Heat Interface Units

Termix – Member of the Danfoss Group

SAV has a long association with Danfoss and its Termix subsidiary, a specialist in heat interface units (HIUs). Termix HIUs are used extensively in district heating systems throughout Northern Europe. 

In response to market interest, a range of cooling modules for use in high specification apartments have recently been developed, which have been supplied to several city projects.

SAV have long realised the importance of flexibility in meeting the various requirements tabled by consultants. If a project arises with ratings for heating or cooling not available from the SAV / Termix standard range, we will commission whatever re-design may be necessary to ensure our offering fits the bill.



FlatStation HIU's: Making centralised plant rooms possible

Danfoss FlatStation HIU's are based on technology by Danfoss, a well-established supplier of District Heating systems. FlatStations are compact, highly efficient and easy to install. Every unit is backed by comprehensive technical support to consultants and site service to contractors.

FlatStations come in 2 main types: Direct and Indirect. In both of these, a Domestic Hot Water (DHW) plate heat exchanger is heated by primary water from the central plant room. Space heating can be arranged either by:


a) Direct FlatStations

The primary circuit connects directly to the apartment space heating. This arrangement can be adapted to either underfloor heating (UFH) or radiator circuits.

Direct FlatStation Heat Interface Units


b) Indirect FlatStations

The primary circuit is hydraulically separate from the apartment space heating, by using a second heat exchanger. SAV can supply everything necessary to serve the apartment ‘sealed system’ heating needs, such as circulator, strainer and expansion vessel.

Indirect FlatStation Heat Interface Units


DHW provided by a plate heat exchanger is available in the same rapid response time as from an adjacent hot water storage cylinder. DHW temperature and pressure at the tap are controlled within tight limits so as to safeguard user comfort. FlatStations have no DHW storage requirement, allowing a considerable saving in terms of cylinder cost, standing heat loss and installation space requirement.

Plate heat exchangers are excellent in producing a high Δt across the primary, therefore helping to achieve low return temperatures. This feature is hugely desirable in reducing heat losses from all return pipework. It reduces pumped volume round the primary system and provides good operating conditions for high-efficiency condensing boilers. Should plant room equipment include LoadTracker CHP, any increase in Δt between flow and return produces a directly proportional increase in thermal storage volume. This is highly beneficial to peak thermal load management and ensures uninterrupted operation over long periods.


Most FlatStation installations are specified with in-built metering. Energy meters can easily be incorporated at works, with output made available by either MBus wiring or radio signal. Individual usage data can be processed using SAV’s Watchman service, with output made available for easy download or for individual billing. Where specified, SAV can arrange for water meters to be installed within each FlatStation.

With applications varying from 1-bed flats to multiple bedroom properties, a FlatStation can be found to suit every eventuality. The smallest output Water Heater is just 35kW, whereas the largest unit can manage 90 kW.

Whether you happen to be taking a first look at FlatStations for a project, or have ‘inherited’ an installation, feel free to sound out SAV on the issues of :

  • Sizing of FlatStation HIU's for different apartment types
  • Recommendations for centralised boiler capacity and buffer storage
  • Help with the estimation of flow rates through riser and distribution pipework
  • Use of coincidence factors to secure equipment supply cost reductions
  • Orientation of FlatStation connections to fit in with site piping layout
  • Advice on the incorporation of energy and water meters (and how best to use the resulting data)
  • Incorporation of ‘first fix’ rails, to provide the hydraulic boundary for phased site works
  • Advice on the use of distribution manifolds to simplify DHW / BCW piping to each tap
  • Commissioning of FlatStation hydraulic and electronic circuits Troubleshooting of heat interface unit installations.


Contact us either:

By phone:

01483 771910

By e-mail: webenquiries@sav-systems.com

And we'll be happy to help!

However, if you’d like to do some preliminary reading, you can download the FlatStation Design & Product Guide full of facts, tips and solid advice on this vital component of district heating systems. Simply complete the request boxes below, and click submit. You can also download other documents and guides on how to achieve efficient systems with low return temperatures in the download area.


Document Downloads 

For immediate access to available Danfoss FlatStation documents please fill out the form below and click submit.