Setting the scene
MEP services are foundational to modern buildings ensuring functionality, safety, and efficiency. Historically rooted in the industrial revolution’s technological advancements, MEP has evolved to integrate complex HVAC, electrical, and plumbing systems. Dubbed the “dark art” by many, MEP services can easily account for around 15% to 30% or more of a project’s value and are generally considered one of the most complex parts of any building project.
Author: Kevin Edge, Technical Director, Coventry, UK
The discipline may often be viewed as ancillary to architectural design. For example, if you consider the MEP requirements of a typical commercial project, such as an office development, when under pressure to provide maximum net lettable or saleable floor area, architects and clients are likely to view this valuable commodity as one of the driving factors behind form, in order to maximise their return on investment. In taking this stance, it can often reduce the available space for MEP plant, risers, and distribution.
It is sometimes viewed as an easy win for architects in the space battle but can result in adversarial debate among designers and contractors as it adds constraints to the functionality, to the point where the MEP design can become compromised. Once this happens, the skill and ingenuity of MEP design engineers is tested due to pressures to incorporate all necessary plant and the associated distribution systems into spaces smaller than they intrinsically need.
Fan Coil Units (FCUs)
FCUs provide localised heating and cooling solutions to enhance comfort, improve energy efficiency, and offer flexibility in design and application. They are an integral part of MEP systems in certain types of buildings.
Consider an office development as an example, where a base build or CAT A[1] design is undertaken. The MEP design for each floor is sized to accommodate a speculative occupancy profile typically of one person per 10m², or one person per 8m² if high density occupation is required, usually in the United Kingdom based on the British Standard for Offices (BCO) specification which sets benchmark criteria for design.
Under a CAT A fit out, the building is usually equipped with a primary mechanical ventilation system which provides preconditioned outside air for the speculative occupancy level at a minimum 12l/s/person according to the BCO, with an allowance of 10% spare capacity to each floor, which would more than satisfy compliance with Building Regulation requirements that typically require 10l/s/person.
The speculative heating and cooling loads are met by either a variable refrigerant flow (VRF) type comfort heating and cooling system, which incorporates a multitude of indoor FCUs linked via refrigeration pipework to externally mounted condensing units, or more often than not, office buildings utilise 4-pipe FCUs in lieu of VRF based systems. This requires central plant in the form of boilers for low temperature hot water which may also incorporate Combined Heat and Power (CHP) plant and chillers, which provide chilled water.
4-pipe FCUs come in various configurations and designs based on them being either water side or air side units. Water side FCUs are common in the United Kingdom and work by controlling their heating and cooling output by adjusting the water side flowrates passing through the heat exchange coils. Air side units work by adjusting the flowrate of air passing over the heat exchange coils.
FCUs usually reside within the ceiling void spaces to ensure floor space is kept clear. FCUs are effectively metal boxes which incorporate filters, water-based heat exchange coils and an internal forward faced centrifugal fan, with associated inbuilt control connectivity.
A typical FCU strategy is shown in the diagram below:
FCUs are usually sized based on cooling demand for commercial developments. The demand is derived from the calculation of heat gains from speculative occupancy, lighting, small power, IT equipment, and solar gains. FCUs can also provide heating where needed. The heat exchange coils are connected to both the low temperature hot water (LTHW) and chilled water distribution network.
FCUs work on the recirculation of air by drawing in air from the selected space (return air intake) to be either heated or cooled via the heat exchange coils. After being filtered, heated or cooled, the air is discharged back into the selected space as secondary supply air to control the temperature.
Primary ventilation is discharged into the ceiling void in close proximity to the return air intake of FCUs, which allows mixing with the return air. The temperature of this return air is sensed by the FCU controls and used to determine if there is a heating or cooling requirement, which in turn controls the water side control valves and hence flowrates. This is why the primary ventilation flowrate taken to FCUs is limited to circa 10% to 15% of the FCU air handling capability.
The supply air discharged from the FCU is distributed by secondary ductwork directly connected to the FCU. The internal fan within the FCU usually has a limited external pressure capability of circa 30Pa set by the characteristics of the fan, meaning that any secondary ductwork connected has to be sized to accommodate the required discharge velocity, usually a maximum of 3m/s, advised by the FCU manufacturer, against 30Pa.
