Get the balance right with boiler sizing

Ideal Commercial, commissioning, boiler sizing, Boilers, space heating, DHW
The problems of oversizing boilers — Darren Finley.

Accurately sizing boiler plant and systems brings many benefits — including lower capital and running costs, ease of commissioning and higher efficiencies. Darren Finley of Ideal Commercial Boilers argues that boilers should not be oversized.

Modern commercial condensing boilers offer the potential to dramatically reduce energy use in buildings, with appliances from leading manufacturers offering up to 110% part-load efficiency. However, like any advanced technology, installation expertise is essential to ensure this impressive potential is actually realised once the appliance is operating as part of an integrated building system, otherwise the dreaded ‘performance gap’ arises, whereby the efficiencies achieved are significantly lower than the product and/or system’s capabilities.

Correctly sizing heating systems takes time, thought and expertise to calculate the correct boiler and heating-system size for each application, but it is well worth the effort if the energy and cost savings offered by modern technologies are to be maximised.

Heating systems will usually be oversized rather than undersized because once a heating system has been commissioned and the building is occupied it is preferable to have more heat available than not enough. This is because there are ways to remove excess heat, but additional heat cannot be generated from a system if its maximum output has been reached. If a building is deemed uninhabitable due to insufficient warmth, those responsible for its design will be liable —which is a situation contractors and consultants would wish to avoid. However, this does not mean that common sense should not go into minimising the size of a boiler system so that it does not become so oversized that costs and emissions are increased disproportionally.

As far as sizing a heating system goes, knowledge is power — particularly knowledge about the building in which it is being installed. The boiler will need to be able to meet the peak heat demand at the coldest time of the year which, in the UK, is generally considered as anything from below freezing to -3°C. Consequently, the most important first step for sizing either a new build or retrofit heating system is to conduct a thorough examination of all the elements that will affect the heat requirement and then use all the calculations available to make an informed decision on the appliances required.

The most important calculations will be to ascertain both the heat requirement of the building and the heat loss — based on criteria such as the building’s current size, materials used in construction, insulation, number of radiators, additional technologies applied to the heating system, humidity, air flow etc. If the project is retrofit, simply matching the output rating of the existing boiler(s) is not best practice, as older buildings may well have undergone a number of modifications since their initial construction — such as building extensions or new pipework. Improvements to insulation could mean the heat demand has significantly reduced, so the old boiler(s) may actually be oversized.

Ideal Commercial, commissioning, boiler sizing, Boilers, space heating, DHW
Although modular boiler installations can modulate their heat output down to very low levels, the rest of the heating installation still has to be sized according to the maximum output.

Conducting a thorough investigation conforms with the guidance in the ‘Non-domestic building services compliance guide’, which acknowledges the practice that a heating system should be oversized rather than undersized, but suggests restricting oversizing to a maximum of 15% — and only following a detailed assessment of the load. The benefits to the end user of abiding by this advice are both short and long term.

It is common sense, but easy to forget, that choosing a smaller boiler system automatically means a lower capital cost. If too large a boiler output is specified, not only will the appliance be more costly but other components in the heating system — pumps, pipes, valves and fittings — will also need to be larger to effectively distribute the increased heat output throughout the building.

Enhanced control to improve energy efficiency is another major benefit. As legislative directives such as the ErP are enforcing the uptake of condensing boilers, it must be remembered that specific conditions relating to flow and return temperatures must be met to ensure the condensing boiler plant operates at peak efficiency, which can also be affected by system sizing. Fuel efficiency will be noticeably reduced if the return temperature is consistently too high to allow significant condensation to form in the heat exchanger.

Ideally, the return temperature should be 54°C or lower. There are methods of reducing the temperature of the return water, such as increasing the size of the radiators or reducing the water flow in order to dispel more heat around the building. However, if the boiler is greatly oversized, these modifications can lead to buildings overheating — which is becoming more common as insulation materials have improved. It is therefore important that the heating system is sized to meet both the heat demand and the return-temperature requirements of the condensing boiler. The boiler manufacturer will be able to advise on the design and maintenance of a space-heating system that optimises the appliance’s performance.

As is the case with many aspects of building construction, the correct design decisions will vary greatly depending on the application, and building-services engineers must always make decisions on a case-by-case basis. This is where expertise should be easily available, so that all the relevant elements can be addressed to ensure that the appliances are not wasting money and fuel as a result of excessive oversizing. Leading boiler manufacturers will be able to guide customers to achieve this balance every time.

Darren Finley is national sales director for Ideal Commercial Boilers

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