Centralised plant and HIUs are on the high rise

The use of centralised plant and individual heat interface units (HIUs) in high rise developments is becoming more popular, with this approach increasing efficiencies, making the integration of low carbon and renewable technologies easier and reducing installation and maintenance costs. Jon Cockburn, head of marketing at Heatrae Sadia, explains more.

In multi-occupancy developments such as high rise flats or apartments, an increasingly common approach to delivering space heating and hot water is to install centralised heating plant and deliver heat to the individual properties through a network of pipes carrying hot water.

Often referred to as communal or community heating, or heat networks, a centralised scheme offers many benefits over the more traditional solution of installing separate heating systems (typically comprising gas-fired combi boilers or a form of electric heating) in each individual property.

For example, using centralised heating plant increases efficiencies and reduces fuel consumption. This helps developers to meet their carbon compliance targets, whereas, in contrast, it has been increasingly challenging to this by using individual boilers.

Furthermore, a number of different fuel sources can be used in a centralised scheme serving a multi-occupancy building, and significant environmental benefits can be delivered by adopting low carbon and renewable technologies such as biomass, solar thermal water heating and CHP. In fact, in its 2013 The Future of Heating: Meeting the challenge document the Government suggested there is great potential to develop heat networks so that they can play a part in the move to low carbon heating.

In most cases, the integration of low carbon and renewable technologies becomes more feasible in a centralised scheme, as it can be difficult to incorporate them into individual properties in a multi-occupancy development, and the diverse thermal loads offered by this type of accommodation present an attractive demand profile against which such technologies can be operated to maximise the benefits.

However, a centralised system based on gas boiler plant should still offer efficiency and carbon savings. In addition, using central gas boiler plant reduces the issues associated with supplying gas to individual properties in a multi-occupancy scheme. The risks linked to gas distribution pipework are reduced and there will be no need to fit numerous flue terminals, plume displacement kits and condensate drainage systems, lowering capital installation and whole life costs. And, if the properties within the scheme are to be rented, the landlord's servicing and maintenance costs will also be significantly reduced, and the need for legally required landlord gas appliance checks in individual properties will be eliminated (providing of course there are no other gas appliances installed).

If a developer decides to adopt a centralised plant scheme (whether using gas or low carbon/renewable fuels) they must consider how the occupants will control their heating and hot water, and how the energy used by each household will be metered and billed.

The Heat Network (Metering and Billing) Regulations 2014: guidance to compliance and enforcement of the legislation, states that final consumers of communal heating and hot water systems (as well as district heating and district cooling) need to be provided with competitively priced individual meters where it is cost-effective and technically feasible to do so (in some cases individual meters must be installed, regardless of cost-efficiency and technical feasibility)*.

HIUs, sometimes referred to as heat boxes, are becoming an increasingly common solution. They not only provide heating or heating with domestic hot water to individual properties, but also enable the occupant to control their supply with either a room thermostat, a separate programmer or individual thermostatic radiator valves, and, importantly, also record the heat consumed, so that an individual household's energy use can be metered and billed.

It is possible for meter reading and energy billing to be carried out remotely from a central location, which can be very attractive for organisations operating a large portfolio of properties. Importantly, metering can also help occupants to lower their energy consumption.

HUIs 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.

When using central boiler plant and HIUs it is recommended that one system is a cascade to allow for wide diversity in loads (so that the boilers can readily modulate across a range of outputs). In all cases the boiler plant must be capable of delivering peak space heating load, but HIUs incorporating a plate heat exchanger for domestic hot water require special consideration, as generally there will be a significant difference between heating and hot water loads.

For example, heat loads for each dwelling could be in the region of 3 to 5kW, whereas 35kW or more is likely to be required for peak domestic hot water. For an economical and energy efficient solution to be achieved, appropriate diversity factors need to be adopted when determining plant capacity - in most cases this will involve consideration of the occupants' lifestyles and typical use profiles.

When selecting HIUs for a scheme, specifiers should look for independent third party approvals to ensure that products comply with industry standards and regulations. For example, the NEMKO mark demonstrates that a product has been assessed for conformity to electrical safety legislation by a competent body. Meanwhile, independent testing for water fittings guarantees that products are safe to use and will guard against the contamination of potable water supplies. Here, the industry standard third party certification is provided by WRAS or KIWA.

The Hi-Max Instant ID from Heatrae Sadia has a compact design (it can fit into a standard kitchen cupboard) and comprises two generously sized plate heat exchangers - one for the hot water and one for the heating system. It is designed to ensure that the primary return temperatures going back to the centralised heating system are very low to maximise the operating efficiency of the system. A controller monitors and regulates the heating and hot water system in order to optimise performance, working with the room thermostat. It is suitable for single and dual zone heating systems.

In recent times there has been a trend towards centralised plant being used alongside HIUs to deliver heating and hot water to individual properties in multi-occupancy developments, such as high rise flats and apartments. In comparison to the traditional approach of fitting individual heating systems, it offers developers many benefits, including increased efficiencies, easier integration of low carbon and renewable technologies and the reduction of installation and maintenance costs.

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