Module 26: Heat interface units

Requirements for low carbon energy solutions for homes have prompted a rethink on how heating and hot water needs are serviced in the UK residential market – leading to a growing demand for heat interface units. This CPD module will cover some of the issues surrounding the design, selection and system integration of these units

Andrews Water Heaters Potterton Commercial

Multi-dwelling development schemes historically have incorporated individual heating plant into each dwelling as the norm, typically in the form of a combination boiler or direct acting electric heating system with hot water storage. As the residential new build sector transitions towards ‘zero carbon’ development by 2016, there are increasingly stringent environmental targets placed upon new developments through the Code for Sustainable Homes, Approved Document L of the Building Regulations and Planning Policy Statement 22.

In the case of multi-storey dwellings, compliance with current requirements poses a set of challenges, as it can be diffi cult to incorporate low to zero carbon technologies in self-contained installations on an individual dwelling basis, and using individual boilers will prove increasingly challenging to apply to meet CO2 compliance terms. With this in mind designers and specifi ers are increasingly looking towards a centralised heating plant strategy, whereby the integration of low to zero carbon technologies can be appraised in a more holistic manner without the constraints imposed by an individual dwelling approach.

Of course, when dealing with a central plant scheme serving multiple dwellings, important considerations are end-user time and temperature control together with metering/billing of individual energy use. These considerations can be addressed with the application of heat interface units (HIUs).

An HIU, sometimes referred to as a ‘heat box’, is an integrated solution for delivering and recording the heat consumed by an individual dwelling served from a centralised heating plant or district heating scheme. HIUs provide localised control and metering in a self contained package, allowing simple integration of individual dwellings into a larger heating and hot water system.

These units can be installed either internally within each dwelling, or recessed into the dividing wall between dwelling and landlord space, allowing ease of access for inspection and maintenance.

HIUs are broadly split into two categories: heating only, and heating with domestic hot water production (DHW).

Heating-only HIU

The heating-only HIU variant is fairly simple (see Figure 1): the central plant feed firstly runs through a differential bypass valve, which provides a hydraulic bypass to the return circuit in the event of no demand from the dwelling system; a motorised two-port valve provides on/off control under the dictates of the dwelling programmable thermostat or single channel programmer; and from here the system flow distributes to the dwelling circuit and heat emitters. On the return side a small circulating pump overcomes the resistance of the dwelling system and a heat meter, measuring volumetric flow rate and temperature difference across flow and return to derive energy use in kWh.

Fig. 1 : A typical heating-only heat interface unit

In this arrangement (see Figure 2) domestic hot water is served from a central plant room, either via calorifiers or direct gas fired water heaters.

Fig. 2 : Schematic arrangement showing heating only HIUs fed from a central boiler cascade with domestic hot water from a central calorifier

There are variants available on the heatingonly HIU principle, incorporating a two heating zone capability for larger dwellings, or the inclusion of an adjustable balancing valve in place of an integral circulating pump, in this instance hydraulic design of risers and distribution will differ (more on this later).

With the heating-only HIU there is no facility to interface or measure DHW consumption, therefore the second category of HIU introduces hot water functionality.

As can be seen in Figure 4, components and hydraulic layout are largely similar to the heat-only variant, but with the introduction of a plate heat exchanger to allow instantaneous generation of DHW at each dwelling, in the same way as a combination boiler. In order to generate DHW at a useful temperature and flow rate, the primary hot water feed onto the plate heat exchanger should be in the region of 75C. This may be at odds with the dwelling heating system operating regime where, in the instance of underfloor heating for example, the flow temperature required may be much lower. For this reason a mixing valve (V2) is introduced to blend primary water from the central plant with return water from the dwelling heating system, thereby allowing the HIU to satisfy differing demands dictated by DHW and space heating. In addition, a three port control valve is required to divert flow between the DHW plate heat exchanger and dwelling heating circuit; this valve is controlled on a hot water priority basis. An example of the system is shown in Figure 3.

Fig. 3 : Schematic arrangement showing heating & DHW HIUs fed from a central boiler cascade

Fig. 4 : A typical heating and DHW HIU with plate heat exchanger

Again variants are available incorporating a two heating zone capability for larger dwellings, or the inclusion of an adjustable balancing valve in place of an integral circulating pump.

In addition to DHW generation via a plate heat exchanger, HIUs offering primary outlets to feed a hot water cylinder within the dwelling are also available. A further variation is a heating only unit with an inbuilt DHW litre counter, thereby allowing a centralised hot water strategy but with the ability to monitor and charge for local usage within each dwelling.

Both of the schematics described so far show HIUs working with a direct connection to centralised pipework distribution; dependent upon the size and height of the building, it may be beneficial to consider hydraulic separation. With a directly connected system there is the potential for a leak, or failure of heating pipework within a dwelling to drain off a proportion of the primary heating system for the whole building. In the case of a particularly tall building, static head may also dictate the need to separate central distribution hydraulically from dwelling systems to protect emitters and fittings from excessive pressure. To address these concerns, some manufacturers provide HIUs that incorporate a further plate heat exchanger to achieve separation of the water content in the dwelling and central pipework systems. When introducing separation, however, it is important to consider that the dwelling side will require provision for expansion, together with a filling loop and pressure gauge. It is always necessary to provide a circulating pump and there is the efficiency loss of heat transfer across the plate heat exchanger to take into account.

