Hot Water Response Time and Heat Network Efficiency

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As the old saying goes – you can’t have it all. When it comes to heat networks, there is a distinct balancing act between heat network efficiency and hot water response time.

In this blog, Helen Gibbons, Technical and Design Manager at Evinox will discuss the trade-offs between hot water response and efficiency in heat networks.


VWART vs. DHW response

Introduced in 2016, the BESA (British Engineering Services Association) UK Standard for HIUs was launched to help designers and specifiers evaluate the performance of a particular unit on their network. To date, of the 7 electronic units with published test results, a trend can be seen where the HIU achieving the lowest VWART delivers the slowest DHW response time, and vice versa. Most units tested achieve balanced results and end up somewhere in the middle for both tests.


The charts shown above display a VWART figure and DHW response time for each electronically controlled HIU that has been through the BESA test (I haven’t included results for mechanical HIUs as they are not able to control the keep warm functionality). These results clearly show a correlation between a low VWART figure and slow DHW response. HIU manufacturers who have put their unit forward for this test have made a choice – what do we think is more important, VWART or DHW response time?

The manufacturer with the impressive VWART figure of 28.0 °C has achieved this partly by allowing their DHW response time to increase to 12 seconds for the high temperature test (and 14 seconds, close to the required limit of 15, in the low temperature test). At the same time, the manufacturer with the fastest DHW response time, which is 6 seconds, has clearly achieved this at the expense of the VWART – achieving 38.4 °C.


What is keep warm functionality and what role does it 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.

(Check out my previous blog for more information about Keep-warm)

Whilst the VWART figure clearly reflects the efficiency of the plate heat exchangers and valves used in the unit, it also shows that improvements can be made by adjusting the keep warm strategy and allowing the DHW response time to increase. It’s obvious when you think about it, if less power is used to keep the plates warm, it will then take longer to receive hot water.

With the new generation of electronically controlled smart HIUs on the market, which feature self-learning, there are a number of settings that can be put in place for the keep-warm functionality, to help provide a fast DHW response with a low VWART. Take our ModuSat XR as an example, the self-learning functionality means the unit can learn what time you get up every morning and make the plate hot and ready for you by that time. Even if this might not show in BESA test results, in real life terms this will further reduce the return temperature.

Additionally, with a Smart HIU the network operator is able to decide the best solution for their specific network and make adjustments to the settings remotely. Is it DHW response time for the residents or the overall efficiency of the heat network or an agreed compromise for both?


Legionella and DHW response

But by how much can you delay the DHW response time without getting into bother? Is the DHW response time a question of convenience and comfort, or are there other aspects at play?

These questions go hand in hand with what I discussed in my previous blog post about timing the keep warm function, where a large energy saving can be made if the system is designed in a way which lets the keep warm function switch off during parts of the day.

One of the arguments out there against a delayed DHW response time, is the possibility of the hot water taking longer than 60 seconds to reach the outlets. The Hot Water Association even goes as far in their Design Guide for Stored Water Solutions [1] as to boldly state that the keep warm function is required for twin plate HIUs to meet requirements of legionella and water regulations, which is simply not true.

This all stems from HSE’s advice on Legionnaires disease, where in HSG274 Part 2 it states that the traditional strategy for reducing risk of legionnaire’s disease, is to store water at 60°C or above, and for this water to be distributed so that it reaches the outlet at 50°C within one minute. It is important to note that the same document classes instantaneous heaters as low-risk systems.

The same advice is replicated in BSRIA’s Legionnaires disease O&M log book, which suggests that hot water should reach 50°C within one minute, but class instantaneous water heaters as an alternative system presenting minimum risk from legionella.

BSRIA Legionnaires disease O&M log book

This advice is replicated in a number of other documents, all referencing back to HSE 274 Part 2.

The above table from BSRIA makes it clear that the “50°C within 60 seconds” school of thought stem from advice based on stored water, but as there is next to no stored water in a twin plate HIU, the risk of legionella is minimised.

We also have the well-respected German Standard DVGW W 551 (Drinking water heating and drinking water piping systems; technical measures to reduce Legionella growth) which states that if controlled properly, a system with a total volume (from hot water production to end use) of less than 3 litres can eliminate the risk of Legionella.

Standards and regulations aside, is there actually a risk of legionella if the DHW takes longer than 1 minute to reach the outlet? From what I have found there is and there isn’t, but the DHW temperature set point at the HIU makes no odds.

At 60°C, it takes 25 minutes to kill the legionella bacteria [1].  If we take this information and think about cold water being heated up to 50°C in the plate heat exchanger of the HIU. The water will be heated for a fraction of a second, nowhere near enough time to kill any of the bacteria present in the water.

