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The Digi HIU
The Digi HIU, Cover Removed
The Digi HIU, First Fix Kit

The DIGI HIU uses the latest electronic stepper motor controls to provide both DHW and central heating. Central heating is provided directly, with a Differential Pressure Valve fitted on the CH feed. The double wall DHW heat exchanger provides complete protection from cross contamination in the case of a plate failure, thereby meeting the most demanding European regulations.

The unit comes in an attractive EPP casing, with overall dimensions of only 570mm (W) x 390mm (H) x 170mm (D), and remarkably low heat losses less than 10W (yes thats ten watts, in case you thought that was a typo). All connections are bottom entry, with a first fix kit as an optional extra. The fixing kit spaces the unit 50mm from the wall to allow pipes to be run up the back.


Installation Instructions

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DIGI User and Installation Manual


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DIGI Datasheet


Click here to go to the HIU Monitor Application Software page

3D Model

Please click here to go to the BIM / CAD File downloads page.

Features & Benefits

  • Compact design with no need for clearance either side for servicing. Allowance made for extra options, yet small enough to fit into a standard kitchen cupboard
  • Instantaneous hot water up to 75kW (at 60kPa Primary Differential Pressure)
  • Direct central heating up to 20kW
  • Option for Inter-networking of all DIGI units within a development to enable site wide adjustments to match loads to primary circuit heat availability -Read More-
  • Option for browser based interface and online dashboard for remote commissioning, monitoring, control adjustments, performance visualisation, fault detection and diagnosis using pc, tablet or phone -Read More-
  • Sensors for temperature, pressure and flow rate allow the system to monitor performance and detect fault conditions
  • The latest stepper motor valve technology allows the system to react rapidly, thereby maintaining accurate control over domestic hot water temperatures
  • Pre-install plumbing jig complete with isolation valves to allow pipework on site to be completed prior to the final installation of the unit
  • Primary circuit trickle flow can be set to maintain heat within the unit. When in Economy Mode it allows the unit to go cold following one hour of inactivity, preventing unnecessary use of metered energy
  • Insulated removable EPP casing with less than 10 Watts standby heat loss; ensures the unit can be maintained at temperature with little impact of metered energy use
  • Sterilisation programme raises the DHW temperature periodically; preventing build-up of Legionella bacteria
  • Site specific factory calibration of controls to predefined settings
  • WRAS approved (components)

As Standard

Electronic Control

Rather than purely mechanical controls, the DIGI uses an electronic controller to read sensors and control outputs to valves. The big advantage of this approach is it enables every control function desired to be implemented through lines of software code, where a mechanical system requires a separate device to perform each function. This is best demonstrated by the list of functions the DIGI can perform using just one valve for each heat exchanger, including temperature control, differential pressure compensation, domestic hot water preheat, heat boost, and security shut-off. A mechanical system to achieve this requires a multitude of valves, and becomes complicated, large, and heavy. The DIGI's electronics also enable such functions as smartphone control, network load management, and remote commissioning and fault reporting, by exposing data and receiving commands via a serial port to the optional IHIU Ethernet/WiFi control system.

Stepper Motor Technology

The DATA makes use of the latest stepper motor technology for both the hot water and central heating control valves. This provides a number of advantages over more traditional thermostatic or motorised valves, allowing significantly faster and more accurate control of flow rate and temperature. More significantly the DIGI can operate the valves to perform a range of functions, rather than simple fixed temperature control. These include sterilisation cycles.

Economy / Comfort Mode

It is common for HIU's to include a thermostatic bypass to maintain heat across the domestic hot water plate heat exchanger at all times, this helps minimise delays in hot water supply when a tap is opened.

The DATA, DIGI, SPLIT and SLIM HIU's all perform a similar function with the Economy or Comfort mode, by momentarily opening the domestic hot water stepper motor to allow small quantities of primary water to creep into the unit at regular intervals. The primary pipework leading to and from the HIU usually loses heat faster than the HIU itself, hence the unit will typically settle trickling in heat fast enough to keep the primary return at or just below domestic hot water temperature. The DATA, DIGI, SPLIT and SLIM HIU's are the only HIU's capable of this level of control with electronics to ensure that primary return temperatures remain below the domestic hot water setpoint at all times.

Note that the units perform a Legionella protection cycle every 24 hours overnight, for 60 minutes over 57°C, enabling the safe use of lower keep warm temperatures.

