All about heat interface units

The use of Heat Interface Units (HIUs) and centralised plant to deliver heating and hot water to individual properties within multi dwelling developments is becoming increasingly common. Jon Cockburn, head of marketing at Heatrae Sadia, answers some frequently asked questions about the approach.

What are HIUs?
HIUs provide central heating, and sometimes domestic hot water, to individual properties within a multi dwelling development (such as a block of flats or apartments, or a larger district heating scheme) served by centralised heating plant. HIUs can be heating only or heating with domestic hot water production. They are sometimes called 'heat boxes'.

The Hi-Max Instant ID from Heatrae Sadia has a compact design and comprises two generously sized plate heat exchangers - one for the hot water, and one for the heating system - to ensure that the primary return temperatures going back to the centralised heating system are very low, maximising the operating efficiency of the system.

What is district and community heating?
District heating supplies heat from a central source to a variety of buildings - which might include houses, blocks of flats or apartments, offices, schools, shops and community centres. The heat is delivered through a network of pipes carrying hot water. Community and communal heating follow the same principle, but on a smaller scale - sometimes serving a single block of residential flats or apartments.

Why are HIUs used in these schemes?
Historically, in multi dwelling schemes, individual heating systems - typically comprising combi boilers or electric heaters - were installed in each property. Centralised heating plant can help developers to meet increasingly demanding new build targets, whether using traditional fuels or low-carbon and renewable technologies.

But if centralised plant is used to serve multiple dwellings without a HIU there will be issues with temperature control and the metering/billing of an individual household's energy use, because there will be no way to control the heating or monitor usage.

In the case of HIUs, the amount consumed by an individual household is recorded by a meter, making them very attractive for councils and those operating a large portfolio of buildings. Meter readings can even be carried out remotely, making HIUs a much more viable option for registered social landlords, who can then bill their tenants. Metering can also be a positive incentive for occupants trying to lower their energy consumption and can help to reduce fuel poverty among vulnerable groups.

A controller monitors and regulates the heating and hot water system in order to optimise performance, working with the room thermostat

Where are the HIUs installed?
HIUs can be installed within each individual dwelling, or in the dividing wall between each dwelling and the 'landlord' space - the latter providing easy access for inspection and maintenance.

Other pluses for landlords include a first fix rail, meaning that installers can minimise risk of damage or theft by completing the installation of the interface unit only at property handover.

At the same time, products like the Hi-Max Instant ID are so compact that they can fit in a standard kitchen cupboard, helping to maximise living space where it is at a premium.

What are the benefits?
In comparison to the traditional approach of fitting individual systems within dwellings, there are many benefits to this approach.

Annual gas safety checks mean that any gas appliance poses a never ending challenge for landlords, who are dependent on tenants providing access to their properties. The use of HIUs removes the need to conduct a gas check on the heating system - and, if there are no other gas appliances in the affected properties, could negate the need for individual gas checks completely. The time and costs involved with boiler servicing and maintenance is also significantly reduced.

It also reduces the problems associated with supplying gas to multi dwelling buildings, decreasing the risks linked to gas distribution pipework and saving costs, and eliminates the need to fit numerous flue terminals/condensate drainage systems. The capital installation and whole life costs are therefore also reduced.

The incorporation of low carbon or renewable technologies (such as biomass, solar thermal water heating or combined heat and power (CHP)) can be simpler and more achievable, as it can be difficult to incorporate these technologies into individual properties in a multi occupancy scheme - helping designers and specifiers to meet 'zero carbon' targets. While the government has recently announced the scrapping of zero carbon targets for 2016, developers will still have to meet zero carbon requirements set by the EU in 2019. The diverse thermal loads offered by multi-occupancy accommodation also presents an attractive demand profile against which such technologies can be operated to maximise the benefits.

How are HIUs regulated for safety?
Reports suggest that electricity causes more than 20,000 accidental domestic fires in the UK each year. As a result, installers should help to educate their customers as to the importance of ensuring that all HIU installations are electrically, as well as hydraulically, safe.

Currently, fixed wiring installations in new build properties are required to comply with Part P Building Regulations, and management regulations for multi-occupancy properties require landlords to arrange for a qualified electrician to inspect and test fixed electrical installations every five years. However, while this would cover the ring main from the fuse box - up to and including the sockets and fused spurs - it does not cover the HIU itself.

Meanwhile, there is no legal obligation to carry out a portable appliance test (PAT) in residential rented accommodation*. While the Electrical Safety Council recommends PAT testing to ensure that landlord supplied appliances are safe at the point of letting, the HIU wouldn't be included as it doesn't fall under the definition of a portable appliance.

Therefore, to ensure electrical safety, wholesalers, installers, and landlords should look to independent third party approvals of the HIU in order to be sure that they comply with electrical industry standards and regulations. For example, the NEMKO mark demonstrates that the product has been assessed for conformity to electrical safety legislation by a competent body.

Equally, independent testing for water fittings ensures that products are suitable for potable water use, guaranteeing that they are safe to use and will guard against contaminating potable water supplies. Here, the industry standard third party certification is provided by WRAS or KIWA.

The Hi-Max Instant ID is KIWA approved and carries the NEMKO N mark. It features a low energy pump and fully modulating pressure independent flow control technology for automatic balancing and fine temperature control, and the scale reduction feature keeps the appliance low maintenance.

Units come complete with a factory set of 55ºC for domestic hot water and 60ºC for the heating circuit, standardising usage for tenants. A two speed pressure independent controller output prevents overshoot of the temperature set point, ensuring more stable control of temperature, and an integral pulsed bypass function kicks in during periods of no heating to ensure that the HIU is quick to respond when it is eventually used.

Suitable for single and dual zone heating systems, the HIU can be specified in new build properties where building regulations require a minimum of two space heating zones.

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