The heat load is the amount of heat that a pool or spa requires at a given time of the year to maintain a desired temperature. Many factors affect the heat load. These include:
- Surface area of pool or spa. Heat is lost from the surface of the water to the atmosphere.Larger surface areas therefore require more heating to overcome the higher heat loss.
- Cover. A pool or spa cover effectively reduces the surface area of the pool or spa. If a cover is used, the heat loss is reduced.
- Wind. The amount of wind over the surface of the pool or spa will affect the heat loss.
- Shade. The amount of sun/shade on the pool or spa will affect the heat loss.
- Location. A pool or spa in the southern parts of Australia or in the Alps will have a higher heat loss than a pool in the tropics.
- Season. The heat loss is higher for a pool or spa in July than it is for September. You need to determine the months of the year when heating will be required.
Computer modelling is the only accurate way to determine the heat load of a pool or spa. This will ensure that:
- Adequate heating is available to achieve the desired temperatures
- The heat pump has not been over specified – a common approach which causes customers to spend more money than necessary
- The customer has a reasonably accurate indication of monthly heat pump running costs
The heat load provides an indication of the heat which the pool or spa requires over a 24 hour period. Other factors which now need to be considered include:
Desired hours of operation. Depending on the installation location, installations in the suburbs may not be able to run 24 hours per day due to noise. Better quality heat pumps can produce low levels of noise. However, pool pumps are required to pump water through a heat pump, and these are generally noisy in comparison. It is usual to restrict the hours of operation to between 7am and 9pm. Reducing the hours of operation means that a larger heat pump is required so that the required heat can be transferred to the pool or spa within a smaller number of hours.
Desired Tariff. A mid rate tariff will reduce heat pump running costs by around 33%. This further reduces the available hours of operation which means that a larger heat pump is usually required.
Importance of Heating. The computer model is based on average weather data and statistics. The calculations are also based on average estimates for wind, shade, etc. This means that there will be days when the actual heat load will be higher than that calculated by the model. The model assumes that it does not matter if the pool is not at the desired temperature for a few days during the colder months. If maintaining a certain pool or spa temperature is vital, an allowance should be made to increase the heat load.
Determining the Right Size
The above calculations and considerations, if performed professionally by properly trained consultants, will give a fair indication of the output power required from a heat pump for a unique installation.
This is now the time where you have to rely on truthful and accurate heat pump performance data from the manufacturers.
A 40 square metre pool has a calculated heat load of 7.6kW per hour (182kW per day) to maintain a temperature of 30 degrees in the coldest month of the year, assuming that a pool cover is used for 20 hours of the day. For this installation, the hours of operation need to be restricted to 14 hours per day. This means that 13kW per hour is required to provide the 182kW required to heat this pool per day. The consultant then needs to look at the performance graph to determine the heat pump size that is most appropriate for the actual air temperatures during the coldest months of operation. In Cape Town the average air temperature in the coldest month would be around 5 to 15 degrees (average over whole day). The Hi Temp WBR14 would provide the required heating for a high proportion of the time. It may not quite get the pool to temperature during prolonged periods of unusually cold weather. This is not generally an issue for a domestic pool. The Hi Temp WBR17 would be recommended when maintaining the pool temperature is more important, or when a pool cover is used for fewer hours of the day.