HIU Keep Warm – What’s the Best Approach?

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If you have an interest in Heat Network design or are involved in the specification of Heat Interface Units (HIUs), you will no doubt be aware of the new BESA (British Engineering Services Association) UK Standard for HIUs, launched in late 2016.

This new standard is regarded as a really important step towards improving the overall performance of British district heating schemes, and its key objective is to enable the performance of different HIUs to be evaluated within the context of typical UK operating conditions. This will enable heat network developers to consider the performance of specific HIUs against design requirements.

The main outcome from the BESA test standard is a Volume Weighted Average Return Temperature – or “VWART” for each HIU at standardised high temperature and low temperature network conditions.

A lower overall VWART represents a lower average annual return temperature from the HIU to the primary network and therefore a better performing HIU. The example below shows published BESA VWART results for our ModuSat® XR-ECO at High Temperature Test Conditions –


Keep Warm BlogToday I want to focus on the “Keep Warm” function, looking at the different approaches and how best to balance the supply of hot water with the overall efficiency of the network.

So what role does the Keep Warm Play?

One of the key impacts on the overall VWART figure is the use and control of the “Keep Warm” facility, which is designed to keep the hot water plate heat exchanger warm continuously so that the HIU’s can provide a fast response as soon as there is demand for domestic hot water. This function also has the added benefit of keeping the risers and laterals warm, although additional controlled bypass valves are sometimes required, for example at the top of the riser or at the end of laterals.

For the BESA test regime, the Keep Warm VWART figure is calculated using a weighted average that assumes Keep Warm is switched ON for an average of 90% of the year (i.e., constantly available while the HIU is not producing space heating or hot water).  And while the Keep Warm facility should only ever bypass a small volume of water per hour, the proportionately high number of annual hours that a Keep Warm bypass might be left “on” for can add up to a significant amount of warm water being bypassed back to the network over the course of the year, wasting energy, raising the primary return temperature and reducing the delta T, as illustrated in the table below:

KWF Table 1

As can be seen, despite the Keep Warm being temperature controlled (in this case), this facility is still potentially responsible for over a third of the annual volume of water by passed back to the primary network.

So, what’s the best approach?

You will find that different HIU manufacturers approach “keep warm” in different ways, typically we see the following for UK heat interface units –

KWF Table 2

By far the most efficient and effective solution, is to provide both time and temperature control over this function, by additionally allowing the keep warm to be timed, it can be de-activated during times of low demand (eg middle of the night), something which is only possible with electronically controlled HIU’s. With a growing understanding in society of the need to be more energy efficient, residents are likely to find it acceptable having to wait a little longer for their hot water at 3am, especially if the cost and energy saving benefits are explained to them. The latest Evinox ModuSat XR models allow this control to be managed either by individual residents themselves; or managed globally across the network by the operator, as the Keep Warm time and temperature settings can be remotely adjusted.

This means that real life average annual return temperatures can be further reduced, with a suitable balance struck between the speed of DHW delivery for the home owner and the overall network efficiency requirements of the operator.

For example, we have calculated that the impact of restricting the operation of the Keep Warm facility to 8 hours per day, can result in a 23% reduction in water bypassed back to the primary network.  Based on the performance of the Evinox ModuSat XR HIU, this would improve the (already impressive) overall VWART figure by a further 7°C – giving an annual Volume Weighted Average Return Temperature of 26°C!

But is there an increased risk of Legionella?

HSE guidelines state that the hot water should reach the outlet at 50°C within one minute. Does this mean that there is an increased risk of Legionnaires disease if the keep warm function is turned off causing a delay in the hot water to come through? No, it doesn’t!

First of all, the “50°C within 1 minute” guideline is there specifically for hot water storage. As there is no storage of hot water in an instantaneous heat interface unit, the legionella risk is virtually eliminated. Additionally, when the keep warm function is turned off in several properties across the network, this will stop unnecessary circulation through laterals which in turn will help prevent overheating of ducts. Overheating of ducts can cause the temperature of the incoming cold water to rise above 20°C, favourable conditions for legionella growth. I will be discussing this in more detail in a future blog.

