The Oxford DHW from Herz is the only HIU is suitable for all community schemes and enables LTHW from a central plant to provide DHWS to each dwelling with complete hydraulic separation between the LTHW primary and the DHW systems.
The HIU is a complete package comprising of all components mounted on a frame, factory assembled and tested.
Domestic hot water is generated via the DHW heat exchanger mounted in the HIU and provides instantaneous hot water on demand.
The temperature of the DHW is controlled by a thermostatic head with an immersion sensor, this head controls the temperature of the hot water that exits the heat exchanger and regulates the primary water flowrate.
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 circulation bypass valve is fitted to maintain a minimum primary temperature when there is no demand for hot water and the heat exchanger is not in use. This provides a quick DHW response and avoids unnecessary energy usage for the occupier.
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 valve 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 ”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 heat exchanger is not in use and ensures a quick response time at the hot tap. The thermostatic bypass valve is installed upstream of the heat meter. Combined with the Pressure Temperature Control Valve, which only allows water through the DHW heat exchanger when there is a demand, ensures that it is impossible to have meter creep-age or standing losses from the Oxford HIU.
The Herz Oxford HIU is fitted with special ball valves designed specifically for the purpose. The ball valves are delivered fitted to the HIU but can be removed and installed at first fix if required. The HIU ball valves are fitted with drain valves to facilitate draining with integral test points fitted on the primary flow and return valves to aid additional temperature or pressure measurement if required. The ball valves have union connections for convenience and to allow the HIU to be easily removed for servicing.
The Herz Oxford 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.
The Herz Oxford HIU has been designed so that it can be installed with the ball valve connections at the top or at the bottom. The fully insulated cover will fit onto the HIU regardless of the installed orientation. An optional ball valve cover is available if required, this can be fitted to the top or bottom of the cover as appropriate to the orientation of the HIU.