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Coping with the modern shift to condensing boilers

With heating and hot water plant refurbishment projects we are seeing a shift from traditional atmospheric boilers to modern condensing models. Stuart Turner suggests that when embarking on this type of project it is vital to assess the current system along with the proposed new scheme in order to gain the maximum benefit from these products and make the best possible use of the existing system including pipework and flue runs.
Taking this approach will ensure an ease of transition from old to new plant with minimal downtime - a crucial factor in commercial buildings that cannot afford to be without heating or hot water for a long period of time.

Taking an holistic approach from the outset of a project gives you the ability to obtain an objective view, whether it be for space heating, hot water supply or heating and hot water services. It is important to understand the clients' philosophy and justification for their approach to identify early on any aspirations for building excellence such as BREEAM ratings or particular performance criteria.

In existing buildings it is beneficial to undertake a thorough plant room survey which will identify factors that, if ignored, will be to the detriment of the project outcome.

This can ultimately save substantial time and cost later in the project. However this must not be explored in isolation, but extended to review the building and entire system. Has the building had a change of use, or occupancy that may affect the demand or is it an old dirty system that will require specific water treatment prior to any equipment being installed? Are there measures that have been, or could be, put in place for increased efficiency and reduced wastage, such as splitting the system into zones, installing external temperature sensors for weather compensation and thermostatic valves on some of the radiators?

There are many factors to consider and these will vary on each project, so there is not one solution that will suit all applications.

Coping with condensate
It's crucial to recognise early on the type and condition of the existing flue system and its route, as many boiler refurbishment projects today will require a flue system upgrade or full replacement - particularly if you are no longer replacing like for like.

Updates to the Building Regulations Part L in April 2014 and European Directives such as the Energy Related Products Directive are driving the change from atmospheric or pressure jet boilers to condensing or high efficiency models, in order to comply with the outlined efficiencies. However, the existing flues on this type of system are not able to deal with a modern pressurised and wet system where condensate will form. Flues for atmospheric appliances will be larger to create a low resistance system allowing the natural draught to pull flue gases through the system. Modern boilers use a fan to push flue gases through the appliance creating a positive pressure at the flue outlet allowing smaller flue diameters, but also typically run at lower flue gas temperatures with the likelihood of condensate forming within the flue.

The flue system must be water tight and pressure tight, and designed to drain the condensate from the flue and prevent flow back into the boiler. Where possible you may be able to make use of the existing chimney with a liner thus enabling it to cope with condensing operation, as well as keeping installation costs down.

A provision must also be made to remove condensate safely from the boiler itself. A 100kW boiler will generate around 13 litres of condensate per hour. The condensate is typically 3.5pH, so slightly acidic but can be disposed of normally through the drainage system.

If the boiler is not located next to a suitable outlet, the condensate trap/drain must be connected to a drainage system using corrosion resistant material. It may be beneficial to install a condensate pump to assist with effective removal, particularly important in basement plant rooms.

Modern boilers with low water content are less tolerant to fluctuating flow conditions in the heating circuit so a low loss header is recommended to safeguard the condensing boilers from low flow conditions on the secondary circuit, which may be caused by a pump being shut off or a motorised mixing valve changing position. The provision of a low loss header ensures that a steady flow of water is maintained through the boilers protecting them from potential damage as a result of firing when there is insufficient or no water flow.

Only part of the benefit of a boiler upgrade can be realised unless a review and improvement to the control system is implemented.

It is vital to identify the existing control regime, control features and functionality, plus find out if and where temperature sensors are fitted. For multiple boiler installations a good boiler sequencer, such as Hamworthy's Merley sequence controller, will help maximise the benefits of the new boiler plant, and since older atmospheric boilers only operated on/off or high/low, compared to the fully modulating operation of condensing boilers, it is unlikely the previous controls will be sophisticated enough to implement effective sequencing strategies.

Ultimately a heating and hot water plant refurbishment project should always be viewed in the wider context; not just the boilers in isolation. This approach will ensure that a best practice solution is achieved and the customer gets value for money, high efficiency compliant products with full system integration.

// The author is the southern regional sales manager for Hamworthy Heating //
10 March 2014


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