Now come the headaches
There are various issues that can be experienced when using 4-pipe FCUs for both air side and water side units. If the secondary ductwork is sized too small, the air velocity at the FCU discharge flowrate will exceed 3m/s within the ductwork with a corresponding increase in pressure drop, resulting in the FCU not achieving the air handling requirements needed to provide the required cooling duty for the space.
Similar issues can arise where excessive lengths of flexible ductwork are used between the FCU connections and ceiling mounted supply air terminals, which is a common problem due to contractors seeking cheap installation methods and materials. It is standard practice to utilise flexible ductwork to some degree; however, there is a UK specification called DW/144 “Specification for Sheet Metal Ductwork” which suggests that lengths of flexible ducts be limited to six times the duct diameter. Also, the Chartered Institution of Building Services Engineers (CIBSE) Guide C recommends that where flexible ductwork is used, it should be kept as short as possible and be almost fully extended, suggesting that if it is installed at circa 70% of the extended length then the pressure drop can be greater by a factor of four.
When considering the secondary ductwork design for FCUs, it is essential to ensure that the fan speed setting and corresponding air volume flowrate, to achieve the cooling duty required, are known and that ductwork is sized to ensure air velocities remain below 3m/s at this design air volume. Flexible ductwork lengths should be limited and installed fully extended so that when the pressure drop associated with the supply diffuser is considered, it remains within the limitations of the FCU fan.
These considerations clearly impact spatial planning, in particular ceiling void depths and consequently the height of the false ceilings when coordinating with the primary ventilation ductwork layout, positioning of all required FCUs and associated secondary ductwork along with pipework, primary electrical containment, and ceiling mounted equipment such as lighting, grilles and diffusers and other ceiling mounted fittings.
In some instances, the effect of these coordination issues is negated by the use of long lengths of flexible ductwork connected to FCU connections, which are sometimes left strewn across ceilings with inadequate support, being potentially squashed at pinch points before their final connection to a grille box. The result of this scenario is that the pressure drop far exceeds 30Pa, resulting in reduced air flow and the cooling requirement not being achievable, ultimately leaving the FCUs to suffer premature fan deck failures and the occupied space to overheat.
CAT A versus CAT B
If a CAT B[2] tenant’s fit out is factored into the equation, then a whole number of other problems can arise if the process is not managed. It is not uncommon for a landlord to provide a lease agreement to prospective tenants which advises its allowances for primary ventilation, chilled water, and LTHW, derived from the CAT A design for the floor they want to lease. These allowances may include a percentage margin over the commissioned state of the CAT A speculative floors which gives tenants some additional capacity for their CAT B fit out without causing issues on other floors. Furthermore, it is not uncommon for the lease agreement to describe how tenants are to accommodate any additional requirements they may have for ventilation, heating, or cooling above the speculative CAT A design, but this is not always the case. In some instances, leases can be silent on this aspect.
Where the lease agreement is silent on how the prospective tenant accommodates its CAT B requirements, a scenario may play out where a tenant installs additional FCUs, relocates others to suit new office layouts and then attempts to rebalance its floor to take higher flowrates than the CAT A allowed. This effectively leaves other floors starved of the primary ventilation, chilled and low temperature hot water commodities they should have. This leaves them in an unbalanced state incapable of accommodating the CAT A design. It is therefore paramount that the CAT A design specifications make spatial allowance for any additional plant, equipment, and distribution that any prospective CAT B tenant may require and that lease agreements provide a contractual basis and specification for how a prospective tenant is to accommodate the potential for higher capacity requirements.
Conclusion
MEP services are integral to efficient, adaptable, and comfortable building environments. Systems such as FCUs play a central role in temperature control and air distribution, especially in both CAT A and CAT B fit outs, where specific customisation levels vary. This highlights MEP’s critical contributions to modern building performance, energy efficiency, and occupant comfort, and the importance of providing adequate spatial provision for MEP installations.
This article was originally written for issue 29 of the Diales Digest. You can view the publication here: https://www.diales.com/diales-digest-issue-29
1. CAT A Fit Out – MEP fit out of an office space based on speculative occupancy and heating and cooling loads.
2. CAT B Fit Out – MEP fit out that updates the CAT A fit out to suit the tenant’s requirements for increased occupancy and heating and cooling loads.