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Herz Dublin HIU
Feel the Water

Herz Dublin HIU


The Herz Dublin HIU is suitable for all community schemes and enables LTHW from a central plant to provide heating and DHWS to each dwelling with complete hydraulic separation between the LTHW primary and secondary heating and DHWS systems. The HIU is a complete package comprising of all components mounted on a frame, factory assembled and tested.

Main Components

Pressure Temperature Control Valve

As a hot water tap is opened the pressure temperature control valve reacts to the difference in pressure via a diaphragm and opens allowing the cold and primary heating water to flow through the heat exchanger. The valve has proportional control and only allows the minimum amount of primary water required to produce the requisite amount of hot water at the tap, maximising system efficiency. At the same time, the hot water priority function within the valve closes the secondary return circuit, thus ensuring maximum flow is available at the domestic heat exchanger. In order to prolong the life of the DHW heat exchanger and prevent lime scale build up, the temperature of the domestic hot water is controlled by the differential pressure control valve combining with the valve’s proportional control to limit the amount of energy used thus maximising the energy efficiency of both the HIU and the primary system. 

Zone Valve

The primary flow to the heating system heat exchanger is controlled by a two port on/off actuated valve linked to a programmable room thermostat. This valve will close when the room temperature setting has been achieved or when the heating system is not in use.

Summer Bypass Valve

The ”Summer bypass” valve utilises a return temperature limiter head fitted to a thermostatic valve installed in a bypass between the primary flow and return pipework within the HIU. This maintains a minimum primary temperature when the space heating is not in use.

First Fix Rail

The First Fix Rail is a pre - assembled unit fitted with all the isolation ball valves required for the various circuits installed within the HIU. The unit is installed at first fix and allows shell and core pipework to be completed without the HIU being fitted. The first fix rail ball valves are fitted with drain valves with integral test points on the primary flow and return to facilitate draining and to aid additional temperature or pressure measurement if required. The first fix rail with ball valves also allows maintenance to be carried out on the HIU as the main unit can be removed easily.

Flushing Bypass


The Herz Caledonia HIU can be fitted with a Herz HIU flushing bypass, this is a custom made valve designed to facilitate the flushing of the primary system without the water circulating through the HIU as recommended in BSRIA Guide BG 62/2015. The flushing bypass valve can be connected directly to the HIU ball valves at first fix so that primary flushing can be carried out before the HIU is fitted.

Main Features

  • Instantaneous hot water and space heating to properties
  • Twin heat exchangers provide hydraulic separation
  • Thermostatic hot water temperature control ”Summer bypass” valve
  • First Fix pre - mounting rail
  • Low primary return temperature maximises system efficiency
  • Suitable for radiators or underfloor heating
  • Option for heat meter (110mm Spool piece provided as standard)
  • 110mm spool piece provided for water meter
  • UK Water Reg 4 Compliant
  • 18mm stainless steel pipe work


a) Residents Heating System

The primary flow to the heating system heat exchanger is controlled by a two port on/off actuated valve linked to a programmable room thermostat (Herz 3 F799 17 ) . This valve will close when the room temperature setting has been achieved or when the heating system is not in use. A Thermostatic head, connected to an immersion sensor in the secondary flow, allows the flow temperature of the secondary heating circuit to be adjusted dependent on whether underfloor heating or radiators are utilised. The secondary heating circuit is provided with an expansion vessel and secondary domestic heating pump which varies the system flow rate automatically based on demand.

b) Residents Hot Water (DHW)

Hot Water (DHW) Domestic hot water is generated via the DHW heat exchanger mounted in the HIU and provides instantaneous hot water on demand. DHW flow rate and temperature is controlled via the Pressure temperature control valve which is temperature compensated and requires no auxiliary power to operate. When a hot water tap is opened the drop in pressure in the hot water pipe will open the 4 port pressure temperature control valve which in turn will allow primary hot water into the heat exchanger. When the hot water demand ceases the pressure temperature control valve will immediately stop the primary flow into the heat exchanger. Therefore there is no drain on the primary heating when there is no demand, so no “extra energy usage” when residents are on holiday for example. A thermostatic “summer” bypass valve is fitted to maintain a minimum primary temperature when the space heating is not in use. This provides a quick DHW response and avoids unnecessary energy usage.

Energy Metering

A battery powered energy meter can be provided as an option. The meter will measure flow using the ultrasonic principle with an accuracy complying with EN1434 and MID in Class 2. The heat meter will have options for M bus or radio. Pre - payment options are available on request. Battery life approximately 11 years .

System Balancing

BSRIA BG 62/2015 states that each HIU should include a differential pressure control device so that the operation of the HIU is independent of fluctuations in the primary pressure. A Differential Pressure Control Valve is included in the Herz Dublin HIU to protect the HIU from pressure fluctuations in the primary system and to govern the primary flow rate to the HIU and control the hot water temperature. This means that the Herz Dublin HIU is effectively self-balancing and requires no other commissioning valves.

Dublin HIU Data Sheet

Dublin HIU BIM File

Dublin HIU Specification