Everyone seems to be talking about district heating. It’s even rumoured to herald the demise of the combination boiler. Peter Gammon of MHS Boilers, believes it may be the start of something big
As the government tightens the Building Requirements with the introduction of the Code for Sustainable Homes, housebuilders and developers are working hard to bring in carbon-cutting technology and innovation to all their developments.

This is especially true in affordable housing schemes and those created for housing associations, ALMOs and local authorities.

Renewable technology is certainly coming to the fore, but the capital costs can be prohibitive - especially when government subsidies are lacking. So what other choices do housebuilders have for heating groups of houses, flats or apartments?

A couple of years ago the obvious choice would have been to put a combination boiler in each residence. But the problem is modern building and insulation standards and a combi boiler sized for instantaneous hot water are no longer ideal in multi-occupancy dwellings such as blocks of flats or apartments.

With the advances in building construction and installation, the demand might only be 4-5kW under design conditions. And, as recent research shows, light load running can be as little as 15% of design (less than 1kW). So, in our scenario, the problem becomes obvious. You've got a 30kW boiler, but the central heating demand is substantially below the turndown capability of many combi boilers. So how does this affect the boiler's performance?

A typical combination boiler will ignite at around 50% of its full load - in our example 15kW. So you've got a boiler starting at 15kW and then trying to turn down to an output between 0.6kW and 4-5kW (relative to the current demand). This typically leads to rapid cycling, risk of nuisance overheat - trip / lockout, and excessive wear and tear on boiler components. This is not only damaging to the boiler but also expensive in terms of fuel consumption and additional emissions - never mind the inconvenience to the householder.

So what's the solution? The better solution, which we will see more of in the future, is district or network heating. This is basically taking a concept, which was used as far back as the Roman Empire, and bringing it up to date. A central heat source - boilers or CHP unit sized with a wide diversity factor (which reduces the overall amount of boiler power installed) - is networked to a group of flats or homes.



The Nexus substation from MHS Boilers, for example, draws heat from a central boiler plant or CHP system via a simple LTHW loop. It then delivers all the functions of a combi boiler - providing heating controlled by the occupants via a programmable room thermostat and instantaneous hot water. This degree of control is in stark contrast to old-style systems. Additionally, the Nexus features a heat meter, which not only ensures accuracy when calculating bills but can also be monitored remotely.

Unlike installing separate boilers in every dwelling within a block of flats, district heating eliminates the need to supply gas to every home. Networked heating also does away with the problem of fluing (especially the unsightly pluming effect of high-efficiency condensing boilers) and eradicates the increasing problem associated with condensate waste from individual condensing appliances.

The absence of gas in the units means that people without Gas Competency certification can carry out maintenance and inspections periodically.

The units are also easy to site - they are small, and the absence of gas means no flue, so they can be installed virtually anywhere within a building.

They can be used with either radiators or underfloor heating and some models can be weather compensated if required.

Thirty six Nexus units from MHS were recently installed in Catherine Mill in Whitehaven. There were to supply heating and hot water to 31 flats, offices and communal areas at the West Cumbria Area Office of the Two Castles Housing Association. It was also the first installation of its kind in the UK to communicate the occupant's energy use direct from the Nexus metering system via a modem to British Gas in Manchester for billing.

The Nexus units replaced old thermal stores at Catherine Mill, which were insufficiently insulated and caused problems with their thermostatic control.

By replacing them, the central boiler plant could be retained and the end-users could still benefit from instantaneous hot water and controllable central heating when needed. And the Nexus units are smaller than the original thermal stores and so extra cupboard space has been freed up for the occupiers.

As energy efficiency in the domestic environment becomes even more important, pressure to provide affordable housing grows, and renewable heat sources such as biomass become standard, networked heating will inevitably become a popular choice in the blocks of apartments and could well spell the end of the combi-boiler.