So, as you can see, it’s important to consider the HIU Keep Warm strategy when designing your next communal or district heating project. Decisions made at the design stage can help to ensure you provide the best system for residents, whilst also maintaining optimum network efficiency.

Helen Gibbons – Design & Technical Manager Evinox Energy Ltd.

Find out more about Evinox’s ModuSat XR range of electronic HIU’s with full time and temperature control

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Top Considerations for Heat Interface Unit Selection

So, what should specifiers – and indeed clients, such as developers and social landlords – be considering when specifying HIU’s for their next heat network project?

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Top Considerations for Heat Interface Unit Selection

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System designers faced with a myriad of challenges within the design of heat networks, can be forgiven for labouring under the misapprehension that HIU’s are one-size-fits-all. In fact, nothing could be further from the truth, as recent studies have shown that the performance of HIU’s can have a significant impact on the overall efficiency of a heat network; and therefore, the very carbon and energy cost savings that the network is intended to deliver.

So, what should specifiers – and indeed clients, such as developers and social landlords – be considering when specifying HIU’s for their next heat network project.  Here are our top tips:

ModuSat XR Twin Plate Heat Interface Unit

1. Independently tested and compliant with industry best practice guides

The new Building Engineering Services Association (BESA) UK test standard for heat interface units (HIUs) has been introduced as a voluntary scheme, which independently tests the performance of HIU’s under “typical” UK operating conditions. Delivering low return temperatures from the HIU back to the heat network is crucial in driving efficiency and reducing energy wastage, and this new standard provides specifiers and clients with a clear benchmark of how any HIU will perform in heating, hot water and “keep warm” modes over the year.

Any unit you specify should have been through this test regime and have its performance results published. (If not, then perhaps ask yourself why not…!).

So, ask your manufacturer for details of their published DHW return temperatures (CIBSE Guide recommends below 25℃); what sort of deltaT’s you can expect in heating/hot water and keep warm modes in “typical” UK conditions; and evidence of this through BESA test results.

2. Designed for modern, low temperature heat networks

Lowering system temperatures is a critical factor in the efficient operation of heat networks. Heat networks with 70/40 primary flow/return temperatures are becoming commonplace.

The best HIU’s will offer the ability to maintain high deltaT’s across the network, even at low primary flow temperatures, while still also offering the capability to deliver (for example) domestic hot water at 50-55°C.  The latest Evinox ModuSat XR-ECO models are able to do just this, delivering 55°C hot water at impressive hot water capacities and flow rates, with a primary flow temperature of just 60°C

3. Timed Keep Warm Facility

The ability to provide a quick response to demand for domestic hot water (i.e. when a tap opens) ensures high levels of user comfort and convenience, while also reducing energy and water waste.

HIU’s often do this using a so-called “Keep Warm” facility, which is designed to bypass a small amount of warm water through the HIU to ensure that the domestic hot water plate does not become too cool.  However, bypassing warm water back to the heat network increases the average return temperature, so this needs to be controlled, which any reputable HIU will, at least do, thermostatically.

Better still, market-leading HIU’s are able to provide further control in this area, allowing the Keep Warm function to be operated on a time schedule, helping to save energy by switching the function off and allowing the hot water plate temperature to drop during times it is unlikely to be needed (such as during the working day or in the middle of the night). This reduces the volume of warm water returned to the primary network, helping to maintain a wide DeltaT and high operational efficiency.

Keep Warm functionality Slide

Evinox believes that timing the operation of the keep warm facility to key period of occupation can save around 6% on the annual energy consumption of the network, so it’s crucial to consider your Keep Warm strategy early on in the project!

4. Electronically controlled

A key differentiator among HIU’s on the market today is the adoption of electronically controlled valves by leading models.

