Assessing the case for condensing boilers
Just how much boilers have moved on in their efficiency, ability to reduce carbon emissions and save money is assessed by Stuart Turner.
If you’re concerned about your energy costs and carbon footprint and are still running high-efficiency non-condensing boilers with on/off single stage burners or, even worse, older traditional equipment such as cast-iron sectional boilers, then it’s time to take action. This means reviewing the economics of modulating condensing boilers and upgraded boiler controls, versus where you are now, and gaining some significant benefits.
At CIBSE’s (Chartered Institution of Building Services Engineers) national conference in April 2011, former CIBSE president, Rob Manning, reminded delegates that 24 million buildings already built are expected to still be in use in 2050. Consequently, an increasing refurbishment programme faces the industry — a point emphasised by Mr Manning who stated that ‘60% of existing non-domestic buildings will need to be refurbished’ in the years ahead.
If you own one or more of those buildings, you may be resisting any refurbishment or new investment in plant until there’s a convincing economic upturn, with the laudable intention of going for renewables later in preference to a half-way house now. In reality though, renewable energy systems cost significantly more than replacement gas-fired boilers. And despite the support available via the Renewable Heat Incentive and Feed-in Tariff, finance is still required to fund those projects, which often integrate gas and renewable energy for the ultimate solution.
Better then, to achieve worthwhile benefits right away by upgrading the boiler plant and associated controls using products such as Hamworthy’s fully modulating Fleet boilers. These offer a gross seasonal efficiency of 97%, making it possible to substantially reduce gas bills and achieve sizable carbon savings. Modulating burners on modern boilers have a turndown ratio of 5:1, so a typical 100 kW boiler module can reduce down to 20 kW to match the boiler load closely to the actual building load without overshooting and wasting expensive gas.
A further incentive to invest in new heating plant is the Enhanced Capital Allowance (ECA) scheme that is available when using products on the approved Energy Technology List (ETL) managed by the Carbon Trust, to give additional tax benefits and improved cash-flow conditions.
Since modulating condensing boilers are generally used to back up renewables, follow-on investment in renewable technologies can be added later. For the immediate future it is quite possible to achieve payback periods of less than three years, along with significant carbon savings, when replacing older boilers.
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| Table 1: Old and new boilers compared — showing clearly just how substantial fuel and cost savings can be. |
I believe that end users with multiple properties, such as in the health sector, education sector, hotel chains and leisure centres have most to gain, and would encourage them to test this through single-building pilot projects. Once delighted with the results, roll out across multiple buildings can be scheduled to multiply the benefit.
Assisting such pilot projects in the capital is the London Energy Efficiency Fund (LEEF). Unveiled in September by London Mayor, Boris Johnson, the £100 million fund has been made available to upgrade buildings in the capital, which are currently responsible for up to 80% of London’s carbon emissions. As part of the funding programme public buildings such as schools, libraries and hospitals will be able to apply for flexible finance to pay for energy saving measures including boiler upgrades.
To illustrate the benefits of upgrading to modern fully modulating boilers, take a building which has heating available on demand continuously, such as residential buildings, hotels, care homes, universities or, even, some hospitals, with a typical annual load profile of 3418 h — 39% utilisation.
The system is operating at 80°C, and the existing boiler plant comprises six atmospheric cast-iron boilers, each 120 kW output with a single-stage on/off burner. The existing boiler plant was oversized by 10%, which can be reduced to just 3% with replacement boilers, contributing to the savings without affecting performance, so the system heat load is 655 kW.
Using the same load profile throughout the heating season, we have prepared an estimate of savings using the new Fleet Vertical F250V-750 floor standing boiler. Using just three modules at 250 kW each (installed condensing boiler plant 750 kW), when operating at 80°C, like the original plant, will provide 675 kW output, which is only 3% oversized, achieving a closer load matching than the original plant.
Table 1 shows the savings that would be achieved. The 97% efficiency is actually the gross seasonal efficiency for the new Fleet boilers and takes account of improvements to the boiler controls. If additional sensors are fitted to the system to allow boilers to run at compensated temperatures, this is the level of performance that can be achieved. If however the new boilers are to continue running at 80°C flow temperature, they will still be capable of achieving a 90% gross efficiency, which is still significantly better than the old boilers at 80%.
If you take a typical installed cost of this new plant at around £44 000 and projected £17 166 annual energy saving costs, you can see that the simple payback would be considerably less than three years.
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| Table 2: Calculating the carbon savings for new boiler plant compared to old. |
By reducing the number of boiler modules from six to three, we have also reduced the lifetime operational cost of the boilers. The labour cost for servicing each module would be similar, so this is a direct operational saving. The load matching that was achieved by turning each of the six original 120 kW boilers on or off in turn is replaced with a far superior modulating boiler system, where each module is 250 kW output (225 kW at 80°C), but will modulate down to around 45 kW, enabling the boiler to match the system load very closely, without over-shooting and wasting expensive energy.
Many users will be equally interested in reducing the environmental impact. Table 2 illustrates carbon savings for our worked example. The results are just as impressive, again on the basis of 39% utilisation, 3418 h over a full year and using the DEFRA carbon-emission guidance factor for natural gas of 0.185 kg/kWh.
Thus the Fleet boilers save 90 734 kg of CO2 each year compared to the old boiler plant. With such substantial reductions in energy costs and carbon emissions, I suggest there is a very strong case for upgrading to modern fully modulating boilers for both environmental and economic benefit.
Stuart Turner is southern regional sales manager with Hamworthy Heating.
