Secondary hot water returns are generally used in domestic hot water applications where the point of usage (i.e. taps) are a distance away from the hot water cylinder. They are used to remove the need to draw off a large quantity of cool water, ( left in the pipe run), before hot water actually reaches the tap.
Secondary returns generally consist of a bronze pump which pumps the DHW around the system and back to the DHW cylinder. This ensures that hot water is always immediately available at the point of usage. To avoid excessive pump costs it is usual to time the pump operation to coincide with the periods of maximum DHW usage i.e. early morning and evening.
Although secondary returns work well with traditional boilers, when used with a heat pump they can lead to the DHW dropping in temperature quickly. The returning water to the hot water cylinder promotes mixing in the cylinder which destroys stratification. This mixing (when combined with the incoming cold water) lowers the overall stored water temperature. As the heat pump is a low temperature device, the stored water is already at a lower temperature than a system using a traditional boiler, resulting in a noticeable drop of temperature in the DHW. As the stored water temperature in a traditional boiler system is higher and the re-heat time usually much shorter, any drop in temperature is less noticeable.
When a heat pump is being used, and if there is a need to avoid excessive water draw off at the point of usage it is recommended that one of the arrangements over is used.
The preferred choice would be arrangement 1, using trace heating tape. This tape maintains the temperature of the water in the pipe, is self regulating and should be linked to a time clock to operate only with the periods of maximum DHW usage. This arrangement removes the need for the water pump and also reduces the amount of installation costs as secondary pipework is not required.
Arrangement 2 uses a return loop, however instead of connecting to the tank it is connected to the tank outlet. A timed water pump is also used and a flow boiler, which makes up any losses to atmosphere from the pipework. The return pipe should also be a smaller diameter than the flow pipe. It is important that an automatic air vent is installed at the highest point of the system.
In both cases, (and for any pipes going through voids) it is important that the pipe is well insulated. This will reduce the losses from the pipe to atmosphere and hence reduce the amount of time that the tracing tape or flow boiler operates, reducing running costs.