In all arrangements one of the primary drivers for the integration of HIUs is metering; the arrangements for this will vary with manufacturer but fall largely into two groups.

Hard-wired HIUs transmit data via a network of data cabling installed in the building fabric to a central hub. Wireless options are also available, using network nodes to boost data transmission over larger buildings, again collecting data to a central hub. At this point data can be collected by a reader device, routed to a network via a PC or transmitted via a node to the GSM telephone network. This then allows either local or remote meter reading with real time energy use, aggregate kWh consumption over pre-determined time periods and, in some instances, fault status or error notification.

With many HIUs the heat meter within the unit can display real-time consumption, which may be accessible by the occupier.

Selection criteria and design considerations

The decision on which type of HIU to employ will largely be informed by the overall building energy strategy. If sufficient CO2 mitigation is included in the space heating provision, then HIUs incorporating DHW production via a plate heat exchanger may be the best option. However, as the predominant energy requirement in new buildings tends to be hot water production, it may be necessary to address separate space heating and hot water strategies. In this instance it can be simpler to integrate low to zero carbon technologies in a central location; for example solar or heat pump to pre-heat the cold water supply to direct gas fired water heaters. In such instances a heat only HIU, perhaps with a DHW meter, may represent the most sensible option.

Regardless of HIU type, it is recommended that one central boiler plant is a cascade system – owing to the possibility of a wide diversity in loads – and therefore able to readily modulate across a range of outputs. In all cases boiler plant must be capable of delivering peak space heating load. However, HIUs incorporating plate heat exchanger for DHW generation require special consideration. It may be the case that space heat load for each dwelling is in the region of 3 to 5 kW, whereas the plate heat exchanger is likely to require 35 kW or more at peak DHW flow rate. Clearly this leads to a huge disparity between heating and hot water loads, so designers must research and adopt appropriate diversity factors when determining plant capacity, in order to arrive at an economical and energy efficient solution. In most cases occupant lifestyles and typical use profiles will determine the appropriate diversity factor.

For example, in the case of private accommodation, it is likely that the morning showering period between 6am and 8am represents the peak demand for hot water generation and this load will be in excess of space heat requirements. In such instances a buffer vessel in the central plant room could be incorporated to smooth out peaks in demand pattern, reduce peak plant loads and reduce boiler capacity.

Required supply temperature will also inform plant selection; if heating-only HIUs are proposed, it is possible to match the main plant temperature regime to that required by the dwelling emitters. If heating and DHW HIUs are proposed, then it will be necessary to distribute from central plant at 75C to 85C.

When deriving pipework distribution and pump sizes, it is recommended that horizontal branches at each floor level are piped in a reverse return layout, both to assist with hydraulic balancing and to ensure even distribution of heat between dwellings (see Figure 5).

Fig. 5 : When deriving pipework distribution and pump sizes it is recommended that horizontal branches at each floor level are piped in a reverse return layout to assist with hydraulic balancing and to ensure even distribution of heat between dwellings

As previously described, HIUs are available with circulating pumps or balancing valves. In the latter case it is necessary to include a dwelling heating Figure 5: When deriving pipework distribution and pump sizes it is recommended that horizontal branches at each floor level are piped in a reverse return layout to assist with hydraulic balancing and to ensure even distribution of heat between dwellings circuit index run calculation, thereby increasing the central pump set duty. Feedback from installation sites would indicate that hydraulic balancing and commissioning is simplified when HIUs including circulating pumps are installed.

Benefits of using HIUs

In comparison to the traditional approach of individual systems within dwellings, there are many benefits with a central plant approach incorporating HIUs:

  • No individual gas boilers in dwellings, reducing risks associated with gas distribution pipework, saving on installation costs and reducing the need for individual flue terminals (with associated pluming);
  • Annual gas safety inspection is typically restricted to the plant room;
  • Reduction in operating costs owing to reduced boiler service;
  • If installed facing into landlord areas, HIUs reduce maintenance access issues;
  • Meter reading and energy billing can be carried out remotely from a central location – this can be very attractive to councils and those with a large portfolio of buildings;
  • Capital installation and whole life costs can be lower for a centralised scheme;
  • Effective integration of low to zero carbon technology can be far simpler with a central scheme and overall energy efficiency can be increased; and
  • Gas consumption is reduced through central plant efficiencies.

When appropriately applied, heat interface units offer a robust and cost effective solution to heating and hot water delivery in modern, high density, multidwelling developments. The application of associated consumption based metering and billing can act as a positive incentive not only to reduce energy consumption, but also to assist in tackling fuel poverty and providing affordable warmth to priority groups.

© Jeff House and Tim Dwyer