Secondary circulation and cylinders for solar and heat pumps

Getting hot water to distant outlets

A secondary circulation loop carries hot water around the distribution system and returns it to the cylinder or calorifier. Properly balanced circulation reduces the amount of cooled water a user must draw before hot water arrives. An outlet branch still needs suitable length and regular turnover; the loop does not circulate through a closed tap.

For a conventional temperature-control regime, keep the hot supply from storage at least 60 °C and the circulation return at least 50 °C. Healthcare systems have additional requirements. Insulate and balance the loop, and verify temperatures at its relevant points. A running pump or a good reading at the cylinder does not show that every branch is adequately controlled.

Cylinders for renewable heat

• Solar thermal cylinders commonly use separate solar and supplementary-heating coils. The solar coil is usually low in the store, where cooler water assists heat transfer. Variable solar input still needs an effective hygiene-control strategy.
• Indirect heat-pump cylinders commonly need a larger heat-exchanger area to transfer the required heat at a smaller temperature difference. Confirm performance against the actual heat source and demand.
• Where the system specifies a thermal hygiene cycle, commission its temperature, exposure and frequency and verify completion. A brief boost to a displayed 60 °C does not mean the whole stored volume has had the required heat treatment.

Manage outlet scald protection alongside these controls. Shorten unnecessary branches, remove redundant pipework and manage little-used outlets; a recirculation loop does not eliminate stagnation elsewhere.

Branches, losses and local heating

• A non-circulating run that has cooled holds water which must be displaced before the required hot supply arrives. Its volume, draw-off flow and the heat taken by the pipe affect the wait. A recent draw or a nearby circulating loop changes those starting conditions.
• Circulation can keep hot water close to several remote outlets, reducing waiting and wasted draw-off. Both flow and return lose heat while maintained hot, so insulate them. Pump schedules must suit the design and hygiene-control scheme; continuous operation and overnight shutdown are not interchangeable assumptions.
• Keep an effective circulation path through each return loop. Short non-circulating outlet spurs can be part of the design; minimise their volume and provide suitable turnover. Remove redundant blind ends: the circulation pump does not refresh them.
• For a remote, rarely used outlet such as a small basin, a local point-of-use heater can replace a long low-turnover branch altogether. In a large building, heating water near groups of remote outlets shortens the distribution runs in the same way. The heater's electrical supply still has to be suitable.
• Size the circulation flow from the loop's heat loss: flow = heat loss ÷ (specific heat × permitted temperature drop).

Worked example. A loop losing 0.84 kW, with the water allowed to fall from 60 °C to 55 °C, needs 0.84 ÷ (4.2 × 5) = 0.04 kg/s: 0.04 litres a second, or 2.4 litres a minute.