Heat Networks

What is a heat network?

A heat network – also called district heating – is a distribution system of insulated pipes that takes heat from a central source and delivers it to a number of domestic or non-domestic buildings. The government has stated that heat networks form an important part of its plan to reduce carbon and cut heating bills for customers, as “they are one of the most cost-effective ways of reducing carbon emissions from heating, and the efficiency and carbon-saving potential increases as they grow and connect to each other.”

Of course, heat networks are not a new approach, especially in delivering heating and hot water to multi-dwelling buildings, such as apartment blocks. In fact, they have been used for two decades or more, but the key difference in recent years has been the opportunity to exploit/incorporate renewable heat sources such as heat pumps.

To support this strategy, CIBSE’s CP1 2020 Heat Networks Code of Practice puts even more emphasis on achieving low return water temperatures. A system designed with this approach will have reduced peak-volume flow rates, requiring smaller pipework which will lead to lower heat losses and reduced pumping energy.

Although heat networks are generally technology neutral, low return temperatures have an impact on the transition to low carbon technologies. This has meant that manufacturers, specifiers and heating engineers are now embracing the renewable technology of heat pumps on the primary circuit – commonly referred to as a fourth generation heat networks.

Fourth generation heat networks

As you would expect with the utilisation of heat pumps, fourth generation heat networks operate at a lower temperature than previously seen in the days of boilers. All of these aspects contribute to higher system efficiency, lower carbon emissions and a reduction in air pollution.

In practice, the commercial heat pump (or pumps) – often installed on the outside of the building – is connected via the heat network to next generation Heat Interface Units (HIU) installed within each apartment. One important thing to note is these new HIUs are designed specifically to work with the lower temperatures associated with heat pumps and optimise performance in a well-insulated, newly built apartment block. They are therefore considered “heat pump ready”.

Heat interface units (HIU)
and BESA testing

Modutherm’s MTA Plus HIU is “heat pump ready” and features the very latest, best performing heat exchangers to ensure superb DHW performance and heating requirements, even at low temperatures. This is crucial; indeed CIBSE’s CP1 set out minimum requirements to use tested HIUs only, and this has been an important step towards improving the performance of UK district heating schemes around the UK. By measuring the performance data of different HIUs within the context of typical UK operating conditions, it becomes valuable in enabling heating engineers and heat network system designers to consider the performance of specific HIUs – not only against design requirements but also competing manufacturers.

The BESA UK HIU standard is the main test regime and this is the key approach for specifiers to compare HIU performance. CP1 best practice states that where indirect HIUs are specified, they should be used with a tested volume-weight average return temperature (VWART) of less that 33oC.

Modutherm’s MTA Plus has been confirmed as the best performing unit on the market, achieving top marks in registering the lowest VWART in both the high and low temperature tests of any heat interface unit, with values of 26oC (high) and 24oC (low) respectively. That’s better than any other HIU previously tested. The ability to reach these figures identifies the MTA Plus as a high performance heat interface unit (HIU) specifically designed for use in 4th generation low temperature heat networks that utilise heat pumps or boilers.

Why does VWART matter?

In simple terms, VWART measures return temperature, which is one of the best ways to gauge a heat network’s efficiency. A lower return temperature results in a larger delta T, which means lower flow rates are required to achieve the same kW delivery. In practice, this allows smaller pumps and pipes to be used - and smaller pumps mean lower capital expenditure and power consumption. Furthermore, smaller pipework also reduces surface area and related heat losses.

So specifying equipment that delivers low VWARTs is the best approach to improving the efficiency of a heat pump-led heat network, remembering, of course, that the CIBSE Code of Practice states, as a minimum requirement, the VWART should be no higher than 33°C.