At first glance, the new requirements of Part F and Part L are in deep conflict. How can increased ventilation be achieved while simultaneously reducing carbon emissions, and all within the cost constraints of the project? Andy Wilshaw of Nordair/Niche argues that it is possible to achieve all three goals by borrowing energy from another source to offset the additional energy burden demanded by Part F.
The comfort, health and safety of people who work in an indoor environment is paramount when it comes to the specification of the building services. As a previous article in BSEE so dramatically pointed out – ‘indoor air kills’. Without adequate ventilation to maintain good indoor air quality, the very least that can happen is a significant drop in productivity. At worst, the well being of anyone living or working in a building will be compromised.
Modern construction provides significantly improved air tightness, which can adversely affect indoor air quality. In high occupancy office accommodation, for example, where there are increasing amounts of computer equipment, there has been an appreciable reduction in air quality levels.
A good ventilation system should both remove particles from the air within the building and replace stale air with fresh air. COSHH regulations require, as a minimum, ‘a good standard of general ventilation and good working practice’. And now Part F of building regulations has raised the bar for ventilation much higher.
Part F regulations demand a 25 percent increase in ventilation – that is, from eight litres per person, per second, to 10 litres pp/ps – within all new buildings. The known negative effects of poor indoor air quality make this higher minimum requirement a no-brainer. However, it is almost certainly true to say that some specifiers and building design engineers are already specifying ventilation levels that exceed the Part F minimum by several litres/pp/ps. Even this may not be sufficient – modern, air-tight buildings need vastly increased input of positively controlled fresh air if they are to remain comfortable and healthy for occupants.
One way of achieving greater ventilation levels is simply to make the ventilation system work harder – specify a system with a higher capacity then turn up the volume and flow to reach the level of air throughput required.
Of course, the major drawback to this solution to achieving Part F requirements is that it is at odds with Part L. Part L building regulations demands a reduction in the overall carbon emission rating of the building, with the ultimate aim of slashing energy consumption of new buildings by 40 percent.
Making a larger ventilation system work harder may well defeat that object, since a higher payload means more energy consumed, which in turn means more carbon emissions. Similarly, as a greater volume of fresh air is introduced into the building, the heating system also has to burn more fuel to bring the air to comfort temperature.
So simply turning up the ventilation system isn’t acceptable in the light of Part L. A lateral, left-field approach is required. Why not, for example, make ventilation work on ‘borrowed’ energy from another source? If two building services can run efficiently and effectively on combined energy, the requirements of both sets of building regulations can be met – reduced carbon emissions and greater throughput of air.
Borrow a little, gain a lot
Advanced heat recovery technology provides the answer. The technology is not new, in fact it has been around for many years, but was not routinely specified for ventilation in the past as the capital cost was perceived to be prohibitive. It used to be cheaper simply to install bigger fans to achieve better ventilation – now building regulations dictate that this is no longer acceptable. At the same time relative costs of heat recovery units have fallen.
The essential element of heat recovery is the cross-plate heat exchanger, which sits within an air handling unit (AHU). Supply and extract air are drawn into the AHU and flow across the cross-plate heat exchanger. The two air streams are kept entirely separate so there can be no transmission of dirt, odours, moisture or bacteria, making it a very clean as well as energy efficient solution.
Up to 60 percent of energy can be recovered from extract (warm) air and used to heat supply (cold fresh) air, which is then delivered into the building to ensure comfort conditions are maintained. In theory, nearly any efficiency can be achieved if the cross-plate heat exchanger is sized and designed specifically for the application, although temperature and humidity of the air stream will affect operational efficiency.
The resulting reductions in energy usage and carbon emissions mean:
• Lower running costs for both heating and ventilation
• Less harmful impact on the environment
• Reduced consumption of dwindling fossil fuel resources.
How cross-plate heat recovery works
The cross-plate heat exchanger is the best technological solution for introducing fresh air with unmixed airflows while optimising energy usage for heating and ventilation.
The heat exchanger consists of multiple light-gauge aluminium plates which are stamped to provide an extensive heat transfer area. The units are designed for maximum heat transfer with low resistance to airflow. The low resistance minimise electrical consumption, an important consideration for compliance with Part L, which restricts the total fan supply power to 2.5W/l/s when heat recovery is used.
Cold external air is drawn into the cross-plate heat exchanger and across the plates. Air filters at the point of entry ensure dirt, dust and other undesirable particles are eliminated. At the same time, warm extract air from the building interior is drawn into the other side of the heat exchanger and across the plates in the opposite direction, being entirely separated from the external air.
Heat from the extract air is transmitted through the plates by conduction. The warm extract air cools down and is discharged from the building; the fresh supply air is heated and delivered into the building.
The cross-plate heat exchanger exploits a relatively straightforward principle of physics – conductivity – to make energy efficiency gains on both heating and ventilation. The technology involves no moving parts or electrical connection – therefore, there are no additional running costs involved, operation is always guaranteed and the equipment cannot fail or wear out.
The fully-modulating burner
To achieve minimum carbon emissions, the cross-plate heat exchanger needs to be combined with a low-energy heat source to temper incoming air. This can be achieved using a high efficiency gas fired heater section which, by introducing heat directly at the point of use, eliminates any transmission or idling losses associated with heat sources served from central boilers.
In order to optimise heat recovery performance, it is essential that the heater section has a modulating control with high turn-down ratio. This will ensure that maximum heat recovery can be achieved all year round without overheating the incoming air, or operating the heat recover by-pass to maintain the correct winter supply air temperature.
Fast response close control to +/-1°C, as provided by Nordair/Niche gas-fired units, enable maximum heat recovery to be achieved whilst maintaining precise air supply temperature.
Money saving solution
Heat recovery via cross-plate heat exchangers is now listed on the Carbon Trust energy technology list. End-users may claim Enhanced Capital Allowances of between £1,320 and £1,800 for each m³/s capacity of the system.
At the same time, heat recovery also reduces the size of the heating section required, allowing further capital cost savings to be made. Fully modulating gas-fired air input units, manufactured by Nordair/Niche, are also eligible for Enhanced Capital Allowances on both the equipment costs and the directly associated installation costs.
Once ECAs are taken into account, the energy efficient option usually costs no more that a standard system that cannot offer the comfort benefits and building regulations compliance of heat recovery and high performance gas-fired heating.
When the building services on a project are considered as a whole, heat recovery will be seen as a major energy and cost reducing solution, and an answer to the significant challenges currently facing specifiers and building design engineers in meeting the stringent requirements of the new Part F and Part L regulations.
The cross-plate heat exchanger recovers up to 60 percent of heat from the air that passes across it. The energy is transferred by conduction to the supply air drawn in from outside.
Air is extracted from the building at 20°C and supply air comes in at 0°C. Both air streams pass across the heat exchanger, without mixing.
The extract air gives up 60 percent of its heat. This is transferred across the plates of the heat exchanger, raising the temperature of the fresh supply air by 12°C. The burner then only has to ‘top up’ the supply air stream by a further 8°C to reach the required 20°C comfort temperature.
An air input unit without heat recovery supplying 4m³/s would produce typical annual carbon emissions of 8,827kg. To comply with new building regulations, the air supply needs to increase to 5m³/s, raising annual carbon emissions to 11,053kg.
Using a cross-plate heat exchanger in combination with a high efficiency gas-fired heater would reduce carbon emissions for a 5m³/s air input to just 4,186kg (ie +25% air for -53% emissions).