Elements of a VRF system from a consultant’s perspective

By Craig Blankers of WSP

Why is going the variable refrigerant flow/volume (VRF/V) route the right option, and what should consulting engineers look out for when designing and commissioning these systems?

WHY VRF?

This question is becoming more common among clients when requesting feasibility studies on projects, as well as budgets for the HVAC installations. The most common response from clients when deciding on which system to install, is what avenue will have the least capital cost for a central HVAC system. A VRF system’s capital cost is typically less than a chilled water installation and with the current economy, clients are willing to overlook the fact that generally a chilled water system has a longer life expectancy.

There are various elements to consider when deciding on what air-conditioning system to design for your building; some examples are as follows (in no particular order):

  1. System efficiency – Electrical consumption versus how much cooling and heating the system can provide.
  2. Building application – What the intentional use of the building will be (for example office block, retail, industrial, or hotel).
  3. Capital cost versus payback period – How much the client will end up paying for the air-conditioning system today and how long it will take them to recover the costs versus resultant energy savings.
  4. System longevity – The serviceable life expectancy of the system before product quality and system performance deterioration.
  5. System flexibility – Will the system be able to cope with any alterations, variations, and possible additions to the initial buildings application?
  6. Location – How far are the nearest subcontractors located to the site and what are their capabilities?
  7. Plant room space available – Are there space restrictions from the architect and is the HVAC plant to be constricted to a specific area?

When referring to the system’s installation, chilled water systems generally are more demanding than a VRF system in terms of installation complexity. Site technicians require different training regimes to qualify for working on the systems. Therefore, it is of the utmost importance that continuous training of staff members is encouraged across the board. 

VRF001 WebA VRF system’s capital cost is typically less than a chilled water installation.
Iamge credit: SmartFlex

IMPORTANT ELEMENTS TO CONSIDER

What should you consider when designing a VRF system? Here are some tips learnt from practical experience …

Positioning of BS/BC boxes: One of the most attractive elements regarding a VRF system is the heat recovery application. When referring to the design of a VRF system, without going into too much technical detail as to how the process works, I believe a common, yet crucial oversight is the location of the branch selector/controller (BS/BC) box. It is important to connect various indoor units with different load requirements to the same BS/BC box (supplier dependent).

For example, an indoor unit supplying a north-eastern-facing corner office will most likely require cooling at 10:00 in the morning (depending on the façade composition), when at the same time, a south-western-facing cellular office could require heating. The BS/BC box can recover the heat from the north-eastern office and provide heating to the south-western office with minimal load required from the outdoor unit; this is known as simultaneous heating and cooling.


The challenges are to find the best suited system for the client’s building and needs, and to determine the sweet spot between system complexity and innovative capabilities with user-friendly interfaces.


With the most effective BS/BC box port allocation and the application of the building, this could further increase the diversity factor of the outdoor unit selection. As a result of increasing the diversity factor of the outdoor units, smaller outdoor units are required, which in turn will result in a lower capital cost of the system. With the effective location of the BS/BC boxes and the reduction in outdoor unit capacity, this will ensure that the outdoor units function at a desired rate and not just idle on minimal performance.

De-rating units for altitude: There are various heat load calculation programmes that consulting engineers, contractors, and suppliers use for the design of VRF systems. Many of these programmes consist of multiple integrated psychometric charts based on the location of the site and adjust for altitude in the project set-up phase. With the design de-rated cooling and heating capacity, it is imperative that the indoor conditions used at design phase correspond to the indoor unit’s selection, as many indoor units’ data catalogues reflect values based on an indoor condition of 27ºC. Depending on the sensible to total cooling load ratio, this could have a substantial impact on the performance of the unit versus the design requirement, as the indoor units could be slightly undersized. 

The risk of the convenient cable ties: Cable ties are very popular on refrigeration piping support and are convenient when installing. To eliminate heat transfer from the refrigeration pipe to the surrounding environment, the material type and thickness of the insulation will dictate the extent of the transfer. With many suppliers’ installation manuals specifying exactly what materials are to be used for the insulation, the use of an over-tightened cable tie will compromise the insulation properties of R3.

