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HPM October 2012

BOILERS:RENEWABLES WWW.HPMMAG.COM Got a story? Ring us on 01732 748041 or e-mail twood@unity-media.com Supplementary boilers explained In the past there used to be a degree of flexibility regarding heat pump sizing. However, with tighter control on this brought in earlier this year, Nu-Heat’s Chris Weightman, discusses what is meant by supplementary boilers (rather than back-up boilers) in relation to heat pump systems... Tbe at least 100% sized for a property forhe latest revision of the microgenerationstandard MIS3005, which came into forcein February 2012, requires heat pumps to 99% of the heating season. This translates to systems being designed to a target external design temperature of between -0.2°C and -3.9°C, depending where the property is located in the UK. This means that the heat pump should be able to entirely heat the house without electric or other back up under all but the most extreme conditions. Nu-Heat always supplies a heat pump that will achieve the 99% target, unless the size of the heat pump is restricted, in which case a supplementary boiler will be needed. This is a possibility where a larger sized heat pump using three-phase is required but the local DNO either does not offer three-phase or it is prohibitively expensive, for example. A system that incorporates a heat pump and boiler is termed ‘bivalent’, but there are two possible modes of operation. BIVALENT PARALLEL Down to a certain temperature the heat pump will be able to cover the whole load on its own. A ground source heat pump This temperature is known as the bivalent point. If the temperature drops below this in bivalent parallel mode, the heat pump keeps working but output. Just below the bivalent point, the heat The fuel source for the boiler may be mains the boiler will work alongside it. This is the pump will do most of the work and the boiler gas, LPG or oil. Renewable heating systems in mode used by Nu-Heat. will contribute a little (to make up the properties that have access to mains gas and When operating in bivalent alternative mode, difference). choose this for their supplementary boiler will below the bivalent point, the heat pump Therefore, it is important that the boiler isn’t not currently be eligible for the Renewable Heat switches off and the boiler does everything. This any bigger than it needs to be, or it will cycle on Incentive (RHI) or the Renewable Heat Premium is really only likely for systems that require a and off under these conditions, and adversely Payment. higher flow temperature (70°C, for example), in affect energy efficiency. The theory is that the However, mains gas remains the cleanest and a poorly insulated house. boiler will never be required to ‘take over’ the cheapest of the fossil fuels and when used to The heat loss parameter of a property needing heating system. supplement a ground source heat pump will a supplementary boiler should be as good as (or With the bivalent parallel theory, the boiler offer a good carbon saving. Taking into better than) where a heat pump is used on its sizing is as important as the sizing of the heat consideration the implications of the mains gas own, usually around 60W/m². The reason for pump in order to maintain the integrity of the tariff or the electricity tariff, a ground source heat this is that the heat loss parameter is directly system efficiency. However, the choice of boilers pump/mains gas system should offer a cost related to the flow temperature required. The on the market dictates that it may well be 12kW saving over a mains gas boiler being used as the boiler exists to extend the power coverage of the or more, so the model must be a modulating main heat source and may counteract the lack of heat pump, but not the temperature range. version. This means that it does not have to give eligibility for the RHI – although this is reliant Nu-Heat believes that a heat pump should be 100% heat supply but can trickle heat as on the homeowner choosing the right tariff. able to cover at least 50% of the power coverage required. Whichever fuel is used, energy figures should at -3°C (so in a property sized for 24kW, the As with other systems, heat pumps give be provided by the supply company or installer heat pump will cover 12kW or more at -3°C). priority to domestic hot water over hot water for before the system is installed, so the customer This is because the bivalent parallel system works heating. Therefore, if the domestic hot water can see the actual figures. by the blending valve taking water from the heat calls, the heat pump will answer that, and if the pump and boiler and mixing them heating suffers, the boiler will pick up the proportionally to get the right temperature difference. enquiry number 146 62 OCTOBER 2012 HEATING & PLUMBING MONTHLY


HPM October 2012
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