BSEE HEAT PUMPS


Graham Hazell of the Heat Pump Association (HPA) attempts to dispel or clarify some of the ‘urban’ myths that inevitably arise from subjects that are not well known.


1. Heat pumps only work with well insulated buildings


This is probably the most common and longest running myth. Heat pumps work better in well insulated buildings but then ALL heating systems work better in well-insulated buildings in the sense that this means less energy, wherever it is derived from, is expended. However, if a building needs heating to a particular level then it will need a finite amount of energy (kWh) each year to achieve that. Hence the most optimal method to provide that amount of energy will need to be sought. However, there are countless examples of low energy, low running cost systems installed in thermally poor buildings including Grade I listed buildings with no insulation and original leadlight glazing. A badly insulated building may require slightly higher flow temperatures than ideally desirable which reduces the efficiency. Verdict: Not true, but all buildings should reduce heat loss through the fabric and from draughts as reasonably and cost effectively as possible.


2. Heat pumps only work with underfloor heating


Again, this is a widely expounded statement that is the slight misapplication of a correct principle! That principle is that heat pumps work better with as low as possible delivery (load side flow) temperature. Underfloor heating can work effectively at flow temperatures of 35-40oC if correctly designed and the building can be effectively heated with a low temperature source (see above). In general, natural or fan convector heat emitters will need to be quite larger than previously or run at slightly higher temperatures, resulting in small reductions in efficiency. Verdict: Not true, they just work even more efficiently at low flow temperatures.


3. Air sourced heat pumps are noisy


uA pond is unlikely to have sufficient heat capacity to support a water sourced heat pump.


Certainly, heat pumps, like almost all moving objects (owls excepted!) emit some sound, especially machines or motors. To classify this as ‘noise’ means it has to be at least noticeable and possibly a bit annoying and these two aspects can be very subjective. It can also be location dependant. For instance, any sound emitted in quiet suburbia will be perceived quite differently if emitted in an inner city setting. In general noise levels from heat pumps are very, very low, however if placed unsympathetically they could become noticeable or worse, perceived as a nuisance. Good practice should be employed and, where it could be critical, specialist advice taken. MCS has developed a Permitted Development (Planning Permission) noise calculation tool (MCS 020) for checking noise levels at neighbouring buildings which is considered by many specialists to be quite exacting with significant safety factors. Verdict: Not true, when sympathetically selected and located. Heat pumps are quieter now than ever before.


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DISPELLING THOSE HEAT PUMP MYTHS Clarification from the Heat Pump Association


4. Ground Source heat pumps need 20m of flow and return tubing for


every kW of heat required This is probably the most risky myth to believe. The amount of active heat exchange element (pipe in the ground to you and me!) varies considerably, based mostly on the soil conductivity. The conductivity of the same soil can vary by a factor of 4 depending on its density and moisture content, so while this figure might be an average, like any average there will be a considerable variation either side of the mean. At best this could mean your array is twice the size it needed to be at worst it could easily be half the size required! Verdict: Not true. Until a full design is undertaken and an assessment of the soil conductivity is undertaken it is very risky to adopt any such rule of thumb.


5. Air sourced heat pumps aren’t efficient in cold weather


Again, there is an element of truth to this ‘myth’, heat pumps work more efficiently the higher the source temperature and the lower the delivery (load side) temperature required. However, the performance of a heat pump should be assessed over a year. This has given rise to “seasonal” figures whether Seasonal Coefficient of Performance or Seasonal Performance Factor*. These seasonal figures either predict the performance over a typical year for a typical weather pattern or actually measure the performance over a season.


*These terms SCoP & SPF are similar but not necessarily exactly the same. This is further complicated by the fact that some standards (e.g. the EU RED) use them incorrectly or interchangeably. In general, SCoP is the predicted seasonal efficiency of a piece of equipment, whilst SPF is the measured performance of a heat pump system but which can include or exclude various components such as circulation pumps etc. Verdict: Statement is misleading.


6. Air sourced heat pumps are not


suited to the UK climate Taking the comments from myth 5 on board it is certainly true that our damp oceanic climate will require higher than average defrost cycles than a drier environment. However, the oceans around us do also tend to keep the winter temperature higher than more land based countries (e.g. central Europe) and this offsets much of the reduced efficiency from defrost cycles which may not be represented in standard tests from other European countries. Verdict: Not true, in many ways they are well suited.


7. Haven’t field trials proven heat


pumps to be inefficient? This is probably the most inflammatory statement possible to the heat pump industry. Field trials have actually been proven to be very unreliable and inconclusive. Again we can extract some ‘truth’– like “ALL heating systems’ efficiencies measured in the test cell are unlikely to be repeated and likely to be significantly lower than those measured in-use”. Results significantly poorer than laboratory tests have been revealed for oil and gas fired boilers as well as significantly for biomass boilers.


The real difficulty arises in the measurement of accuracy, since it is much easier and likely to be more accurate in the laboratory than out in the field. This is even more so if there is inadequate training and poor installation for the installation of metering as demonstrated by the government backed RHPP field trials.


Field trials have been most inconclusive as to the reasons for the figures calculated but proved they are NOT directly related to the ‘obvious’ things like manufacture/book performance


26 BUILDING SERVICES & ENVIRONMENTAL ENGINEER OCTOBER 2017


uHeat pumps can deliver efficient residential heating.


figures, flow temperature, type of


installation/building etc. Verdict: Most definitely NOT true.


8. If the ground area/volume is insufficient just put more tube in the ground or reduce borehole spacing


Unfortunately, there are some ‘specialists’ who still promote this, and although it might mean a little more energy can be extracted from the ground it is likely to be disproportional to the extra tube required and the cost this will incur. In addition, the ground has a reasonably finite amount of heat that it can store and re-charge and placing excessive quantities of tube will only waste money and risk causing a near perma-frost! Verdict: Not true: Select the correct area/volume of soil with the right amount of tube for the anticipated heat demand.


9. I have a pond in my garden and want to install a water sourced heat pump


Great news if you like garden aquatic life but a pond is unlikely to heat much more than a well insulated dog kennel! Any source needs to have sufficient heat capacity (in this case volume) to deliver the required amount of heat energy until the heat is replenished. A pond 4m in diameter and 0.5m deep is likely to hold just 37 kWh of energy meaning it could supply a heat pump of 8 kW for just over 4.5 hours full load. And this is assuming that the heat is replenished in the pond by the next time heat is required. Verdict: A pond is unlikely to have sufficient heat capacity to support a water sourced heat pump.


RHI update T


he preliminary regulations received parliamentary approval on 31st July 2017. Part of the reforms outlined on the 14th December 2016 in the government response to the consultation of February 2016 were laid before parliament on 30th August and were expected to be passed on 20th September 2017 as we went to press.


They include tariff increases, significant for Domestic ASHP tariff (7.3-10.08 p/kWh) but minor for GSHP (19.44-19.86 p/kWh) and also Heat Demand Limits to limit the payments made to ‘large’ installations as well as new budget caps to prevent premature reduction in tariffs due to potential ‘over’ expenditure.


Yet to be laid before parliament are the more radical changes requiring a ‘positive’ resolution process from the Houses. Hence for example Tariff Guarantees for Non-Dom RHI will not be introduced until late autumn at the earliest. Hence, the scheme remains very much open for business with budget allocated for new registrations up to March 2021 (£1.1 bn) which would secure income for seven years for DRHI and 20 years for Non-Dom RHI as announced at the 2016 Spending Review.


The effect of this announcement is that applications made between between 14 December 2016 and 20th December 2017 will benefit from tariff increases but without the caps imposed by Heat Demand Limits.


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