Each HIU has two domestic hot water keep hot modes, Economy or Comfort. These can only be selected during commissioning or by a trained engineer.

The default setting is Eco mode, and this can be specified to run for 0 minutes (which turns the feature off all together), 15, 30, 60 or 120 minutes after the last hot water draw off. In Eco mode the heat exchanger is kept to a specified temperature (between 25 and 60 Deg C) for the selected period of time and after this period of inactivity has elapsed the stepper motor closes completely and the temperature in the domestic hot water circuit is allowed to drop through natural dissipation to minimize energy consumption. If a hot water draw off is made any time throughout the Eco mode function the unit will deliver hot water as usual and the Eco mode function will begin counting down for the selected time once again.

In Comfort mode the heat exchanger is continuously supplied with a trickle of primary water for quicker hot water delivery, however energy consumption will be slightly higher than if the unit was set in Eco mode.

Commissioning Options

The DATA, DIGI, SPLIT and SLIM HIUs all offer a choice between two modes of keep warm function:

  • Comfort Mode maintains heat up to the HIU at all times
  • Eco Mode maintains heat up to the unit for a set period of time following hot water use, then goes cold for up to 23 hours. The period of time is selectable.
  • Keep Warm Temperature determines the temperature maintained in the HIU.

Comfort Mode

In Comfort Mode, the HIU maintains temperature into the HIU by pulsing in a calculated volume of primary flow water every few minutes. The amount of water let in is very small, and not enough to pass through the plate heat exchanger. The heat then dissipates throughout the plate.


Reduced Comfort Temperature

The latest firmware provides for a separate temperature setpoint for the keep warm mode, that can be set lower than the hot water temperature setpoint. This is to allow the system to be setup to maintain heat in main distribution pipework, but let branch pipework drop significantly in temperature.

The Comfort temperature setting must be high enough to maintain enough overall flow to maintain heat in the primary flow pipe.


The following shows how the use of reduced comfort mode at 26°C settles to a state where heat is maintained in the main pipework in readiness for a draw-off, however the branches see considerably lower temperatures and lower heat losses. Return temperatures are just above room temperature at all times (outside of heating season and Legionella cycles).

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Economy Mode

Economy mode allows HIUs to turn off during periods of inactivity, only coming up to heat once per day as part of the sterilisation cycle.

The following diagram shows how the network can go completely cold, however there is significant risk that one system may call for hot water overnight, and experience a significant delay before heat can arrive from the plant, as the entire volume of cold primary flow pipework will need to be expelled.


To overcome this, a thermal bypass can be fitted on the end of the primary pipework. Given the main primary branches are well insulated, the heat losses in maintaining them at temperature this way is far lower than if HIUs are kept warm also. The primary return will reflect the setting of the thermal bypass.


Anti Legionella Function

After 24 hours of inactivity, the HIU will bring the domestic hot water plate heat exchanger up to temperature to prevent legionella from growing. The heat exchanger is heated to 57°C for a period of at least 60 minutes.

It is not possible to specify the time at which this occurs, however it will stop immediately upon a hot water draw off.

Moulded EPP Insulated Casing

The best way to reduce heat losses is typically to insulate. In an HIU this is especially important as there is very little water content and so even a small heat loss results in temperatures dropping and the controls cycling the primary supplies to reheat. To significantly reduce this, the DATA, DIGI and SLIM HIU's makes use of engineered EPP (Expanded Polypropylene) moulded enclosures that encase individual components preventing any heat bridging to the environment, or to other components within the unit. The resulting heat losses are less than 50 Watts across our range, helping to ensure that cupboard temperatures do not rise significantly, and although the finished moulded casing looks good in itself, there is also an option for an outer metal cladding.

Advanced Heat Exchanger Technology

District heating pipework should generally be sized for temperatures as high as the plant can typically generate, to satisfy those days when it is -5C outside. Most of the time however, there is considerable space to run the network at lower temperatures during periods of reduced demand. In addition, modern heating loads are becoming very small indeed, making it difficult to maintain turbulence (and efficiency) within heat exchangers. The Data makes use of Asymetric plate heat exchangers to best match the characteristics of modern dwellings, while maintaining efficiency.

Metering & Security

A selection of heat meters are available to be factory fitted. The heat meter display is mounted in an aperture in the top of the HIU casing, providing access to the display and buttons, but preventing removal.