The use of anThe use of an over-tightened cable tie will compromise the insulation properties of R3.
Image credit: WSP

With the width of the average cable ties only being a few millimetres, when over tightened, the cable tie could cut into the insulation. Not only will this have an impact on the system’s performance as mentioned above, but you could run the risk of condensation forming, which would naturally lead to multiple complications.

An alternative for using cables ties is using Velcro straps. Velcro straps are much wider and the risk of damaging the insulation is less (it is still important not to over tighten). Due to the thicker strap, the force exerted on the insulation is more evenly distributed than that of the more concentrated cable tie.Insulating the top of the galvanised trunking cover: When refrigeration piping is concealed in galvanised trunking that is directly exposed to the sun, the surface temperature of that trunking will reach undesirable temperatures. Insulating the top layer of the galvanised trunking cover incorporated with an air gap between the top of the insulated trunking and the refrigeration piping, will not only aid in the protection of the piping, but also eliminate unnecessary heat transfer to or from the refrigeration piping.

insulating the top layer MMInsulating the top layer of the galvanised trunking cover incorporated with an air gap between the top of the insulated trunking and the refrigeration piping, will not only aid in the protection of the piping, but also eliminate unnecessary heat transfer to or from the refrigeration piping.
Image credit: WSP

Product supplier’s sign off: Many HVAC subcontractors are approved installers of various VRF systems. It is recommended that the manufacturer of the product goes to site, inspects the work, and approves the installation before the system is switched on. Insisting on the manufacturer inspecting the installation will not only improve the quality and the longevity of the installation, but is also imperative for the warranty of the system that the manufacturer provides to the customer.

Over and above the quality inspection from the manufacturer, it is important that the manufacturer commissions the system to ensure that pressures are sustained, the system is vacuumed and charged correctly, the power is stable, and the system is functioning optimally. This, however, does not take away the responsibility of the HVAC contractor commissioning the system; instead, it facilitates a lower installation risk advantageous to both parties, and most manufacturers offer this service free of charge.


It is recommended that the manufacturer of the product goes to site, inspects the work, and approves the installation before the system is switched on.


Centralised controller and its benefits: The additional cost of installing a centralised controller is easily justifiable when looking at the added benefits you get from this function. Having the entire VRF system monitored and controlled from a central point allows clients and facility managers to monitor the individual performance of the units at any given moment, adjust set point temperatures, monitor the system’s performance holistically, obtain an indication of electrical costing for multitenant billing, as well as assist with troubleshooting when there is an issue on site, to name a few. The resolution of disputes in an office space, for example, can easily be resolved by using the centralised controller to lock set point temperatures to various spaces and it can monitor space temperatures for validating these disputes.

The centralised controller gives a holistic view of the air-conditioning system and, depending on the manufacturer, can incorporate other disciplines such as electrical, wet services, and fire. The centralised controller can further be easily connected to a building management system (BMS). However, it is recommended that the centralised controller is used for the non-critical day-to-day operations, and critical operations (that is, fire systems) are controlled separately.

THE TIME IS NOW

Should a VRF system be applicable to your building’s application and a suitable air-conditioning system, there are some really exciting and innovative products on the market.

From switching your air-conditioning system on and off from a hardwired controller, to adjusting the comfort level from your desktop or smartphone, the possibilities are endless. With the technological advances in the market and the innovative products available, the time is now for everyone to get on board with the futuristic monitoring and controlling of the systems.

It is often the case that a high-level multiple integrative system is installed in the building, but it is not user-friendly. A common ground is to be established between the two, as there is absolutely no point in having a complex control system but when the subcontractor walks off site, no one can operate the system and unleash its full potential. Training and handing over the system to the client or facilities management is essential and may be required more than once.  

This is an exciting time in the air-conditioning industry, as innovation and technological advances are growing at an exponential rate. The challenges are to find the best suited system for the client’s building and needs, and to determine the sweet spot between system complexity and innovative capabilities with user-friendly interfaces.


Click below to read the February 2018 issue of RACA Journal

RACA FEB2018

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