How to ensure heat pumps will meet hot water demand

As heat pumps become part of the mainstream, how can you ensure that they deliver on efficiency levels while meeting your customer’s expectations for hot water delivery? Isaac Occhipinti from the Hot Water Association explains.

The length of time householders are prepared to wait for hot water following a large draw-off from a cylinder (e.g. filling a bath) is now far shorter than it used to be. This has an impact on the recommended domestic hot water (DHW) cylinder volume, particularly when the cylinder is being reheated by a relatively low power heating device such as a heat pump. 

The overall system efficiency of a central heating system is influenced by the hot water cylinder. For heat pumps, this is a particularly important issue because they can be less efficient when operating at the temperatures required to produce DHW.  For this reason, it is particularly important to investigate how hot water cylinders work under different operational regimes, and how these heat losses may be minimised. 

Growing market
Over the last number of years heat pumps have continued to increase their market penetration, to provide space and water heating to dwellings and commercial properties of various sizes and applications.

With the focus on discontinuing the use of fossil fuels in the UK, the future of the heat pump market represents an exciting opportunity for the hot water industry. The general consensus is that technology, the industry and the installer base have reached reliable levels of technology deployment. The UK is clearly on a trajectory where heat pumps will be part of the heat generation mix of the future.

Heat pumps use energy contained in generally low temperature ambient sources such as the ground, the air or water, and transfer it to useful temperature levels for space and water heating. The heat distribution system can be based on water or air for heat transfer purposes. 

A wide variety of heat pump models, types and operation modes are available, but for the purpose of the considerations in this article only system heat pumps using the ground and the air as energy source with water as heat transfer medium are being considered.

 Within legislation, standards and certification schemes a lot of effort has been made to ensure the deployment of heat pumps for space heating applications is completed correctly and efficiently, because in the majority of the existing housing stock, space heating requires considerably more energy than water heating. However, with ever tightening Building Regulations with regards to the conservation of fuel and power, the water heating energy demand becomes a bigger proportion, and due consideration should be given to the effectiveness of the provision of hot water using heat pump technology.

Varied output
Both ground and air source heat pumps vary their output depending on the source temperature. The issues arising from this output variation can, in space heating applications, be mitigated against by the introduction of a suitably sized buffer vessel and by the fact that high ambient temperatures may revoke the need for space heating.

In stored hot water systems, however, the output rating of the heat pump does affect the time in which attainable hot water temperatures in the cylinder are achieved and therefore affects directly the comfort level of the user or running costs should other heat sources, such as direct acting immersion elements, be used to top up the hot water to comfortable temperatures. 

In particular, the output temperature of air source heat pumps and their efficiency can vary considerably with source temperature. In most cases, the units will have been specified for the heating design temperature of the dwelling, i.e. -2°C, which means that at low external ambient temperatures the heat pump has more workload placed upon it and the ability to heat both the house and the hot water cylinder simultaneously is testing. 

Contrary to fossil fuel fired systems, from a thermodynamic point of view, heat pumps do not work independently of the load connected. The operation parameters of the refrigeration circuit internal to the heat pump are affected by the external load connected to the condenser of the heat pump. It was therefore correctly recognised in SAP that the heat exchangers used in hot water cylinders have to be designed accordingly to ensure correct system operation. They also have to be placed in suitable hot water volumes, with the temperature control and safety devices positioned correctly.

Inverter driven
Whereas the correlation between heat pump output and cylinder heat exchanger requirement is reasonably straightforward for fixed speed compressor heat pumps, they have become much less transparent for inverter driven heat pumps. Depending on the control algorithm developed, the control strategy of providing hot water can vary considerably.

Inverter driven heat pumps can, similar to other heating appliances, modulate the output from the heat pump with the load on the system. For DHW preparation, this means that the closer the temperature gets to the hot water temperature set point, the lower the output from the heat pump. This allows the heat pump to ‘edge’ its way to the set point without tripping out and the hot water temperature set point can be within a few degrees of the maximum heat pump flow temperature, thus eliminating the need for the use of auxiliary heating systems to top up the hot water temperatures.

Meeting expectations
Although heat pump technology has been ready to go mainstream for some time, the above considerations show that certain knowledge is required to ensure that the systems operate efficiently. It also important that end user expectations are being met both in respect to comfort and running costs, and greenhouse gas emissions reductions are actually realised. 

Compatibility of products is of utmost importance when designing any DHW system. One way of achieving overall system compatibility is to utilise complete system packages. In these cases individual component characteristics are fully understood and combined in the most efficient and all round beneficial way.

As ever, HWA members will be pleased to offer advice on best practice. We are at present working on a HWA heat pump cylinder specification guide, to aid installers when specifying cylinders.

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