The use of Pressure Independent Control Valves (PICV’s), in combination with advanced control logic algorithms provides a myriad of benefits in terms of HIU performance, optimisation for heat network efficiency and ongoing customer support, including:

  • PICV’s provide differential pressure control, so no need to fit additional differential pressure control valves elsewhere on the network;
  • Aids primary system balancing and is “set and forget” as no additional adjustments are required once the heat interface unit is initially commissioned;
  • System can easily be adapted or modified to include additional heat interface units, if required, with no PICV adjustment;
  • Minimises system pressure fluctuations and ensures efficient operation of the HIU by close control of the heat energy required;
  • The PICV also reduces overall equipment costs and the time required to carry out system commissioning.

A historic criticism levelled at HIU’s featuring PICV’s is that they are slow to respond to changes in heat demand and therefore potentially wasteful in terms of energy consumption.  But independently produced results from the BESA test standard clearly shows that HIU’s that include the latest fast-acting valve actuators and self-learning control logic to PICV’s, will deliver fast response, close control and low return temperatures.

5. Smart Connected Control

Furthermore, electronically controlled HIU’s provide many additional benefits which are simply not possible without the use of electronic control, including the ability to facilitate a 2‐way communication network, which when connected over the internet to remote servers allows the HIU to become an IoT device, offering a wide range of benefits to both consumers, facilities managers and heat network operators alike.

For example, the heating and hot water system in each dwelling can be remotely metered, controlled, interrogated and tested, both during the initial set up/commissioning phase and ongoing operation.

Remote access to the HIU also allows domestic hot water and/or space heating set‐points to be altered remotely after the HIU’s have been commissioned, which allows the system operator to maximise the efficiency of the whole system and end user queries to be quickly and effectively resolved without the need for an engineer to attend site.

6. Metering and Billing Ready

The requirements of the Heat Network (metering and billing) regulations now mean that all end consumers on a district or communal heating network must be billed for their individual heat consumption.

For social landlords (and increasingly in private schemes), pre-payment billing systems are common place and PICV’s offer a further advantage over mechanical HIU’s by acting as the energy shut-off valve and thus allowing automatic disconnection of the heating and hot water when credit has been exhausted. This allows the HIU to be delivered as “pre-payment ready”, removing the requirement to fit an additional external shut‐off valve and avoids the associated cost and installation time.

7. Weather compensation

Weather compensation allows secondary heating flow temperatures to be reduced, based on outside temperature. Lowering the secondary flow temperature whilst maintaining a consistent deltaT across the secondary circuit means lower return temperatures to the network can be achieved, helping to optimise overall heat network efficiency and further reduce resident energy bills.

Smart, connected control allows outside temperature data to be sent to each HIU, which can then adjust heating flow temperatures accordingly. Operating in this way, only a single outdoor temperature sensor is needed to provide information to the entire building.

8. Energy display devices

Smart metering systems are increasingly commonplace, and the ability to display real time and historical heat, power and even water consumption is often demanded by specifiers, either to meet planning regulations or developers added value specifications.

The ability to provide this information direct from the HIU through its own wall mounted display (which also acts as the system programmer and thermostat) is a key attribute where energy display is a requirement, reducing the cost and complexity of providing additional devices, as well as minimising the amount of wall space taken up by various controls and displays.

ViewSmart ENE3 controller with 12 months display


So, I think you will agree that we have seen some serious innovation in heat interface unit design and technology in recent years, and units are certainly not “one size fits all”.

Consider the following before you choose your next HIU –

  • Can the units perform efficiently on a low temp network?
  • Are the units Independently tested to the BESA standard with published results?
  • Do the HIUs operate using electronically controlled Pressure Independent Control Valves? And if so, does the HIU employ fast-acting actuators and advanced control logic to deliver fast demand response?
  • Does the HIU have the ability to control the Keep Warm function by temperature and time?
  • What control and communication functionality do the units provide?
  • Are they Metering and Billing Ready without the need for extra equipment?
  • Can they be supplied with HIU compatible Energy display devices?

You can find more information on the latest Evinox ModuSat® XR and ModuSat® XR-ECO Heat Interface Units in the product area.

Chris Davis – Head of Sales & Marketing at Evinox Energy Ltd.

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