072 HPM 1012

HPM October 2012

BOILERS:RENEWABLES WWW.HPMMAG.COM Got a story? Ring us on 01732 748041 or e-mail twood@unity-media.com Getting ready to pump it up Heat pumps offer great potential for reducing energy consumption and carbon emissions but many installations fail to achieve maximum benefits. Graham Rodd, of MHG Heating, considers some of the key factors... Wa number of options to choosehen it comes to helping buildingoperators reduce their energyconsumption, heating installers have from. The most obvious and most commonly used of these is an efficient condensing boiler and this is set to be the first choice for most installations for years to come. However, there are other things that can be done in parallel to make use of low and zero carbon technologies in such a way that the boiler’s contribution to the heating and hot water are minimised. The key is to establish the optimum control strategy and hydraulic design in support of what needs to be achieved. This will vary from one project to another so A typical heat the purpose of this article is to explore the pump set-up options that can be applied to suit the specific requirements of the project. To that end, the tend to like a bit of peace of mind so it’s far limited recuperation time. article will consider boiler/heat pump hybrid more common to have at least half of the load However, if an ASHP is also brought into the units and how they can be configured, as well as being covered by the ASHP combined with a mix it will reduce the running time of the GSHP, opportunities for using air source heat pumps boiler sized to meet the entire load. Again a so that less recuperation time is required and the (ASHPs) to dramatically reduce the installation modulating boiler with a good turn-down will extent of the groundworks can be reduced costs of ground source heat pumps (GSHPs). help to meet variable heating loads with accordingly. To put this into perspective, our maximum efficiency. experience shows that running the ASHP only OUTDOOR TEMPERATURE A potential complexity that arises from using when ambient temperatures are above the In our experience, there is growing interest in the heat pumps and condensing boilers together is bivalence point will reduce groundworks by use of combined boiler/ASHP units but how that if you pre-heat the water with the heat around 25%. And if the ASHP is used below the these are configured in relation to the bivalence pump it will raise the return temperature of the bivalence point to supplement the GSHP, point. At the risk of teaching granny to suck eggs, water and reduce the amount of condensing that groundworks can be reduced by as much as 50%. the bivalence point describes the outdoor can be achieved. The way round this is to use a This functionality is achievable with all our temperature down to which the heating load is thermal store as a buffer with very good control units due to their high level of controllability exclusively covered by the heat pump. of stratification. In this way, the different and there is also a single unit that combines So the bivalence point for an ASHP might be temperature layers are maintained and each heat both ASHP and GSHP to deliver these benefits in at an ambient temperature of 5°C. Below this source is used to maximum benefit. Good one package. temperature, the Coefficient of Performance thermal stores incorporate devices such as Underlying these examples is the key principle (COP) of the heat pump will be at an perforated plates to prevent turbulence and have that each project will have different unacceptable level if the design water flow multiple temperature sensors for efficient requirements, so the skill of the heating engineer temperature is to be maintained. In a typical monitoring of the system. The information from comes in applying the technologies in the best scenario, therefore, the ASHP will be turned off the sensors is then used by the control system to way. For example, retrofitting new heat sources to at this point and the boiler will be used to meet optimise performance. Incidentally, when an an existing system means you need to design all heating loads. ASHP is being used solely for domestic hot water around the existing distribution system and heat However, with a good control strategy the there is no need for a thermal store. emitters, so that the old flow temperatures may ASHP can be configured to work with a variable Ground source heat pumps have found less need to be maintained. bivalence point, whereby an acceptable COP can favour in the UK than their air source In other circumstances, such as with be maintained. Here the ASHP effectively meets counterparts, mainly because of the cost of the underfloor heating or oversized heat emitters, it part of the load leaving the boiler to provide the installation – a ballpark figure would be around may be possible to work with lower flow remainder so that the use of fossil fuels is £1,000/kW for the heat pump and groundworks. temperatures, which increases the scope for minimised. However, as mentioned earlier, there are increasing the COP of a heat pump and the This means that if a building requires 30kW of opportunities for combining GSHPs with ASHPs amount of condensing in the boiler. heating it is possible to meet the heating loads to reduce the groundworks. This is why the control strategy and the with a 10kW ASHP and a 20kW boiler. For With GSHPs the extent of the groundworks is hydraulic design are so important. Get those optimum control and, therefore efficiency, a proportional to the heating loads, as when heat right and the end client will end up with a high modulating boiler with a good turn-down will is extracted from the ground it requires some performance, low energy system. And it does no help to meet variable heating loads. time to recuperate. Thus a GSHP on its own may harm to work with companies that have However, human nature being what it is, we require considerable groundworks to allow for specialist expertise to ensure you do get it right. 72 OCTOBER 2012 HEATING & PLUMBING MONTHLY enquiry number 152


HPM October 2012
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