The DIGI has been designed with pre-pay billing systems taken into consideration, where HIU's need to be fitted with a security shut-off valve, and designed with tampering in mind. The DIGI includes pipework allowance for a heat meter. To simplify the process of securing bolts and access points on HIU's, we make use of unique moulded security clips and caps that once assembled, make it impossible to access a bolt head or connection nut without breaking the cap. To secure casings, and any bolted on components that one would like secured, we use a Bolt Security Cap. Bolts are inserted through the lower part of the security cap assembly, and screwed into position. The upper part is then pushed into the lower part until it locks, at which point the cap cannot be removed, and the screw head cannot be accessed.

Reliability and Confidence

Ask yourself, is it possible for installers or users to modify any settings in an HIU that could affect performance? Then ask yourself, of all the problems you have ever had with HIU's, were any related to inconsistent settings or tinkering?

The performance settings on the DIGI can only be altered by an approved engineer with the right kit and software, and is typically setup both in the factory, and again confirmed at commissioning. These settings are locked in software, with a report confirming all settings generated at commissioning. Following setup, no-one can change the settings, override DHW settings, open bypasses, or set the central heating temperatures too high. The whole system actually performs in reality as it does on paper.


  • Case and heat meter tamperproof security fixings
  • Various heat meter models
  • Landlord security valve or relay kit
  • Heating pump kit
  • Underfloor heating blending valve and pump kit
  • ModBus communication module (externally mounted)


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Differential Pressure Control Valve

As the DIGI HIU has a direct hydraulic connection to the apartment heating system, it is fitted with a Differential Pressure Control Valve (DPCV) across the Primary flow and Return to the heating circuit. The valve has a range of 5-25kPa and is factory set to 20kPa. This enables control over the differential pressure in the heating circuit separate to that of the hot water circuit, which is controlled electronically, as explained below.

The DIGI, DATA and SLIM electronic HIUs, all come as standard with a function to learn the differential pressure of the primary system during operation, and adjust their characteristics to suit.

Differential pressures (DP) can vary significantly across a network. The properties nearest to the plant room will see the maximum pressures generated at the plant, where as the index flat, furthest from the plant, will see much lower pressures - enough to work at full network load. The typical range of pressures is from 0.5 bar up to 1.5 bar, but can be higher on some networks.

The traditional approach is to fit a Differential Pressure Control Valve (DPCV), that knocks out excess pressure so the HIU only sees a constant DP. This is a perfectly adequate solution, however it has three disadvantages compared to using an electronic self-learning system:

  • They add cost, taking up more space and hence increasing HIU size and heat loss.
  • They introduce pressure loss of their own, so that the index flat will in fact need a higher DP to achieve maximum load.
  • One more component to go wrong.

The self learning feature works by examining the effect of a valve movement compared to known characteristics, and hence determining what the DP actually is. The self learning feature takes three seconds to perform, and is done every time a tap is opened and temperatures stabilise.

The following graph shows how the self learning feature works during an independent test. The test shows the commissioning process where the unit is turned on for the first time. The preset DP value is 0.6 bar. The actual DP of the system is 2.5 bar. The first test draw-off shows how the system would react without self-learning in place. The hot water temperature overshoots the target on first draw-off, and a spike in DH flow (red dotted line) can clearly be seen. The second draw off starts with heat in the system, allowing advanced functionality to start and enables the system to learn the new DP value. From there on you see no spikes in DH flow or significant overshoot of temperatures when taps are opened.


DP values will vary with load for any one property, so once the initial commissioning has been performed (by running hot taps a few times) then the system is setup to go, and will continue to fine tune itself every draw off.

Electronic Technology

Electronic Technology

DHW Performance - DIGI CW6

DHW Performance Table

Primary Return Temperatures for 10-55°C (DIGI CW6)

Primary Flow Rates for 10-55°C (DIGI CW6)

Primary Pressure Drop for 10-55°C (DIGI CW6)

Domestic Hot Water Pressure Drop for 10-55°C (DIGI CW6)

DHW Performance - DIGI CW5

Primary Return Temperatures for 10-55°C (DIGI CW5)

Primary Flow Rates for 10-55°C (DIGI CW5)

Primary Pressure Drop for 10-55°C (DIGI CW5)

Domestic Hot Water Pressure Drop for 10-55°C (DIGI CW5)

CH Performance - DIGI (All Models)

CH Performance Table

Guarantees and After Sales Backup for HIU's

Heat Interface Units (HIUs) carry the following guarantees as standard:


1. The warranty begins on the date of delivery. A dated delivery note will be issued to the customer and a copy will be stored by Thermal integration.

