Warming to condensing

Powrmatic, condensing warm air heater, space heating
Anticipating legislation — Phil Brompton.

As the energy-efficiency requirements for warm air space heaters tighten, requiring the use of condensing units, it’s important for building services engineers to be aware of the benefits. Phil Brompton explains.

Warm-air space heaters continue to be a popular choice for large, high-ceilinged buildings such as factories, warehouses and retail sheds. Furthermore, as the operators of these buildings continue to improve the insulation and air tightness of their facilities, the advantages of warm-air space heating become even more apparent.

Clearly, it is important to ensure that these heaters operate efficiently, and minimum efficiencies are dictated through Part L2 of the Building Regulations. The current part L2 requires a minimum net efficiency of 91%, which can be met with non-condensing designs.

However, as legislation continues to demand ever-higher energy performance it is clear that condensing technology for warm-air space heaters will become mandatory within the next few years. Given that the current criteria for including warm-air space heaters on the Energy Technology List require minimum net efficiencies of 101% at full and part load, it is possible that this requirement will be mirrored within Part L2.

While some people will be tempted to focus on the negative aspects, I would suggest this is actually going to be a positive development because of the cost savings and environmental benefits it will deliver. When you consider that estimates suggest around 18% of the world’s carbon-dioxide emissions arise from non-condensing boilers and heaters, it seems clear that a move to condensing units will certainly be of benefit to the environment. Also, of course, reduced energy consumption will deliver significant cost benefits to building operators.

Condensing warm-air heaters can quickly repay their extra costs in fuel savings compared with non-condensing units.

In fact, condensing warm-air space heaters are not new — they have been available for several years and tested in a wide range of applications using either natural gas or liquefied propane gas (LPG). As with the more familiar condensing boilers, they are equipped with a secondary internal heat exchanger which extracts residual heat from the flue gases. The residual heat is transferred into the warm-air stream for heating the space, thereby increasing the unit’s overall efficiency.

Again, as with condensing boilers, the use of a secondary heat exchanger does result in slightly higher costs for condensing warm-air space heaters, compared to non-condensing units. In addition, there is a need to incorporate a system for safe disposal of the condensate. In new installations a condensate-removal system can be designed in from the start and will have a very small impact on installation costs. In a retrofit situation the need to install additional condensate pipework will make the project slightly more complex, which will be reflected in a slightly higher cost.

Nevertheless, as illustrated below, there is an overall cost saving in using condensing units.

To understand the impact of these factors it is useful to consider the running costs of a gas-fired warm-air space heater with an output of 140 kW operating for 10 h per day, 5.5 days per week during a typical heating season.

Basing prices on typical commercial gas tariffs, a non-condensing heater meeting a minimum net efficiency of 84% will cost £5073 per annum to run. A more efficient non-condensing heater, with typical efficiency levels of 91.5% net, will cost £4660 a year to run. A condensing heater with 101% net efficiency will cost £4222 per annum. So, depending on the type of heater that is being replaced, the annual cost savings may vary between £413 and £851.

Clearly, it is also important to take account of installation costs — though it’s important to note that these will vary considerably from one site to another. For the purposes of this illustration, however, we can use a modern well insulated building with a floor area of 2500 m² and a volume of 13 500 m3 using the 140 kW heater described above.

The typical installed cost for a non-condensing warm-air space heater would be £6500, while a typical installed cost for a condensing heater would be around £7900. Therefore there is a cost difference of £1400.

As shown in the table, an installation based on these figures would deliver a return on investment within five years, though as energy prices continue to rise many facilities will achieve a payback within a shorter period. It is not unreasonable to expect at least a 10-year life for a warm-air space heater, so over the life of the unit the savings will be considerable.

There are clear parallels between this situation and the way in which legislation has resulted in the wider use of condensing boilers. Given the right system design, condensing boilers have been shown to deliver significant energy savings, so there is every reason to be positive about the use of condensing warm-air space heaters in the future.

Phil Brompton is managing director of Powrmatic.

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