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Multi-site HVAC: Large sites – larger questions!

Stewart Purchase, managing director of Viessmann UK, explains the Durham Standard and its impact on the future
LARGE scale projects, especially in the domestic market, pose their own problems on top of any technical issues. For example, an on-going boiler renewal programme in social housing may involve the client and his suppliers taking the longest possible view on the heating equipment, its cost in use and the effect on tenants.

In the past decisions were straightforward. Within the lifetime of the equipment there was one energy choice, gas, and its price looked pretty predictable. With conventional, especially cast iron boilers, boiler life was rarely an issue.

Now life isn’t so simple. Well within the operating lifetime of a boiler installation undertaken now, these factors could be in play;

• Gas prices and availability – prices are likely to reflect the geo-political nature of future supplies, a gas OPEC is being set up, for example.

• Green concerns are already bringing a greater use of renewable energy into the mix – especially solar heating linked to conventional systems.

• Condensing boilers are more costly and their replacement, if problems occur, could knock capital and repair budgets for six.

Let’s look at the last of these factors first and set a standard for costs.

In 2005 Durham City started a programme to update heating in its housing stock.

Durham had prepared for the introduction of Part L changes to the Building Regulations, the requirement for Sedbuk A listed boilers, by piloting condensing boiler installations. This showed a lack of proven sustainability in the service life of many condensing boilers.

This suggested to the housing team that a different selection approach should be taken. The team wished to ensure the choice of boiler should give longer life and greater choice and efficiency for the tenant and so they set what might be called the “Durham Standard”.

The key issues were service life and reliability as the cost of boiler changes and tenant disturbance in a programme involving some 4,000 boilers would be disastrous for the housing maintenance budget.

The target was a 10-year total service cost of no more than £200 per boiler. An average 15 year life before the new boilers have to be replaced is the Council’s expectation.

To set the “Durham Standard”, a panel was made up of the officers from housing and other technical departments, finance and also two of the tenants. Invitations were sent to 11 manufacturers to make presentations and the panel had a scoring system covering each candidate boiler’s operating effectiveness, efficiency, long term service life and back up. From the first round a shortlist of four was made and then the final manufacturer chosen which was Viessmann and the Vitodens 100 30kW Combi. Key to the panel’s decision was the 10-year warranty on the Viessmann Inox Radial stainless steel heat exchanger.

Many authorities are now beginning to see that ensuring the boiler lifetime service cost is only the first stage.

What of future energy costs? While new housing can be built and equipped to approach or reach a zero carbon figure, for the millions of existing homes in the UK conventional heating systems based on gas is the only economical option. Moving to condensing boilers as quickly as possible makes sound sense by offering a drop in energy use of 20% to 25%.

There is a further step that can add significantly to those savings; better control and especially weather sensitive control.

In brief, weather compensation control, linked to the boiler, controls the water flow temperature in line with weather demand.

Thanks to the fact that in the UK only around 15% of days in the heating season are at or near the design temperature of 0ºC , the boiler can be run well below its design flow temperature of 80ºC.

Result – the boiler condenses most of its operating period instead of during the few minutes at the beginning of a firing cycle. This means a further reduction in running costs – perhaps to 35% to 40% – rather than the 25% without control. All this for a further capital cost of around £100.

Finally, the issue of renewables. Again most existing house sites will have to rely on conventional heating for their central heating and most of their hot water. But solar water heating can provide around 55% of year round water requirements. However this level of saving can only be made if the boiler and water heating system work together rather than fighting each other. The answer is an integrated control system where the solar control, the boiler and the hot water cylinder are linked and so can optimise the solar input. Viessmann boilers and solar systems include control links to enable this to be installed easily.

Better yet, the newly introduced Vitodens 343 includes in one neat package the boiler, control, cylinder and the solar connections already to use, as the computer people say, out of the box.

When it comes to large multi-site schemes Viessmann has to add fore-sight to technical decisions but having done that the equipment choice and back is available.
1 July 2007

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