The HIU Revolution

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This article has been inspired by the recent publication of independent test data on a spread of Hydraulic Interface Units tested against the first ever UK standards, as well as by the ensuing technical discussions.

The tests have shown one particular HIU to stand out from the crowd in terms of expected efficiency in the field.

This article explains why its time to throw out the existing book, and embrace this revolutionary new technology.

Teaser Video

To view our teaser video please go to

Accuracy of Data

The data used in this article is extracted from the published test data from recent tests carried out at SP Technical Research Institute of Sweden under the new UK testing standards funded by DECC, and supported by (among others) Engie, EON, SSE and six of the big names in HIUs.

See the Fairheat website for more information

We are the only HIU manufacturer we know of to publish ALL the raw test data.

We have nothing to hide, but a lot to explain...

Key Advances in Technology

At the core of the revolution is the shift from mechanical systems to electronic. Believe it or not, the district heating industry still clings to the concept that mechanical is best and more reliable. This is simply down to having not yet experienced an electronic system from a reliable manufacturer.

There is however no doubt that the ability of electronic systems to deploy advanced control strategies, using just the one valve per circuit, enables systems to become more advanced without the need to add more and more valves (or valve components).

The introduction of electronics also enables networked communications. It allows remote management of settings, remote fault alarming, and holistic heat network functionality.

The nice thing is, there is usually all the hardware already in place to allow this networking to take place, in the form of a billing system, and we will in part cover the latest advances in this arena.

The real breakthrough that has been made in the HIUs, enabled by the use of electronics, is the way the systems manage the volumes and times that primary heat is drawn, in order to manage the heat network in its entirety. The effects on heat loss and volumes circulated is, as the independent data proves, quite revolutionary.

Efficiency Categories

Saving on wasted energy is the key to it all. Paying less for your energy, and pumping less carbon into the atmosphere than we need to. Important stuff in our book.

We believe inefficiencies (or losses) can be split into various categories:

  1. Generation efficiency - how efficient the plant room is at generating heat, and typically a function of flow and return temperatures.
  2. Heat loss from main network flow pipework - these are the distribution pipes that carry heat to homes and need to be maintained at temperature.
  3. Heat loss from branch flow pipework - these are the pipes in the vicinity of properties, that can drop in temperature (conditions apply) without affecting performance.
  4. Heat loss from HIUs - unnecessary losses directly paid for by occupants.
  5. Heat loss from return pipework - this includes both branch and main network pipes and reflects the losses of heat from unused heat returning to plant.
  6. Unbalanced Radiators - inefficiencies caused by poorly balanced heating loads.
  7. Commissioning Errors - guesswork, fidgeting with controls, and a lack of measurement are the root causes of many inefficient systems.
  8. Pipe over-sizing - unnecessary losses incurred by oversized pipework.
  9. Sterilisation losses - these are heat loses incurred as a result of Legionella policies that require pipework to run hotter than the minimum required for performance.

These are each described below in more detail, along with an explanation of how the use of revolutionary technology helps solve them all.

Generation efficiency

Many heat sources rely on low temperatures to be efficient, and one should be able to obtain data relating to exactly how efficiency is affected by return temperatures for any particular type of heat generator, or boiler.

A condensing boiler, for example, may experience a 10% gain in efficiency by dropping return temperatures from 55C to 25C.

The biggest generation efficiency gains come from the use of low grade heat sources. Heat pumps benefit greatly from reduced temperatures, as do waste heat recovery systems, including CHP.

This article is not so concerned however with the mix of fuels and which is most efficient, rather to demonstrate how it is possible to ensure the lowest possible return temperatures are provided with our technology.

Weather compensation is a feature enabled by electronic control that allows us to drop district heating temperatures and improve boiler inefficiencies further still. We do weather compensation in our HIUs by tracking the district heating flow temperature and targeting a secondary temperature to radiators 5 degrees lower. No additional sensors or valves are used in the HIU, with the entire process controlled by varying flow temperatures from plant.