This is backed up by the following graph illustrating how long it takes to decrease or increase the growth of legionella bacteria at different temperatures [2]:

Growth of legionella bacteria Graph

The y-axis is showing the number of bacteria colonies per ml H2O and the x-axis is showing hours lapsed. This graph shows that after one hour at 50°C, the bacteria has reduced by 60%, but that during the small amount of time the water is heated to 50°C in the PHE, the level of bacteria is unaffected.

What about if the water is heated to 50°C and kept at 50°C during the one minute it takes for it to get to the tap? Nope, it still doesn’t look like one minute at 50°C will be able to kill much of the bacteria that would potentially be present in the water.

The conclusions I draw from this information are that getting the water to the tap at 50°C within 1 minute has no effect on the legionella risk when it comes to instantaneous twin plate HIUs. The risk is where the water is stored, ie. in the cold water booster tank and the cold water distribution pipework. This is where there is a risk of the legionella bacteria multiplying if the temperature of the cold water is too high, and if the bacteria has been given the chance to grow in the cold water pipework. Heating it to 50°C or even 60°C in an instantaneous water heater will not kill it or even noticeably reduce the levels.

Despite this information, I can still understand if system designers would like to stick to the 50°C within 1 minute rule in the absence of any firm advice regarding instantaneous heaters. Even BS 8558:2015 quotes the 50°C within one minute rule, but again references back to HSG274 Part 2. A discussion needs to be had with HSE to see if this piece of advice can be rewritten to clarify that this rule should not apply to instantaneous heaters.

If legislation stipulates a high DHW temperature with a very fast DHW response time, this will lead to the continuation of unnecessarily oversizing heat networks under a false pretence that this somehow protects the end user from contracting Legionnaire’s disease.

There is of course still the comfort argument to be had, but that’s for another day.


Hot water return and heat network efficiency

In line with the above, we often see projects where a specific domestic hot water return temperature is detailed in the specification for instantaneous twin plate HIU’s. When we see this, it is important to have a discussion with the client to understand the reasons behind the required temperature and explain how this might affect the heat network efficiencies.

As with the keep warm facility, the DHW return will increase convenience for the resident but will come at an expense of the heat network. The return temperature of the primaries cannot physically be lower than what they are cooling down against, and if they are constantly cooling down against a hot water return of 50°C, the primary return will always be above 50°C in this mode.

The above will also mean that the HIU will constantly be bypassing primary water, so both the total volume of water, as well as return temperature, will increase.


Alternative designs that offer a happy medium

When I said earlier that you can’t have it all – this is of course not true. There are a couple of ways in which you can have a fast DHW response time which will not come at the expense of the heat network efficiency. Solutions we recommend include the following;

  • Trace heating of the hot water pipework. If a trace heating cable is placed on the pipework underneath any lagging, you will still get the fast delivery of hot water. And as the trace heating is only used to maintain the temperature of the water in the pipe, rather than heating it up from cold, the energy consumption from applying this is low.
  • Reduce the size of your hot water pipework. Let’s say the pipework required from the HIU to your hot water outlet is 6 meters. If you use 22mm copper to deliver this water, and the required hot water flow rate is 0.10 l/s, the velocity within the pipework will be approximately 0.31 m/s and it will take the water almost 20 seconds to get to the outlet. Reduce the pipework dimension to 15mm and the delivery time is more than halved to 9 seconds. And if you are able to use 10mm pipework the velocity increase to 1.64 m/s with a delivery time of only 4 seconds! This means that even if the HIU takes 55 seconds to get the hot water up to 50°C, you are still within the golden minute.

If this is not possible, an alternative to using small dimension pipework is to install a potable water manifold at the HIU, and to have a single pipe serving each outlet.



In almost all cases – fast hot water response time will come at the expense of the heat network efficiency. As an industry, it should be our priority to make these networks as efficient as possible to avoid getting negative press and secure the position of heat networks in the UK infrastructure. It is the price per kWh and the carbon emissions that will get measured at the end of the day – not how long people have to wait to have a shower, especially not when it is a question of seconds.

In high end developments, where comfort and convenience are especially critical, the alternative design solutions should be used, i.e. trace heating or small-bore pipework. As long as there are ways around this, let’s allow our systems and HIU’s to perform as efficiently as possible.

But most importantly of all, let the operator of the heat network decide what they want to do, after all, they will know their system and residents better than any of us. Is efficiency and cost important? Is fast hot water response important? As long as we give the operator the information and benefits of different options, why not let them decide? With a smart-HIU they can even try different options out over time, until they find the ultimate strategy for their building.

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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.

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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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