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Put simply, the lower your design working temperature, the better the COP. If you are in any doubt about the suitability of the heating system, stop and seek the advice of a qualified heating engineer or experienced system designer. 6 Calculating Radiator Sizes CalculatingRadiator 15 Most existing wet heating systems will use radiators as emitters. When the original system was installed, the radiators would have been sized according to the manufacturer’s specifications. Typically, this would have been 82.C flow and 71.C return with the connections being flow at the top and return at the opposite bottom corner. Existing systems With the advent of condensing boilers, most installations were found to have oversized radiators and as such, little or no adverse effects were found when the system temperatures fell to 70.C flow and 50.C return. However, as heat pumps work at temperatures lower than even this, it is important that each radiator is checked again for its suitability and replaced with one of the correct size/output if necessary. As can be seen, the size of radiator required will be larger than conventional systems. This can be controlled to an extent by choosing a suitable design water temperature. The trade off will be a slightly lower COP. As we have already discussed, the higher the running temperature, the harder the heat pump has to work to reach the desired temperature. Please advise the customer that, in any case, the radiator will not get ‘hot’. The perception may well be that the system is not working correctly because the radiators are only ‘warm’. Below is a typical radiator conversation factor table* and a worked example of sizing radiators for use with a heat pump A typical heat pump operating to feed radiators will run at a flow temperature of 50.C and a return temperature of 40.C – giving a mean water temperature of 45.C. In the case of a system using both radiators and Underfloor heating (UFH) a flow of 40°C and a return of 30.C – giving a mean water temperature of 35.C – would usually be preferred. For a living room with a design temperature of 21.C and heat loss of 1.8kW. The .T = 45.C – 21.C = 24.C. From the radiator manufacturers correction factor table: for .T = 24°C factor .0.406. For a design heat loss of 1.8 kW: the required corrected output is 1.8 / 0.406 = 4.43kW. Select a radiator from manufacturer’s information that would give 4.43kW output (at 75°C mean water temperature) – this will give the required 1.8 kW output at 45°C mean water temperature produced by the heat pump. Similarly, for a bedroom with the same design heat loss but design temperature of 18°C. The .T = 45°C – 18°C = 27°C. From the radiator manufacturers correction factor table: for .T = 27°C factor .0.46. For a design heat loss of 1.8 kW: the required corrected output is 1.8 / 0.46 = 3.48kW. Thus, select a radiator from manufacturer’s information that would give 3.48kW output to give the required 1.8 kW output at 45°C mean water temperature. For an UFH system with a mean water temperature of 35°C. For a design heat loss of 1.8kW and a design room temperature of 18°C. The .T = 35°C – 18°C = 17°C. From the radiator manufacturers correction factor table: for .T = 17°C factor .0.26. For a design heat loss of 1.8 kW: the required corrected output is 1.8 / 0.26 = 6.92kW. Thus, select a radiator from manufacturer’s information that would give 6.92 kW output to give the required 1.8 kW output at 35°C mean water temperature. * Where possible reference should be made to radiator manufacturers own information for the correction factors for different types of radiator. .C Correction Factor 5 0.050 10 0.123 15 0.209 20 0.304 25 0.406 30 0.515 35 0.629 40 0.748 45 0.872 50 1.000 55 1.132 60 1.267 65 1.406 70 1.549 75 1.694 7 Sealed Systems 16 Sealed Systems The following components are required to use the Grant Aerona heat pump as part of a sealed heating system. Due to the lack of space these components are not located within the heat pump, but have to be fitted external to the unit. a) expansion vessel (of the correct size to suit the volume of the system) b) Pressure relief valve – 3 bar c) Pressure gauge d) Filling loop e) Tundish These items may already be installed on the existing system. If so, they should be checked to ensure the integrity and suitability of the components before proceeding to re-use them. Refer to Section 14 for details of the Grant sealed system kits designed for use with the Grant Aerona heat pump range. The expansion vessel can be fitted to either the flow or return pipes but ensure that there is no automatic or manual valve in line that may prevent the heat pump utilising the expansion vessel. The filling loop can be sited anywhere in the system, but it must always be sited within visual distance of the pressure gauge. The nominal filling pressure for the system when cold is 1 bar. Before filling the system check the expansion vessel charge pressure. This should be 0.2 – 0.3 bar higher than th...

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