DHW generation generally requires heat above 55C at peak loads (pipe/pump/heat exchanger sizing being the limits) so this can be taken as a lower limit. Between, therefore, 55C and 90C one can vary the DH flow temperature and the HIU will adapt accordingly - varying radiator flow between 50C an 80C, while keeping return temperatures as low as possible, and under set limits.


The chart opposite shows how the output of a typical radiator varies with temperature difference between the average radiator temperature and the room temperature. A factor of 1.0 is used with radiators at 80/60C. With the DATA HIU, dropping the heat network to 55C would drop average radiator output to approximately 40% of peak output.

This form of control has added benefits. With the output from radiators proportional to actual heating requirements, actions such as leaving a window open will result in a property getting cold, not wasting energy. It means that heating systems need less intervention from users to get room temperatures correct, and it does a lot of the work of a TRV, reducing the impact of oversized radiator valves.

Other HIUs on the market work differently. The have a fixed central heating setpoint that is derived to achieve peak heating load with -5C outside. Lets say, for example 70C, requiring a 75C feed from the network to drive it. Now, for 99% of the time, when its not -5C outside, it would be beneficial to drop this temperature. Run the network at 60C, lets say. The problem is, as soon as the network temperatures drop below 70C, the fixed control valves open up fully, passing huge amounts of water from the flow to the return in a vain attempt to heat water hotter than is possible. The whole system goes pear shaped, with return temperatures flying uphill - not down as they should. This is shown below in a comparison (not test data).


Heat Losses

Heat loss from main network flow pipework

There are two means to reduce heat loss from main distribution pipework:

  • Insulate - these pipes are hot all the time and are the main source of heat loss.
  • Weather compensate as far as DHW performance allows (as described above) - the cooler the pipework, the lower the loss.

Heat loss from branch flow pipework

Pipework leading up to an HIU needs to be warm enough to deliver hot water within a satisfactory time after opening a tap.

The way to save heat loss in branch pipework is to let it drop in temperature as far as you can without sacrificing DHW response times, and this is where some of our more advanced functionality comes into play.

Our trick is to set a keep warm temperature just above ambient temperatures, maintaining a very minimal flow through the branch pipework, in the region of a litre per hour. This maintains a temperature gradient in the pipework, rather than letting it all go cold, or all stay hot.

For this to work to maximum effect we use another trick; as soon as a tap is opened, if the district heating temperature into the HIU is lower than required, the HIU draws a boost flow of up to 20 litres per minute. This brings heat from the main network pipework that is over 50C into the HIU quicker.

Now compare this level of performance to other HIUs.


The graph shows how even with our reduced temperature keep warm system our HIU performs better than other HIUs in terms of the time it takes for DHW to reach set-point temperature.

Legionella regulations require DHW to achieve 50C within 60 seconds. 55C for care homes (a number of HIUs on the market do not achieve this, although they officially passed test).

Weather compensation will also provide a benefit on reducing heat loss from branch pipework during non-peak heating season.

Heat loss from HIUs

If branch pipework is insulated then the greatest heat loss savings can be had at the HIU. Insulation thicknesses are typically lower than on pipes, but the surface area is considerable.

Even a well insulated HIU can loose in the region of 100W maintaining DHW at 55C keep-warm.

Heat loss from an HIU registers on the end-users heat meter, and they pay directly in terms that a pre-pay meter will gradually run out of credit.

Keeping the keep warm at just above ambient temperature means the heat loss from the HIU is tiny by comparison.

It is still worth mentioning the technology that goes into the insulation on the DATA HIU. We use moulded EPP (Expanded Polypropylene) to provide a fully insulated enclosure, with compartments to keep electronics separate from hot areas.

Heat loss from return pipework

With properly managed keep warm, as explained above, the temperature of return pipework should be near ambient temperatures, like the HIU. Removing return heat losses (outside heating season) will have a significant impact on overall system losses.

It raises a question. With no significant heat loss from return pipes, is it more economical to spend budget instead on better flow pipe insulation?

Common Errors

Unbalanced Radiators

We have direct experience, from both research and working on a DECC funded analysis, of how radiator balancing can have a considerable impact on return temperature performance. A single unregulated radiator can wreak havoc.

