Pump it up

It is well established that heat pumps offer great potential for reducing energy consumption and carbon emissions, yet many installations fail to achieve maximum benefits. Graham Rodd of MHG Heating considers some of the key factors that building services engineers should consider.

When it comes to designing an energy efficient heating system, building services engineers may not be spoilt for choice but they certainly have a number of options available. Of these options, the most obvious and most commonly used is undoubtedly an efficient condensing boiler – and we can expect this to be the first choice for most projects for years to come.

Nevertheless, it is possible to design the system so that it makes use of low and zero carbon (LZC) technologies in such a way that the boiler’s contribution to the heating and hot water are minimised. Which may sound obvious but there are still many projects where such opportunities aren’t taken. Often this is because the performance expectations of the LZC technologies are relatively low, so that a lengthy payback is predicted and their use is considered non-viable.

I would suggest that optimising the system design to maximise the performance of the LZC element will radically alter the return on investment calculations and increase the use of LZC.

The key is to ensure that the control strategy and hydraulic design support what needs to be achieved. This will vary from one project to another so the purpose of this article is to explore the options that can be applied to suit the specific requirements of the project. To that end the article will consider boiler/heat pump hybrid units and how they can be configured, as well as opportunities for using air source heat pumps (ASHPs) to dramatically reduce the installation costs of ground source heat pumps (GSHPs). It will also discuss how to maintain low return water temperatures when using condensing boilers and heat pumps together.

In our experience there is growing interest in the use of combined boiler/ASHP units but how efficiently these work depends on how they are configured in relation to the bivalence point. At the risk of teaching granny to suck eggs, the bivalence point describes the outdoor temperature down to which the heating load is exclusively covered by the heat pump.

So the bivalence point for an ASHP might be at an ambient temperature of 5°C. Below this temperature the Coefficient of Performance (COP) of the heat pump will be at an unacceptable level if the design water flow temperature is to be maintained. In a typical scenario, therefore, the ASHP will be turned off at this point and the boiler will be used to meet all heating loads.

However, with a good control strategy the ASHP can be configured to work with a variable bivalence point, whereby an acceptable COP can be maintained. Here the ASHP effectively meets part of the load, leaving the boiler to provide the remainder so that the use of fossil fuels is minimised.

This means that if a building requires 30kW of heating it is possible to meet the heating loads with a 10kW ASHP and a 20kW boiler. For optimum control and, therefore efficiency, a modulating boiler with a good turn-down will help to meet variable heating loads.

However, human nature being what it is, we tend to like a bit of peace of mind so it’s far more common to have at least half of the load being covered by the ASHP combined with a boiler sized to meet the entire load. Again a modulating boiler with a good turn-down will help to meet variable heating loads with maximum efficiency.

Stratification

A potential complexity that arises from using heat pumps and condensing boilers together is that if you pre-heat the water with the heat pump it will raise the return temperature of the water and reduce the amount of condensing that can be achieved. The way round this is to use a thermal store as a buffer with very good control of stratification.

In this way, the different temperature layers are maintained and each heat source is used to maximum benefit. Good thermal stores incorporate devices such as perforated plates to prevent turbulence and have multiple temperature sensors for efficient monitoring of the system. The information from the sensors is then used by the control system to optimise performance.

Incidentally, when an ASHP is being used solely for domestic hot water there is no need for a thermal store.

Combining air and ground

Ground source heat pumps have found less favour in the UK than their air source counterparts, mainly because of the cost of the installation – a ballpark figure would be around £1,000/kW for the heat pump and groundworks. However, as mentioned earlier, there are opportunities for combining GSHPs with ASHPs to reduce the groundworks.

With GSHPs the extent of the groundworks is proportional to the heating loads, as when heat is extracted from the ground it requires some time to recuperate. Thus a GSHP on its own may require considerable groundworks to allow for limited recuperation time.

However, if an ASHP is also brought into the mix it will reduce the running time of the GSHP, so that less recuperation time is required and the extent of the groundworks can be reduced accordingly. To put this into perspective, our experience shows that running the ASHP only when ambient temperatures are above the bivalence point will reduce groundworks by around 25%. And if the ASHP is used below the bivalence point to supplement the GSHP, groundworks can be reduced by as much as 50%.

This functionality is achievable with all of our units due to their high level of controllability and there is also a single unit that combines both ASHP and GSHP to deliver these benefits in one package.

Horses for courses

As with so many building services projects, the individual characteristics of the project will favour some solutions and mitigate against others, so the skill of the building services engineer comes in applying the technologies in the best way. For example, retrofitting new heat sources to an existing system means you need to design around the existing distribution system and heat emitters, so that the old flow temperatures may need to be maintained.

In other circumstances, such as with underfloor heating or oversized heat emitters, it may be possible to work with lower flow temperatures, which increases the scope for increasing the COP of a heat pump and the amount of condensing in the boiler.

This is why the control strategy and the hydraulic design are so important to obtaining the maximum performance from a low carbon heating system or, indeed, in ensuring that it is low carbon in practice. Get these aspects right and the end client can be assured of a high performance, low energy system. And it does no harm to work with companies that have specialist expertise and can help to point out potential pitfalls, as well as opportunities for further refinements.

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