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4.5.3 Manual calculation of heat loss and heating requirements for a building Heating loads can also be calculated manually using a whole-house approach or performing room-by-room load calculations as follows. To calculate the size of heating required, you need to know: • the required indoor temperature • the lowest likely ambient (outdoor) temperature that can be expected • the direct heat loss from the building envelope • how much natural or mechanical ventilation heat loss there is. The difference between the ambient (outdoor) and internal temperatures is called the ‘temperature lift’: temperature lift (°C) = internal temperature (°C) – ambient temperature (°C) Calculate heat loss for each surface of the building envelope (walls, floor, ceiling) by calculating the area and multiplying this by the thermal transmittance coefficient or U-value (U = W/m2 °C) of the surface: surface heat loss (W/°C) = width (m) x length (m) x U-value (thermal conductivity) Deduct window and door areas from the wall area they are in and calculate their heat loss separately. The total surface heat loss for the building is the sum of all surface heat losses: total surface heat loss (W/°C) = walls loss + roof loss + floor loss + windows and doors loss Allow for ventilation heat loss such as air leakage through badly fitting doors and windows and any damage to the surfaces, and so on. Estimating these losses can be difficult – figures may range from 20% to 70% depending on the type and condition of the structure: total heat loss (W/°C) = total surface heat loss x ventilation heat loss Calculate the heating required by multiplying the total heat loss by the temperature lift: heating required (W) = total heat loss (W/°C) x temperature lift (°C) 4.6 Step 4: Select a system to meet requirements You should now have all the information needed to select the right heat pump system for the home’s needs. Things to keep in mind: • Inverter type units are recommended in most cases for their increased efficiency. • Consider suitability for the environment (i.e. seaspray, sulphur from geothermal activity). Some manufacturers provide performance guide charts for selecting a heat pump system similar to Table 4.5. EECA recommends that particularly in colder climates, you should use products with this information available. Table 4.5: Example of a heat pump performance guide chart Model A: Capacity 4.0 kW rated output Indoor (°C) Outdoor (°C) 20 -10 -5 0 2 7 Output Input COP Output Input COP Output Input COP Output Input COP Output Input COP 2.4 0.77 3.12 2.88 0.91 3.15 3.7 1.1 3.31 3.8 1.12 3.4 4.1 1.18 3.5 Model B: Capacity 6.0 kW rated output Indoor (°C) Outdoor (°C) 20 -10 -5 0 2 7 Output Input COP Output Input COP Output Input COP Output Input COP Output Input COP 3.6 1.3 2.76 4.32 1.54 2.8 5.1 1.71 2.98 5.2 1.75 3.0 6.1 1.9 3.2 Model C: Capacity 8.0 kW rated output Indoor (°C) Outdoor (°C) 20 -10 -5 0 2 7 Output Input COP Output Input COP Output Input COP Output Input COP Output Input COP 4.8 1.85 2.59 5.65 2.05 2.75 6.2 2.25 2.8 6.4 2.4 2.7 8.1 2.6 3.1 22 For a heat load design requirement of 5.6 kW and an ambient temperature of 0°C, by referring to the performance guide chart in Table 4.5, an 8 kW capacity heat pump is rated to deliver 6.2 kW at 0°C, which will meet the heat load design requirement.PDF Image | Heat pump installation Good Practice Guide
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