‘Boilers’ that are more than boilers

MHS Boilers, HIU, heat interface unit, boiler, space heating, DHW
Presenting the benefits of network heating — Peter Gammon.

Multi-storey residential buildings have historically incorporated individual heating units for each dwelling. However, Peter Gammon, of MHS Boilers, explains, how centralised heating plant with heat interface units is now a more pragmatic approach for compliance with the latest industry requirements.

Utilising low-carbon energy in new homes is becoming more important than ever before. And as the new-build sector moves closer to zero-carbon development by 2016, specifiers are faced with even more stringent targets through the Building Regulations (Approved Document L), Code for Sustainable Homes and Planning Policy 22.

The ideal scenario for network heating is residential blocks several storeys high.

Complying with these areas of legislation, guidelines and codes can be challenging in multi-storey developments — especially when the typical heating plant has traditionally been individual combination boilers or conventional wet-heating systems with hot-water storage. In recent years, the industry has started adopting a different approach based on centralised heating plant connected to individual heat interface units (HIUs). This setup facilitates the use of low- or zero-carbon technology and on-site energy generation (such as biomass and/or CHP units or) alongside condensing gas boilers for the main plant to create a highly efficient network heating system.

An HIU draws heat from the central plant via a simple low-temperature hot-water (LTHW) loop. It then delivers all the functions of a combi boiler, with both heating and instantaneous domestic hot water (DHW) supplied to an individual property at temperatures and flow rates that suit user demand. HIUs can be installed inside a dwelling as fully cased units or built into walls to allow easier access for inspection and maintenance in a communal area.

There are a number of benefits to a network heating system compared to separate combi boilers.

The Nexus heat/hot water Interface unit can provide metered service to multi-dwelling developments such as district heating schemes or blocks of flats or apartments.

First, they remove the problem of flueing (especially the unsightly pluming effect from high efficiency condensing boilers) and condensate pipework. Plus, with the absence of gas in the units, installers without Gas Competency certification can carry out maintenance and periodic inspections.

However, one of main advantages is apparent when the heating demand for individual apartment is considered. Quite often, this might only be 3 to 5 kW under design conditions, and at light load running can be as little as 15% of design (i.e. less than 1 kW). Consequently, the system needs to react quickly and be able to turn-down significantly, which is not possible with standard 30 kW combi boilers. These are likely to ignite at around 30% of full load, starting at 10 kW and then attempting to turn-down to an output between 0.45 to 5 kW (relative to the current demand).

This disparity between the power needed to generate instantaneous domestic hot water and very low space-heating loads (made even greater by seasonal reductions), can lead to excessive wear and tear on boiler components, rapid cycling and a higher risk of nuisance overheating problems with the boiler. HIUs eliminate these problems.

Plus, setting up the main plant to cascade allows for a wide diversity of system loads and the capability of modulating across a wider range of outputs. This is especially important for residential buildings where there are likely to be peaks in demand at certain times of the day for generation of hot water.

Despite the advantages of HIUs over combi boilers, one constraint affecting the centralised heating plant has always existed — the likely need for large-capacity buffer vessels on the primary system to cope with peak demands. With space at a premium in modern residential buildings, a tight plant room can make accommodating the required vessels difficult.

Adding hot-water storage to a heat-interface unit can reduce the need for large buffer vessels in the central plant room of a network heating system.

However, one solution developed in recent years has been combining individual HIUs with an integrated unvented storage cylinder. These units, such as the Nexus SFS from MHS Boilers, can reduce the need for large buffer vessels on the primary side.

Inevitably, installing an HIU raises some important design considerations. So, whether it is a wall-hung unit, built into the wall, or an HIU with integrated cylinder, allocating the necessary cupboard space needs to be considered early on in the plans. This allows for effective unit siting, while also providing ease of access for maintenance and metering.

Overall, the developments in network heating systems make them an effective and highly efficient system for multi-storey residential buildings. With carbon reduction a key factor, a network system can make the most of renewable heat from the main plant, such as a biomass boiler with condensing gas boilers as a backup.

With powerful HIUs being available in a variety of sizes and configurations, occupants are sufficiently supplied with heating and hot water when they need it. When compared to other options, not many systems will stack up against the overarching benefits from network heating.

Peter Gammon is technical manager with MHS Boilers.

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