Whole life costing: Mechanical ventilation in offices
In many instances mechanical ventilation
may be the only option to manage office
Peter Mayer of Building LifePlans explores the options
Although there is an emphasis on passive ventilation to minimise carbon emissions, the reality is that offices often include mechanical ventilation and space heating to create a comfortable internal environment.
There are many options in the design of ventilation systems to minimise costs. Ventilation can account for a third of building services costs and have high energy demands, which can be reduced by specifying systems with lower air flow rates and pressures. Systems that use less energy tend to have higher capital costs. Areas that can improve whole life costs are:
There is an optimal time to replace filters based on the degree of dust collected. Replacing filters too frequently leads to high filter and labour costs. If replacement is not frequent enough, the filter gets clogged and creates pressure drops which require higher fan power and generates higher energy costs.
Electrostatic filters operate with fewer pressure drops than fabric filters, but capital costs are higher and charging the filter has energy costs too.
Fans and motors
Centrifugal fans are more efficient at higher static pressures. Axial fans are more efficient at lower static pressures so are more likely to be specified for a low pressure, low flow rate, low energy systems.
Approved Document Part L2A gives limits for specific fan power for system types, for example, central mechanical ventilation with heating and cooling should not use more than 2 W per litre per second. Whole-life costs may be further reduced by attracting the tax benefits of enhanced capital allowances for high efficiency motors and variable speed drives.
Direct Current (DC) brushless motors are more efficient but may be twice as expensive as an alternating current (AC) induction motor.
The more efficient the heat recovery unit, the higher the capital cost. While heat recovery efficiencies of between 50 – 90% can be achieved, fan power needs to be increased and the cost of the air handling unit may increase by 50%. A separate whole life cost calculation will confirm if the additional unit is cost effective. The longer the period the unit is run, the more likely it is that a heat exchanger will be cost effective.
Air handling units
Expensive air handling units offer lower pressure drops with slightly lower energy costs.
Consider including an air handling unit bypass for when cooling is not required. This reduces pressure drop and saves fan energy. A variable speed fan is required for this option.
Design the system to minimise pressure losses caused by dampers, grilles, diffusers and louvres.
Long radius bends minimise pressure drops but have high capital costs. Minimising the variation of fitting and achieving economy of scale may be a more cost effective design strategy.
Reducing duct leakage, by in situ duct seals, enables fans to run at an optimum level.
Calculate the optimum duct diameter for lowest life cost.
Remote monitoring for condition-based maintenance is proving to be a cost effective maintenance strategy.
Inspection and cleaning of ductwork should be facilitated by reasonable provision for access.
Further guidance and acknowledgement
The Chartered Institution of Building Services Engineers (CIBSE), Building Services Research and Information Association (BSRIA) and Heating, Ventilation and Cooling (HVAC) have published guidance on whole-life costs, design, installation and maintenance of ventilation systems. The CIBSE’s TM30 Improved life cycle performance of mechanical ventilation systems provided information for this article.
Net present value for 60 years£/m²
Low velocity air handling unit with heating coils filters and fans, internal location 2m3 at 350 Pa, including louvres, dampers, ductwork to louvres, attenuators
15 – 20
Galvanised steel ductwork to DW 144 tested to BS EN 12237, supply and extract ducts, dampers and attenuators.
30 - 35
Axial flow fan, 500mm diameter 2m3/s duty at 500Pa, 415V, 3 phase with anti vibration mountings; 2 fans
20 - 30
Centrifugal duct mounted in–line fan; duty: 3m3 at 500 Pa Impellor backward curved. 415V, 3 phase; 2 fans
20 - 30
• Costs modelled on a nominal office building of 5 storeys with a GIA of 2,000m2.
• Costs include for maintenance, cleaning, testing, filter replacement, minor repairs component replacements and energy as applicable.
• A discount rate of 3.5% is used to calculate net present values.
• A cost analysis based on project specific information is essential for a realistic best value appraisal.