The Dublin Indirect HIU (Heat Interface Unit) from Herz 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.
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. At the same time, a hot water priority valv e closes the primary feed to the secondary heat exchanger, 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 a thermostat fitted to the pressure temperature control valve . Using an immersion sensor, this thermostat controls the temperature of the hot water that exits the heat exchanger and regulates the pressure temperature control valve thus maximising the energy efficiency of both the HIU and the primary 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. This valve will close when the room temperature setting has been achieved or when the heating system is not in use.
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.
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.
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 differential pressure control valve should be fitted on each HIU or across the primary flow and return circuits in the distribution pipework or to protect the control valves from excessive DP. The secondary heating circuit is provide d with an expansion vessel and secondary domestic heating pump which varies the system flow rate automatically based on demand.
Domestic Hot Water (DHW) 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 away 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.
As an option the HIU can be provided with a built in battery powered energy meter mounted in the primary heating return pipe. The meter will measure flow using the ultrasonic principle with an accuracy complying with EN1434 and MID in Class 2 with dynamic range of 1:250 (qi:qp) The heat meter has options for pulse, M bus or radio to allow remote reading with hand held scanner, drive by or remotely via GPRS. All necessary system hardware and software is available on request. If a cold water meter is fitted this can be pulse linked to the energy meter. The heat calculator will display energy usage in kW hours . Battery life approximately 12 years.
High differential pressures could affect the performance of some components in the HIU and in particular cause temperature spikes in the hot water. Excessive differential pressures can also cause noise and reduce the lifetime of components within the HIU. We would recommend that DPCV should be installed on each HIU to protect the components from excessive differential pressures and to control flow. If this option is not utilised then differential pressure control valves must be fitted at branches/floors to stabilise pressures and temperatures.