To protect the heat network from the effects of unbalanced radiators requires the use of return temperature limitation. The test data shows how the electronic control in the DATA throttles back the heat input under these conditions, limiting the return temperature to a value set at commissioning.

Commissioning Errors

Despite there been numerous settings that can be adjusted on electronic HIUs, they require the use of dedicated commissioning software installed on a laptop or handheld system, and this is only provided to approved engineers.

Furthermore, the software provides an engineer with a host of sensor and error information that can be used for records as proof of correct function.

HIUs are typically factory set to the customers specific requirements - they are typically known at design stage. Without any controls to set on site, this removes the need for site commissioning other than a basic check of services - something that can easily be carried out in seconds by the installers, and avoiding the need for an additional layer of site commissioning charges.

Going to the next level, once the HIU is plugged into a communications network, it becomes possible to check and adjust settings remotely, as well as view all the sensor data in one second resolution.

Pipe over-sizing

Larger pipes lose more energy. They also introduce longer delays in system response times, cost more, and take up more space.

Even with less efficient HIUs its is apparent that the majority of systems end up considerably oversized, in large part down to the combination of generous diversity calculations as well as safety margins.

A lower return temperature - an increased temperature drop - directly results in reduced flow rates around the district system. Sizing pipes for return temperatures of 30C is in a different ball park to sizing for a return at 70C.

The removal of excessive keep-warm flow around the system drops flow rates in general. This is one of the more marked findings from the SP test data with our DATA HIU only requiring 77.4 cubic metres per year of primary water. All but one of the other HIUs tested were more than double this, and it is common to see HIUs in the field more than five times this.

It is worth noting that we have a patent pending on Networked Hot Water Priority. This is a feature where HIUs respond to peaks in DHW demand by reducing central heating output. The effect can (theoretically) be a 50% reduction in peak network flow rates (on top of the reductions currently demonstrated). We are currently applying for DECC funding to demonstrate this technology in the field.

It is clear that pipework sizes need to be specified differently to they way they have been historically, using proper models that deploy the latest strategies.

Maybe then the use of more economic smaller bore plastic pipework will become the standard.

Sterilisation losses

Definitely one for discussion.

  • Is it a legal requirement to maintain HIUs at temperatures over 55C to kill Legionella?
  • For how long and how often?
  • Has there ever been a recorded case of Legionella from a combi-boiler, or an HIU?
  • Does a higher keep warm between 30C and 40C make Legionella more of an issue ?
  • If taps are fairly regularly drawn so systems are flushed with temperatures over 50C, can we forget about sterilisation?
  • Given a Legionella cycle just sterilises the HIU, whats the point given shower heads and warm mains supplies are the most likely sources for bacterial growth.

Questions we would all like some closure on, but until we have a consensus that Legionella can be ignored on HIUs one needs to look at its impact on efficiency in any models developed moving forwards. The impact will generally be small on overall efficiency, unless you opt for maintaining DHW at 60C all the time that is.

The DATA HIU provides a Legionella function as standard that can be disabled at commissioning. This takes the system to over 57C for a minimum of an hour. The cycle starts after the last draw off of the day. If a tap is drawn during a cycle, then it stops immediately and moves back until after the draw off has finished. In this way the timing quickly settles typically into the middle of the night.

To be done

Counter Arguments

Reliability of Electronics

Given we already use electronics in heat meters, programmers, pumps, billing systems, routers etc etc etc, this argument reduces to whether the specific electronics used in the product are reliable.

Our controller has been in the field now for years, with thousands sold, and with no recorded failures. Over half a million pounds went into its development and it is a solid piece of electrical engineering.

Electronic control actually makes systems more reliable. There are fewer mechanical components and valve seals that are known to be where failures occur. Also we can view the operation of systems through the commissioning software and confirm operation.

We can say no more, other than emphasise - NO recorded electronics meltdowns to date.

