Heat pump AHUs: creating the right atmosphere


14 June 2023
The 3DxHP Series fully packaged heat pump AHU range is ideal for displacement ventilation and other air conditioning applications

Combining temperature control with improved air quality, heat pump air handling units boast high efficiencies and year-round comfort and are particularly well-suited to high-demand applications such as healthcare establishments or data centres. Tim Mitchell, Sales Director of Klima-Therm, explains.

Fresh air has always been crucial in occupied buildings both for health and wellbeing and to maximise productivity, but the recent COVID19 pandemic highlighted its need in the most powerful way possible.

As the UK government has pointed out: “The amount of respiratory virus in the air can build up in poorly ventilated areas. This increases the risk of spreading COVID19 and other respiratory infections… Bringing fresh air into a room… helps remove older stale air that could contain virus particles and reduces the chance of spreading infections.”

So, there is the need for some sort of air handling system to ensure the right fresh air ratio in commercial properties. Normally, fresh air is supplied using a heat recovery system or an air handling unit (AHU) system carefully designed in terms of filtration, humidity, and recovery for the building requirements.

A heat recovery system – also sometimes called mechanical ventilation with heat recovery (MVHR) – employs a heat recovery ventilation unit usually located in the roof space or plant room of a building.

Rather than simply extracting air from an internal space and replacing it with outside air, a heat recovery system is designed to draw the heat from the extracted air and passes it to the air filtered in from outside.

Increased flexibility

Extract and supply air flow within separate pipes so there is no cross contamination of the different air flows. The heat recovery unit will generally be linked to room air valves through a network of ducting that runs throughout the building.
A typical standard AHU has two coils, one for cooling and the other for heating. Water is provided to the coils through inline pumps from the chiller for cooling and from the boiler for heating. This traditional system is generally very stable but has a high installation cost compared to a fully packaged solution. 

The AHU is generally installed in the basement, roof, or plantroom, from where the processed fresh air is fed to the building interior.

However, along with fresh air, a source of cooling and heating will be required, often via a variable refrigerant flow (VRF) or heat pump system – thus, the heat pump AHU.

As chiller technology has improved over the past few years – in particular, the development of variable speed fans on the air handing units together with variable speed compressors (inverter driven) on the chiller side – air volumes can be varied, which gives the client a lot more flexibility, energy savings and, therefore, better overall system efficiency.

Heat pump technology is, however, far more cost effective in terms of initial capital cost and installation cost, often using reversible VRF condensers for a seasonal change-over. The simplest solution for direct expansion (DX) heat pump operation in an AHU is with a reversible DX coil inside the AHU.

This technology requires a single coil providing cooling and heating (not simultaneously, but with a seasonal changeover). The AHU coils is called a reversible DX coil. This option is very simple and compared with the traditional system. 

It includes an outdoor unit so space in the plantroom can be significantly reduced since there is no boiler or water-cooled chiller. On top of this, having a single coil, the length of the air handling unit can be also reduced making the overall system smaller compared with a traditional one.

Any refrigeration system will need to absorb energy from a heat source and dissipate that heat to a heat sink. In winter, the refrigeration system absorbs heat energy from the outside air (which could already be close to or below freezing point) or exhaust air from a building.

When the outside air is close to freezing (or heat has already been recovered via a heat recovery device such as a heat wheel or plate heat exchanger) then the refrigerant must be evaporated at a temperature that is low enough to cause ice to form on the coil surface. This ice builds up and compromises the operation of the evaporator, which reduces the amount of heating we can provide to the internal space of our building.

If left unchecked, the refrigeration system performance will degrade towards zero output and the refrigeration safety switch for low pressure will stop the compressor operation. 

To combat this, a defrost cycle is included. This reverses the operation of the refrigeration system so the evaporator becomes the condenser, and the condenser becomes the evaporator, which melts the ice on the outdoor coil.
In winter this means the coil in the supply deck actually cools the air leaving the heat recovery device and provides cold air into the conditioned space.

This happens with both an external condensing unit (whether VRF or multi-stage type) and with a unit with built-in refrigeration system. It means the poor building occupants get cold air blown on them when they least want it, that is, in the depths of winter.

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There is, however, a solution that effectively processes fresh air without expensive external thermal production: the 3DxHP Series AHU, which has a unique built-in refrigeration system.

The built-in refrigeration system minimises the refrigerant content, eliminating pipe runs through the building and dramatically reducing the refrigerant content in comparison to variable refrigerant flow (VRF) systems.

The 3DxHP series eliminates the defrost cycle, thus ensuring there is no interruption of thermal output or sudden increase in energy consumption

Innovative design

Heat pump AHUs’ weak point has always been the threat of loss of performance during defrost cycles, just when heat is really needed. The 3DxHP series AHU overcomes this by eliminating the defrost cycle altogether, thus ensuring there is no interruption of thermal output or sudden increase in energy consumption during a defrost cycle. 

The device’s innovative design uses the heat pump’s own energy to prevent any need for a defrost cycle. The system uses less energy than a traditional system and provides the guarantee of useful heat when it is needed most. It also helps to eliminate cold spots.

Moreover, installation costs can be significantly reduced as there is no external heating source such as boilers or external heat pumps or cooling source like chillers that must connect to the AHU, because the refrigeration system is integral to it.

Without these additional thermal plant devices there is more usable area in the building, to be used for lettable space or for the installation of other desirable attributes such as rooftop gardens and spaces for occupants to enjoy rather than avoid.

A fully packaged AHU with built-in heat pump refrigeration system that allows more air to be circulated in applications such as offices without adding more central plant, the 3DxHP’s applications for this technology include any buildings that require conditioned air such as commercial buildings as well as schools, colleges, and universities, leisure centres, and healthcare facilities.

IIt incorporates a variable frequency drive scroll compressor for enhanced part-load performance. And, the fact that the unit is factory assembled and tested ensures the quality is consistently high, unlike some site-based system assemblies.


Packaged heat pump AHUs with zero defrost

The 3DxHP Series fully packaged heat pump AHU range from 0.5 to 24 cu m/s is ideal for displacement ventilation and other air conditioning applications. The zero defrost system ensures an uninterrupted supply air temperature in winter, with no need for a defrost cycle.

It can be adapted to most applications, particularly where space is a premium – only ductwork, electrics and an optional BMS connection are required.

An innovative three-coil design allows air to be returned effectively during winter heat pump operation and incorporates heat recovery that is active all year round. A heat rejection coil located after the heat recovery device provides some heat dissipation without affecting heat recovery.

The unit is ideal for displacement ventilation applications as it is capable of providing a constant summer/winter supply air temperature (+/- 1K tolerance) due to the three-coil design and a variable speed drive scroll compressor.