Heat Interface Unit (HIU) - EvoFlat 4.0 PRO

The Danfoss Heat Interface Unit (HIU) concept "EvoFlat 4.0 PRO" comprises an installation in which flat stations supplied from a central energy source are built into each flat, allowing the central source to distribute heat and domestic hot water services to each home through the HIU. These units normally incorporate a compact plate heat exchanger, which delivers instantaneous hot water on demand, and a differential pressure control valve to control the heating flow to the tenants’ radiators or underfloor heating system.  

This centralized approach can operate at higher efficiency than individual boilers and helps landlords manage apartment block heating more effectively.  

The main difference between traditional heating systems and decentralized ones with heat interface units is that in decentralized systems the process of domestic hot water preparation is moved from central heat source (traditional systems) to flat station units in each apartment. Eliminates the need for continuous DHW circulation, avoiding year-round energy losses and reducing pumping energy requirements. By reducing the need for high-temperature operation, decentralized systems support low-temperature heating networks and heat pumps, improving overall system efficiency and lowering CO₂ emissions. In addition, fewer distribution pipes are required, reducing installation complexity, material costs, and ongoing maintenance requirements.  

The total flow is formed by the heating loop flow rate and the primary supply flow rate for the water heaters, reduced by the relevant coincidence factor and a flow rate to compensate for other heat losses.

There are two typical applications, depending on the type of apartment heating installation and the central heating systems serving the building: radiator heating and floor heating. In retrofit projects, these HIU applications can also support a turn from gas heating to heat pumps in multi-family buildings.

In the EvoFlat 4.0 PRO concept for decentralized system, the heat exchangers are used mainly for producing domestic hot water. The operating principle of instantaneous water heating/the flow principle can be described as follows:  

• Heating water circulates on one side of the plate heat exchanger transferring heat to cold water passing through on the other side. The heated water is then fed straight to local hot water taps and showers.  
• To control the domestic hot water capacity of the unit an electronic intelligent controller, especially developed for this purpose, is used to regulate the flow across both sides of the plate heat exchanger.  
• The electronic by-pass and circulation functions learn the residents' schedules and secure instant domestic hot water when needed and energy savings when hot water isn’t needed.  

Electronically controlled flat stations have automatic domestic hot water priority, and the electronic control supports maximum efficiency during domestic hot water priority operation.

In mechanical stations, if the DHW control valve turns off mechanically the supply of heat for the radiators/floor heating, the existence of 100% DHW priority may be assumed. In case control valves for DHW without mechanical changeover function are used, it must be calculated, whether DHW priority may be assumed (Hydraulic priority).

• when ΔpDHW < Δp Heating DHW priority in the unit can be assumed
• when ΔpDHW ≥ ΔpHeating DHW priority cannot be assumed

An estimation of the domestic hot water requirement is part of the total evaluation, which is the reason why the term coincidence factor is relevant in this context. Briefly stated, only a part of all tap points is used in actual practice, based on a 99.9 % confidence interval in a statistic probability distribution.

In the Danish standard for domestic hot water installations DS439 and the specifications from TU-Dresden the consumption pattern of a “standard flat”, which includes 3.5 persons, constitutes the basis for the factors. Deviations may occur if the dwelling for example is inhabited by residents for which an atypical consumption pattern can be expected. Therefore, the application of these factors must be supported by an estimation of the composition of the residents of the building.

In the EvoFlat concept for the decentralized system the heating circuit of a flat station can be configured in 3 ways:

• Directly – controlled only by a differential pressure controller and optionally by a zone valve controlled by a programmable room thermostat. This principle is applicable mainly for radiator heating in supply systems with maximum PN10.
• Directly with mixing circuit – controlled by a differential pressure controller and a mixing circuit. This principle is applicable mainly for floor heating and systems with maximum PN10.
• Indirectly - a plate heat exchanger is controlled by a 2-way thermostatic valve and optionally by a zone valve, controlled by a programmable room thermostat. This principle is applicable mainly for floor heating and systems with maximum PN16.

Some projects compare these hydronic layouts with electric systems, although HIUs remain focused on central heat and water distribution rather than electricity-based space heating.

By contrast, all-electric services can eliminate the need for hot water pipes, and electricity can offer the lowest carbon intensity among energy options depending on the supply mix.