I don't like change

As a manufacturer centred on quality and reliability, we understand that once you find a reliable supplier where nothing really goes wrong, you need to be sure that changing is worth it, and wont just create problems.

We hope that the data demonstrates clearly that the change is worth making, if it doesn't cost more in terms of money, time or problems, and levels of service are maintained.

The following points are how we would hope to offer piece of mind:

  • Experience - We patented our first electronic HIU in 2000, and have numerous other patents in thermal storage and heat exchange systems over the years. We authored the sections on plate heat exchange and thermal storage in the 2002 Institute of Plumbing's 'Plumbing Engineering Services Design Guide'. We have spent over 25 years designing, manufacturing and backing up both standard and bespoke plate heat exchange and storage systems. We have supplied nearly every major house builder, and a considerable number of local authorities.
  • Quality - Our HIUs are manufactured to the highest quality levels using an assembly line manufacturing system, tested by computers, CE approved, and with a proven track record in the field.
  • Service - We provide on-site commissioning and service cover throughout the UK. We pride ourselves that we have never left a customer unhappy, and treat any problem that arises as an opportunity to show our worth, and improve. We always support contracts with stock, and offer next day delivery via our fleet of vehicles.
  • Training - We have the UKs most advanced renewables and district training centre. It was also the UK's first HETAS biomass centre, and remains the largest. The addition of numerous types of renewable energy, thermal storage, and district heating equipment, makes the centre a true gem. We also carry out a number of BPEC approved courses.
  • Cost - We are very competitive, but in addition to our competitive prices, take into account the energy savings.

Change is inevitable. The key is making sure that when you make the change, it's with the right partner.

Keep-Warm Temperature

Possibly the biggest counter argument so far for reducing keep warm temperatures, stems from the fear of reducing comfort levels, and introducing delays in DHW production.

Despite the test data showing it has little effect on time to target temperature, it does take longer to reach 40C, as there is no longer hot water sitting in the HIU or in branches. This will be seen at the first draw-off of the day on a branch, and while there is occasional activity the branches will respond quicker.

This still as fast as a combination boiler - and will be significantly quicker most of the time, but goes to the heart of the efficiency issue. You can speed up response time to 40C by a few seconds by setting keep warm at higher temperatures, but in doing so introducing heat loss to the HIU and return pipework, and raising return temperatures.

If you were to take the official stance - the requirements to deliver hot water to taps over 50C within 60 seconds - and allow 20 seconds margin for delivery to tap from HIU, then only the HIUs achieving 50C within 40 seconds pass the mark. Only 3 HIUs do this. The DATA is one of these - on either keep-warm setting.

One could still argue the test rig itself is not representative of a real life system where larger pipe volumes are involved.

To resolve this we have provided a worked example of pipework temperatures, flow rates, and heat losses for a top floor on a riser. It shows how as the number of HIUs connecting into a branch increases the temperature drop reduces (as you get nearer the main network). While the flow rate in the pipes increases significantly, the heat loss does not, so the temperature drop reduces significantly.

Riser 20 26C keep warm.jpg

The rig itself had 8 metres of 35mm pipework (approximately) so with regards to water content of a branch is fairly high for a single HIU.

VWART Figures

One of the key parts to the SP test process was the production of a VWART (Volume Weighed Average Return Temperature) figure that represents the average return temperatures from an HIU over a year, taking into account heating, hot water and keep-warm for a typical UK dwelling.

It is a scientific process based on test data that has real meaning, and should be properly understood by anyone responsible for specifications, as well as, one could argue, those who take energy efficiency into account when looking to rent or buy properties.

So when we managed to obtain figures lower than others thought even possible (the original graph stopped at 25C), and then they were published in the public domain by agreement with all manufacturers involved, it did cause a few sparks.


For the VWART figures to be meaningful, they need to represent the efficiencies that can be achieved with a particular HIU, without dropping below a specified performance level. This is where the DHW response tests are so important. They confirm the performance of keep-warm functionality.