2. 12 Months Parts and labour - Parts or labour proven to be defective will be replaced / repaired free of charge for a period of 12 months from date of delivery, provided the HIU is installed by a qualified engineer within 6 months of date of delivery.

24 Months Parts Only - Parts proven to be defective will be supplied free of charge (for fitting by others) for a period of 24 months from date of delivery.

3. Replacement of parts under warranty does not extend the duration of the warranty.

4. Any other costs are not covered by this warranty. All other damages of any nature whatsoever and howsoever arising, are expressly excluded from this guarantee.

5. The warranty conditions above only apply:

a. In the UK only on items provided exclusively by Thermal Integration.
b. If the product is installed by a Thermal Integration approved engineer in compliance with the installation instructions.
c. The installer must complete the commissioning checklist in full at the time the HIU is installed. This checklist must be returned to Thermal Integration within 30 days of commissioning. Checklists are enclosed in the HIU installation instructions.
d. If the installation complies with all current and relevant building regulations and codes of practice. (including the requirement to clean the primary and secondary heating systems and add corrosion inhibitor in line with BS7593:2006)
e. If the product is used and maintained exclusively according to the manufacturer's instructions and proof of periodic inspection / maintenance by a Thermal Integration approved engineer is available.
f. If the returned item is accompanied by a fully completed Thermal Integration Warranty Return Form.

6. Excluded are defects caused by:

a. Failure to maintain in accordance with manufacturer’s instructions.
b. Improper use.
c. Any attempt at repairs / maintenance by un-qualified persons.
d. where parts other than Thermal Integration Genuine Parts have been used in any service or repair.
e. Lightning, fire or natural disasters.
f. Deterioration and / or pollution from the district heating system or water network, either domestic hot water or heating side.
g. PH values of the primary medium being less than 7.5 or greater than 9.0.
h. Harmful additives to the heating water.
i. Consumables as specified by us, including but not limited to: hoses, gaskets and batteries.

7. Report any faults to your installer, service engineer or Thermal Integration directly. Faulty parts must be accompanied by a fully completed Thermal Integration Warranty Return form, to be returned to Thermal Integration. Returned items remain the property of Thermal Integration Ltd.

The Thermal Integration Warranty Return Form can be requested by phone or email. Returns are not accepted unless expressly agreed in writing.
Transport risk of returned items lies with the sender. The shipping of replacement parts are the responsibility of the supplier.

8. If the HIU breaks down, we may ask you to pay us a deposit before we visit you to repair it. We will return the deposit in full if we find a fault that is covered by this warranty. We may keep the deposit if we cannot access your property at the time we had arranged with you to visit or we find other conditions of this warranty have not been met. A responsible adult must be at the property to give our engineer this access to the HIU.

Thermal Integration offer commissioning services across the UK. The service includes:

a. The fulfilment of the defined commissioning instructions.
b. Completion and return of site wide commissioning paperwork.
c. Rectification of any HIU problems.
d. Confirmation of HIU performance to contract specifications.
e. Engineer travel and subsistence.

Commissioning rates are charged on a whole day basis, and it is the responsibility of the client to ensure:

a. Safe access to properties and HIUs
b. Correct operation of central plant with delivery of heat to HIUs at specified temperatures, flow and pressures.
c. All pipework has been correctly tested and flushed.
d. Credit on billing system to enable security valve.

A minimum 2 weeks notice is required in writing to Thermal Integration before the requested commissioning date.

Technical assistance and engineer backup can be obtained by calling our offices:


The Heatweb Wiki website at contains information on all aspects of HIU design, function, and servicing, and is always the best place to visit for additional documentation or how to guides.

Other Products in our range

On this page you will find technical datasheets for many of the products in the Thermal Integration range.



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DATA Datasheet



DATA SPLIT and MIX Datasheet


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DIGI Datasheet


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SLIM Datasheet

The SLIM Extra HIU

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SLIM Extra Datasheet

Storage HIU's

The Amazon HX-DATA Storage HIU System

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Amazon HX-DATA with Recirculation

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Amazon HX-DATA

The Amazon HX-SLIM Storage HIU System

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Amazon HX-SLIM with Recirculation

The Amazon HX-SLIM Extra Storage HIU System

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Amazon HX-SLIM Extra

The Amazon HX-SPLIT Storage HIU System

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The Amazon HX-DIGI Storage HIU System

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Amazon HX-DIGI