SAV-i Issue 34

Danfoss FlatStations – making Central Plant and District Heating schemes work

Danfoss FlatStations (or Heat Interface Units – HIUs) greatly facilitate the use of centralised plant rooms delivering heating and domestic hot water (DHW) to separate spaces, such as apartments or retail units.


Jan Hansen, SAV Sales Director

The complete solution for peace of mind

Historically, HIUs within district heating schemes have been treated as accessories, rather than a key element within the system. The results have frequently been disappointing for the end user – and embarrassing for the specifier.

Danfoss Direct FlatStations are different! They have been specifically designed to provide a complete solution with a built-in water heater and a differential pressure controlled heating system.

As such, Danfoss FlatStations deliver dramatically improved heat transfer efficiency and, thanks to their ability to automatically divert energy to where it is most needed, eliminate the need for separate hot water storage within the space.

Each FlatStation is pre-fabricated with a differential pressure controller, a fitting piece and a sensor pocket for insertion of a heat meter as well as strainer and ball valves – making it ideal for use in district heating applications.

SAV also provides Indirect FlatStations, where the primary circuit from the central plant room is hydraulically separated from the apartment space heating by a plate heat exchanger. This maximizes the options available to the building operator in terms of circuit working temperatures.


Danfoss FlatStations for heating and domestic hot water


Danfoss FlatStations are now available pre-insulated

“Danfoss FlatStations are now supplied complete with a fully insulated cover, enabling faster installation while also minimising heat losses. They also feature an integrated idle temperature controller which keeps the supply line warm.”


Facing up to the challenges of designing low carbon systems


Standard heat interface units often lack useful control functionality, which leads to a number of problems. These include lack of responsiveness and poor temperature control of domestic hot water.


Danfoss FlatStations incorporate patented valve technology to ensure precise control of both space heating and domestic hot water.

The heating circuit is designed for direct generation of heat. The differential pressure controller sets the optimum operation conditions for radiator thermostatic valves enabling individual temperature control in each room. In order to enable a time-dependent temperature control program, a zone valve with actuator and a room thermostat can be included as an option.


Danfoss FlatStation – Series 5

Domestic hot water (DHW) is heated in the heat exchanger and the temperature is regulated with a flow-compensated temperature controller with integrated differential pressure controller. Heat is transferred from the flow water to the DHW via a heat exchanger, ensuring that DHW is delivered at a safe temperature, while the control valve compensates for variable loads, supply temperatures and differential pressures. This protects the heat exchanger against overheating and lime scale formation.



With district heating HIUs there can be long delays in the supply of sufficiently hot DHW at times when space heating demands are low. DHW temperatures may also fluctuate in relation to DHW usage in other parts of the building.



FlatStations incorporate an integral idle temperature controller in the control valve. This ensures that water in the supply pipe remains warm, so that the DHW is highly responsive – even at times when space heating loads are low.

Also, Danfoss FlatStations provide thermostatic control of hot water temperature at varying inlet pressures. Hence, hot water can be supplied at whatever temperature setting is required, and this temperature is unaffected by the subsequent opening or closing of additional taps on the same system.



It can be difficult to achieve maximum efficiency of the centralised plant when serving a district heating system.


Danfoss FlatStation and Manifold assembly.


Danfoss FlatStations enable low heating return water temperatures, which are crucial to maximising the energy efficiency of heat sources such as combined heat and power (CHP), biomass boilers, heat pumps or gas fired condensing boilers. For Part L compliance, it is recommended that the return water temperature from a community heating scheme should not exceed 40˚C for hot water systems and 50˚C for radiator systems. During hot water generation, Danfoss FlatStations typically return heating water at between 15-30˚C, greatly facilitating compliance with Part L.

In incorporating integral FloCon ‘ultra low flow’ balancing modules, Danfoss FlatStations ensure that temperatures are balanced in each apartment, as well as reducing flow rates in heating circuits to increase heat exchange and maximise the ΔT.



District heating systems often waste considerable energy through standing losses in the distribution circuits.