The test regime calls for DHW up to full temperature within 90 seconds. This is too long in most people's view, and is a long time to wait for heat. The fastest HIU in the tests was the DATA with a keep-warm at 55C, at about 12 seconds. The next fastest was a draw between the DATA on a keep-warm of 25C, and another make, at about 32 seconds. All the others took longer.

It should be noted that the published VWART figures may not be representative of the actual return temperatures seen at a plant room. There are still factors, some already mentioned, such as riser bypasses, Legionella cycles, unbalanced radiators, or exceptional pipe lengths or losses, that will have some impact on return temperatures if present.

We stick to our position that taken in hand with DHW response results the VWART results are only possible with the technology we deploy, they are backed by test data, and they are therefore very important.

They mean, excluding factors external to the HIU, and from a perspective of specifying HIUs, you know a VWART of 23.2 is achievable. Its been independently tested and verified by one of the world's best test houses according to a methodology and regime arrived at by consensus in the UK, for the UK.

Experience counts, and now we have a UK test regime, and the DATA, decades of experience have finally been heard over the crowd.

We have a level playing field and a standard test against which other manufacturers can have a go at beating our figures. So far none have come close.

Other manufacturers can also set keep warm lower

Maybe one or two but none have demonstrated officially that they can without sacrificing performance.

It's not just about keep-warm temperature. The initial boost flow is just one function required to make a low keep warm temperature function as required. This is the explanation why all the other HIUs took considerably longer to achieve temperature, despite having much higher keep-warm temperatures. If they set keep warm any lower performance would become an issue.

And then there is the question of Legionella. The sterilisation cycle on the DATA HIU allows one to deploy any type of DHW and keep-warm temperatures you choose, without fear of introducing an environment for bacterial growth.

HSG274 - Part 2 - The control of legionella bacteria in hot and cold water systems.

To be done

HIU Communications and Networking

2016-02-27-201055 1920x1080 scrot.png

Well done if you made it this far. We have saved the best for last.

So far, all the benefits discussed are a result of the technology built in to every DATA HIU, as tested in Sweden, and no mention has been made of the benefits the system provides external to the HIU, and not demonstrated in the tests.

The HIU communicates with an attached device through an RS485 port. This would typically be either a laptop for commissioning, or a Linux computer or billing system.

We provide various interfaces to connect the HIU into additional networks, including M-Bus, Ethernet & WiFi, with BLE coming soon.

Once connected to the network, a world of more advanced features opens up. As revolutionary to district heating as the internet has been to us all in general.

Every sensor in every HIU is available for control and monitoring the entire system, as well as all the extra calculated information and error codes. The following list gives an idea of whats on offer:

  • Remote commissioning and confirmation of correct operation.
  • Differential pressures around the system can be mapped in real-time, and used to control central pumps. No more riser sensors required, with far more relevant information fed back from index flats.
  • Differential pressure reading together with flow readings allow pipework losses to be mapped and blockage located.
  • HIU keep warm temperatures can be altered automatically to match the keep-warm requirements of particular network, should this be required.
  • Central heating flow and return temperature limits can be adjusted on the fly to match weather conditions and control the heat drawn by a property.
  • Pre-pay billing shut-off can be done on-board without an additional security valve.
  • Alarms can be setup so the whole thing manages itself, and just emails you or your engineer with a diagnostics report when there is a problem - or potential problem.

The Implications

We have a new UK standard against which manufacturers and specifiers, can be challenged. It has already been signed up to by DECC, utility companies, manufacturers, and is gaining momentum rapidly.

It cannot be ignored, and nor would you want to. The test data shows what is actually possible if you pick the right equipment.

From the data, one can draw the following conclusions:

  • Existing district heating networks are oversized for what they need to do, if used properly.
  • Plant and pipe sizing can be lowered using the latest methods.
  • Existing networks can be significantly expanded if retro-fitted with modern HIU technology.
  • Heat networks can be a lot more efficient that they are now, and cheaper to install at the same time.
  • Heat networks will be alive with operational information, and any inefficiencies identified immediately.
  • Waste heat and heat pumps can be used more efficiently than previously thought possible.

All it takes is to believe the independent published test data and change what you specify.


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