Durability - Offices - mechanical ventilation
In
many instances mechanical ventilation may be the only option to manage
office environments. Peter Mayer of Building LifePlans
explores the options
Introduction
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:
Filter systems
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.
Heat recovery
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.
Ductwork
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.
Maintenance
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.
Specification options
| Capital cost £/m² |
Net present value for 60 years£/m² | Service life Years |
|
| Ventilation components | |||
| Low velocity air handling unit with heating coils filters and fans, internal location 2m3 at 350 Pa, including louvres, dampers, ductwork to louvres, attenuators | 9,000 | 35,950 | 15 – 20 |
| Galvanised steel ductwork to DW 144 tested to BS EN 12237, supply and extract ducts, dampers and attenuators. | 7,500 | 26,110 | 30 - 35 |
| Fans | |||
| Axial flow fan, 500mm diameter 2m3/s duty at 500Pa, 415V, 3 phase with anti vibration mountings; 2 fans | 2,000 | 15,840 | 20 - 30 |
| Centrifugal duct mounted in–line fan; duty: 3m3 at 500 Pa Impellor backward curved. 415V, 3 phase; 2 fans | 4,000 | 19,980 | 20 - 30 |
Table notes
• 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.
First published in Building 2007
Further information
BLP provides latent defect warranties for buildings www.blpinsurance.com
Further information contact peter.mayer@blpinsurance.com
or telephone: 020 7204 2450.
