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Heat Pumps Technology Guide SEAI Heating system design (including the hot water cylinder and the distribution pipework) must minimise the risk of Legionella. Design strategies include avoiding dead-legs (where water accumulates in a pipe that is seldom used, e.g. a shower at the end of a building that is no longer used). Where dead-legs cannot be avoided they must be flushed regularly. Thorough assessment is important when retrofitting a heating system. The hot water cylinder should be sized for the hot water load. A heat pump system may need a larger hot water cylinder than a natural gas boiler system, and will require a larger heat exchanger, so this should be checked if the original cylinder is to be retained. An alternative approach is to have a thermal store, typically in the same hydraulic circuit as that of the heating loop, with instantaneous water heating upon demand via a heat exchanger. This avoids hot water storage and reduces the risk of Legionella. The temperature of the hot water generated is typically 2 °C to 4 °C cooler than the temperature of the water in the thermal store. The heat exchanger should be sized for maximum flow rate, making it less suitable for very high flow rate applications. 4.6. Process and other heat use Where heat is used for process applications, the requirement of the process plant must be met. As when using a heat pump system for space heating, reducing the flow and return temperatures to the heat pump increases efficiency. Therefore, heat pumps are suited to processes with low temperature heat uses (e.g. swimming pools). Higher temperatures can be met efficiently at a larger scale (e.g. a seasonal performance factor of 3 can be achieved at 70 °C if the heat pump is greater than 1 MW, or at 90 °C if the heat pump is greater than 10 MW). These are specialist applications and should be discussed with a consultant. 4.7. Pipework and pumps In any heat distribution system, the circulating pumps must provide the required flow rate of water while overcoming the pressure loss across the system. If the flow rate increases, the pressure loss will also increase, often significantly. Heat pumps tend to operate at lower temperature differences than many other heat sources. A smaller temperature difference means that an increased flow rate is needed. Energy efficiency measures reduce heat load, this needs to be factored in when retrofitting, and increasing the size of the pipes should also be considered as part of the retrofit. It is important to minimise the pressure losses across the pipework and components. Circulating pumps have to be correctly specified and sufficient to distribute the heat required, to make sure the heat pump will stay within its optimum design. Minimisation of pressure losses can be tackled in a number of ways. Assuming that the length of the circuits and the pressure loss across all other items (e.g. radiators) in the existing heating distribution system stay the same when the heat pump is installed, then the choice is likely to be between: 1. Increasing the diameter of some or all of the pipework, or 2. Increasing the capacity of the circulation pumps. Changing the pumps is likely to change electricity consumption (it will drop if the new pump is more efficient). In practice, the pressure loss across the other system components is also likely to change, as some of the heat emitters are likely to change. Usually it is necessary to reassess the entire system. A pressure test of existing pipework and a power flush helps to determine what repairs or replacements are necessary. 22PDF Image | Heat Pumps Technology Guide
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