Issues and advice

An air conditioning system is essentially a larger version of the domestic refrigerator. Inside the building, cooling is created as air is fanned over a ’chilled’ radiator filled with compressed refrigerant. The refrigerant flows to an outdoor unit where it expands and gets warmer than the outside air. The refrigerant in the outdoor unit is cooled by the air and then chilled further as it is compressed ready to flow back to the indoor unit.

In a heat pump system the process can be reversed to make the outdoor unit cooler than the surrounding air and the indoor unit give heat into the room.

Inside a refrigerator, cooling is created by a series of pipes carrying compressed (chilled) refrigerant. The refrigerant flows to a radiator behind the fridge where it expands. As it expands it gets warmer than the outside air and so is cooled by the air. The refrigerant is then compressed (by a compressor) which chills it further ready to flow back inside the refrigerator.

Initial costs for modern energy efficient split systems for any given installation might be a third or more less than the capital costs of an equivalent VRF system, but in some applications, multiples of split system might be more expensive to install. High performance split systems and VRF systems qualify for Enhanced Capital Allowances (ECA), giving certain tax advantages to the user.

The latest generation of VRF solutions, including the new KX series from Mitsubishi Heavy Industries, offers increased design flexibility, reduced installation costs, and a range of compact lower output systems. It means that VRF can now be the most cost-effective choice for some smaller installations. Add to that the increase in connectable capacity (the diversity maximum has been increased to allow up to 21kW of connectivity to one 14kW outdoor unit) and the KX6 VRF Compact looks like the smart choice for a whole range of smaller installations.

Cost is obviously a major driver, but it is not the only criterion. Local authorities are becoming more restrictive when it comes to the visual impact of outdoor units, particularly those in a roof-top location, and are demanding reductions in both the number and size (especially height) of outdoor units. A larger sized split system installation, with, perhaps, thirty outdoor units, could well run into planning difficulties, making a VRF system a viable alternative.

In the past, very large VRF outdoor units could still fall foul of the planners. The latest generation of VRF systems, however, have radically reduced dimensions. MHI’s KX6 outdoor unit, for example, is only 845mm high, making it a much more flexible and attractive proposition than in the past.

The choice for specifiers, then, is far from clear. However, there is an answer. Talk to the technical team at MHIE or our distributors who have a wealth of experience and a long track record of providing the optimum solution.

In split systems an outdoor condensing unit serves one or more (up to four) indoor units. Simple split systems are used only for cooling the indoor environment. Heat pump splits may be ’reversed’ to provide indoor cooling or heating as required.

With VRF (Variable Refrigerant Flow) systems an outdoor unit serves up to 40 or more indoor units. In simple (2 pipe) heat pump systems all the indoor units are controlled to be in either cooling or heating mode simultaneously. More complex heat recovery systems (3 pipe) allow indoor units to function individually or in zones to serve the local requirement for cooling (eg on the sunny side) or heating (eg on the north side) with minimum energy consumption by the system.

The choice between VRF and split systems is no longer a simple one for specifiers, but help is at hand, says Mitsubishi Heavy Industries’ General Manager, David Lettis.

When specifying an air conditioning system, the decision used to be a fairly easy one to make - splits were for small installations, VRF systems were for larger operations, and chillers/AHUs were the only solution for the largest commercial projects. At either end of the spectrum, this still holds true in many cases. However, it is in the middle zone – medium-sized systems – where system selection becomes a more complex process, as VRF becomes ever more attractive in both smaller and larger projects.

Until recently there was one simple fact in selecting the air conditioning system for small to medium sized projects – splits were cheaper to buy. VRF systems, whilst potentially more energy efficient in operation and energy recovery, were more expensive to install. Splits were simpler and cheaper, and in many applications a series of split systems gave greater design flexibility than earlier generation VRF systems.

In more recent years the balance has changed. The relative cost of VRF systems has come down as a result of production efficiencies and increasing skills and knowledge among specialist installation contractors. Most premium brand split systems now include inverter technology (to optimise energy consumed by compressor operation) which has existed in VRF specifications since their inception in the 80s.

Typically a system which consumes only 10kW of electricity for powering fans and compressors can achieve over 35kW of cooling energy or over 40kW of heating energy in the building. This is generally described as the Coefficient of Performance (CoP). At MHI we claim that our systems offer industry leading standards of energy efficiency, ie some of the highest CoPs achievable, which ensure minimum running costs and reduced environmental impact.

Enhanced Capital Allowance

Many of our products qualify for ECA, which allows the purchaser of the installed Mitsubishi equipment to claim 100% tax relief in the first year on the total cost of the installation, including the design, installation and commissioning costs. Please check the Carbon Trust web site for a list of eligible products at: www.eca.gov.uk

Because air conditioning systems only use energy for powering compressors (to compress refrigerant) and fans (to blow air over chilled or heated pipe coils), rather than for electrically heating the refrigerant, they can achieve extraordinary levels of efficiency.

Other factors

The specifier should consider unit size at the design stage. Even with larger VRF systems most MHI outdoor units are designed to occupy minimum footprint and even to fit into a standard lift for transporting on to the roof. The specifier should also consider noise levels of both indoor and outdoor units. MHI outdoor and indoor units incorporate several design details to ensure minimum noise levels in operation.

Design flexibility

Once the appropriate location for outdoor units (eg outdoor wall or roof) is decided the specifier must consider the design of pipe runs. MHI VRF systems allow maximum design flexibility in terms of height difference between units and maximum pipe length – over 500m.

Fresh air ventilation

The inclusion of the SAF energy recovery ventilation units in the building design will reduce the total amount of carbon emissions and will make a significant contribution to compliance with Building Regulations Part L2.

Controls

While air conditioning systems continue to grow in capability and sophistication, getting the best out of the system by effective controls has been a bit of a black art. Mitsubishi Heavy Industries Air Conditioning division believes that there is an optimum solution for each of the three system categories: standalone aircon control, web gateway control, and full BMS solutions. See the controls section.

Availability, acceptability and reliability

MHI is an established market leader; product is readily available through a network of distributors; installers know that MHI kit is installer friendly; and users report high levels of satisfaction with system reliability.

The specifier for any air conditioning solution should consider a series of issues in a logical sequence:

Cooling only or heat pump system

As the term suggests a simple cooling only split system can be used to provide indoor cooling. A heat pump system can be ’reversed’ to provide indoor heating and cooling as required.

Heat recovery system

A simple (2-pipe) heat pump Variable Refrigerant Flow (VRF) system supplies either cooling energy or heating energy at different flow rates to each indoor unit according to need. In a heat recovery (3-pipe) system inter-connecting pipe work allows simultaneous cooling of warm areas and heating of cool areas within the building with minimum energy demand at the outdoor compressor unit.

Right mix of units

At Mitsubishi Heavy Industries we have probably the widest range of systems available to allow specification of the right type and size of system. This is especially the case in larger and more complex projects which may require, for example, VFR heat recovery systems for major open plan areas and specific split systems for computer rooms.

Energy efficiency

At Mitsubishi Heavy Industries we claim to have industry leading energy efficiency across our entire portfolio of products.

As well as systems guidance they can also assist with site surveys and preparing detailed drawings; for each project our distributors can produce full system drawings, wiring schematics and pipe work drawings.

MHI also offers monitoring and service software (Mente PC) to allow on-site system interrogation via a PC laptop or PVA palmtop simply connected to a port on the outdoor unit PCB.

Our network of experienced distributors is second to none in the advice and service they provide to contractors and installers. As a result, customers can be sure that we can deliver the right system and the best advice to get the optimum solution for any project.