

  1. Calculate the heat losses from the flow side of the network when at primary flow temperatures.
  2. Estimate average return temperatures from HIUs.
  3. Estimate the return temperature losses.

Return losses are estimated by using the calculated flow losses and scaling against the calculated (or set) HIU temperatures under standby. It is assumed that return pipework will generally be decided by keep-warm temperatures, however it is planned for future versions to perform a more thorough time weighed monthly return temperature analysis.

Standby Pipework Analysis

The next stage is to calculate the heat losses from the network, when in standby conditions. This gives us a minimum possible figure for heat loss (the maximum losses been when the network is at full temperature).

User inputs tell us the keep warm strategy, which will be either based on fixed flow rates (via HIUs or riser bypass) or temperature based at HIUs. Where fixed flow rates are involved the calculation is as follows:

  1. Calculate flow rates and through every section of pipe with system under standby.
  2. For each pipe, starting from plant, calculate the temperature drop at the calculated flow rate. This done by iteration until temperature drop from known flow matches temperature drop from the LMTD losses.

Where a temperature at the HIU is required the calculation is as follows:

  1. Guess the flow rate at each HIU.
  2. Calculate flow rates and through every section of pipe with system under standby.
  3. For each pipe, starting from plant, calculate the temperature drop at the calculated flow rate. This done by iteration until temperature drop from known flow matches temperature drop from the LMTD losses.
  4. Check how the achieved temperature at every HIU compares to the target, and adjust the guess appropriately.
  5. Repeat steps 2 to 4 until the temperature delivered to each HIU is on target.

This iterative process is required because of the interplay between HIUs, and can take up to 500 cycles for larger networks. It provides the limit on the software capabilities, and for very large numbers of HIUs the calculator will simply time out if too large to calculate.

Efficiency Calculation

We now have figures for the heat loss from all parts of the network under standby conditions, and under load. We also have details on annual heating loads as well as hot water loads, so it is possible to generate an initial efficiency figure.

  1. Using the figure for % time at load, calculate the annual heat losses from the network pipework, using the fixed primary flow temperature.
  2. Calculate annual losses from HIUs
  3. Compile totals, using heat losses at standby, heat losses at load, HIU losses, and plantroom heat losses, to arrive at efficiency figure.
  4. Generate pie chart.

This figure is however based on a fixed primary flow temperature, and doesn't yet take into account seasonal fluctuations in loads as indicated from the EST trial data sets.

Seasonal Efficiency Analysis

To obtain a more realistic efficiency figure the next phase of calculations examines the network month by month, and pulls in the options for weather compensating the network. It also draws on EST data that indicates how hot water volumes an temperatures, and mains water temperatures vary month to month, and from the SP VWART calculations the hours of central heating at various loads.

The analysis generates a table of calculations, as well as a bar chart to detail the proportions of energy use and losses. The table does allow the inputs to the calculations to be varied from the assumptions, and enables different temperature profiles to be tested, as well as various heating loads.

This more detailed analysis then provides us with an efficiency rating from A-F, indicative of the overall seasonal efficiency of the system.