HIU Comfort and Economy Function

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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 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 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 DIGI, DATA 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).

Riser 20 26C keep warm.png

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.