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Local Authority Heating: The barriers to weather compensating boilers

Mike Freeman, national specifications manager, at Broag, questions whether, in our quest to achieve even greater energy efficiencies, we should be looking towards Holland, where weather compensating boilers are standard. He also discusses the barriers heating contractors’ social housing clients will face in adopting this approach
Local Authority Heating: The barriers to weather compensating boilers
MUCH has been written about the energy efficiency of condensing boilers and with the recent changes to Part L of the Building Regulations the Energy Savings Trust has estimated that about 7.6 million condensing boilers will be installed by 2010, many of those within the social housing sector, resulting in a reduction in carbon dioxide emissions of 1.3million tonnes a year.

This is great news, and condensing boilers are certainly effective. In some cases advances in boiler technology have meant condensing boilers are now operating at efficiencies above 90%. However, the choice of operating system can improve efficiencies even further.

Graph 1

Graph 1 typically represents the way an on/off boiler control controls the room temperature. Here, the room temperature will hunt above and below the design temperature by an average of 3°C because of the inaccuracy of standard heating industry temperature controls and the need to dissipate the heat from the system, which is likely to be at maximum output when the boiler is turned off, before the control turns it back on when the temperature falls below design conditions.
Graph 2

Graph 2 shows the way a modulating boiler control controls the room temperature. In this case the room temperature will modulate above and below the design temperature by as little as 1°C, demonstrating how this form of control more closely follows the design temperatures required, thereby reducing the energy wasted owing to the boiler hunting as in an on/off system.

Because of this, modulating boilers are growing in importance and currently the most popular choice, particularly in the public sector, are those based on output, where the boiler control increases or decreases the boiler output to maintain a pre-set constant flow temperature (system design conditions).

However, this method only takes into account design conditions, not the effect that other external influences can have on the temperature of a particular room. This means that, although the boiler is working at optimum efficiency to achieve the design conditions, the overall temperature of the room may be far greater than necessary, and so the system is still not working as energy efficiently as possible.

This situation arises because of the variation in outside temperature. When a system is designed the conditions are based on a worse case scenario outside temperature (usually around -3˚) and the level of heat loss expected from the building under those circumstances. If you take into account how many days of the year the outside temperature is at or below -3˚ it soon becomes apparent that for a large proportion of the year all central heating system are oversized.

Graph 3

In Holland, they have recognised this and few boilers operate without a weather compensating control. This works by monitoring the actual outside temperature and adjusting the boiler flow temperature accordingly. Using this method, the system will work out the flow water temperature required to give a level of heat emitter output necessary to maintain a room at a certain temperature, usually around 20˚C/21°C. This calculation is centred on knowing the design conditions of the building and understanding how much heat loss will occur based on the temperature outside.

In the case illustrated (graph 3), the design conditions are 81°C flow at -3°C outside. The heat emitters (radiators etc) are designed to release XkW of heat into the room at these conditions. When the outside conditions change, eg the outside temperature rises above -3 °C, the weather compensating control reduces the flow temperature to the heat emitter accordingly, as the heat emitter no longer needs the full 81°C flow temperature to satisfy the room demand (heat loss is less because outside temp being higher). This reduction in flow temperature continues as the outside temperature rises until it reaches a point where no heat loss is occurring (20°C flow at 20 °C outside). These design temperatures provide the min and max points on the graph that the weather compensating control reads to set the desired flow temperature at any outside temperature (called the compensation slope).

It is therefore clear to see the additional contribution a weather compensating control makes to energy efficiency. However, the challenge for heating contractors is how they can support their social housing clients in overcoming some of the misconceptions a weather compensating system can create.

Educating social housing tenants is an important part of any new central heating installation. From the initial set up through to discouraging tenants from 'tinkering' with the system and manufacturers can play an important part, right from the initial design of a boiler.

The Broag Remeha Avanta Plus condensing boiler is designed to be easy to install and maintain, with all the major system components built-in, resulting in a neat unit less predisposed to tampering. The boilers are delivered fully live, tested and pre-set, with the aim that once installed by the contractor the tenant shouldn't need to handle the system again. Broag also offers contractors training on its Avanta Boiler to ensure installation is as hassle free as possible.

Additional problems can arise when a weather compensating control is employed. As the system adjusts the flow temperature in response to the outside temperature, it will mean that for some periods the radiators will be cooler, as less output is required to maintain the room temperature stipulated. This often leads tenants to think that the system isn't working, as they expect the radiators to always be hot. This can result in unnecessary 'tinkering' with the system or service requests, all at a cost to the housing provider.

When selling in a weather compensating controlled system contractors therefore have to be aware of the additional support their client may require. This could include providing briefing information for tenants, with their installers talking tenants through how such a system works and providing a user instruction / information sheet. A frequently asked questions / answers sheet for those in charge of dealing with complaints is also useful, listing some basic questions to enable them to ascertain whether there is actually a problem with the system, before an expensive call out is authorised.

With principles from both Egan and Gershon now heavily influencing the way in which social housing providers work, there are now real opportunities for contractors that provide expert advice and added value services. Providing information on achieving the highest levels of energy efficiency and offering a working framework to ensure a smooth transition is a sure fire way to secure high value, long term partnerships.

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1 June 2006


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