Danfoss FlatStations are available fully pre-insulated, including a fully insulated cover, thus minimising heat losses. This ensures that all of the heat delivered to each apartment is used for useful heat production and that there are no uncontrolled heat emissions during summer months.

It also makes installation faster and easier and eliminates reliance on onsite lagging, as well as avoiding the problem of insulation being disturbed during commissioning and not being replaced.


Danfoss FlatStations cover a wide range of sizes and functions



An often oversized estimation of DHW peak loads leads to the design of larger than necessary pipe work and thermal stores. Oversized boilers will not exploit condensing capabilities because of high return temperatures. Energy is wasted.


SAV engineers carefully design pipe work and flow temperatures by means of the most advanced and updated European standards, advising design managers on the best technical solutions improving efficiency and promoting carbon free performance.



Where buildings are sub-divided, splitting heating bills equally between different users can cause dissatisfaction amongst tenants/users.


Danfoss FlatStations can be fully integrated with the SAV Energy Management Service (see below).


Heat meter


SAV Energy Management packages

Where separate metering and billing of energy use is required, Danfoss FlatStations can be combined with SAV Energy Management for a complete pre-payment and billing package, provided in conjunction with Insite Energy. Tenants can be provided with pre-payment or credit-based systems.

Pay-As-You-Go pre-payment systems provide tenants with a card reader which communicates with the heating network. When the credit has been used the FlatStation shuts down the system, preventing the user going into arrears, though emergency credit facilities can also be programmed in.

With credit billing systems, payment options include direct debit, internet and telephone, as well as cash payments through the Post Office and Payzone networks.


Danfoss FlatStations in action

West Bridge Mill Social Housing

West Bridge Mill in Kirkcaldy is an award-winning social housing facility operated by Link Group Housing Association. The former rope mill has been converted into 16 separate flats, all of which are served by SAV LoadTracker mini-CHP.


Each of the apartments is fitted with a FlatStation to provide efficient heating as well as instantaneous domestic hot water (DHW) when it’s needed, without the need for hot water storage within the apartment. As hot water is only generated when it’s needed, combined with the pre-insulation of the FlatStations, there are no standing heat losses and overall efficiency is greatly improved.

“For the people living in the flats it’s controllable. It’s instant heat, it lasts all day and if they’re cold they just turn it up a little bit.”

– Colin Reed, Energy and Sustainability Officer at Link Group Housing Association.

The FlatStations also ensure low return temperatures to the SAV LoadTracker CHP to maximise the efficiency of heat production in the plant room.

This combination of CHP and FlatStations has helped to reduce energy bills for tenants by nearly 70%, as well as ensuring they have heat and hot water when they need it. The SAV system also features integrated energy meters and a datalogger for monitoring of energy consumption by each apartment.


Highbury Square Luxury Apartments

Highbury Square is a new development of 719 one, two- and three-bedroom apartments on the site of former Arsenal home ground Highbury Stadium. The apartments are served by a central boiler room and district heating system that delivers underfloor heating and domestic hot water.


Each apartment is fitted with an Danfoss FlatStation to ensure that temperatures are balanced and to provide instantaneous hot water. The FlatStations were pre-fabricated off-site to improve quality control and reduce onsite work, and different sizes were supplied to suit the requirements of each type of apartment.

“One reason why this sub-station is more energy efficient than a traditional boiler, which would be the alternative, is that there is no need for a hot water cylinder. Here, you only have hot water on demand and don’t store any water.”

– Simon Angold, Project Manager, N G Bailey

Another major benefit of the FlatStations is that they ensure a low return temperature to the central boiler house. In summer, for example, the primary flow temperature is 70°C but the return temperature falls to 25°C. This is then re-heated to around 55°C by solar thermal panels on the roof. The result is a considerable reduction in energy bills for tenants, compared to the traditional combination of a boiler and hot water cylinder in each apartment.

Each FlatStation has an in-built energy meter that monitors consumption of heat and water for billing and fault-finding. Data is collected remotely through an integral radio link so there is no need to enter apartments to read meters.