Optimisation of a recirculating domestic hot water system to minimise wait time and heat loss

Abstract

Many houses suffer from a delay in hot water arrival, after opening the hot tap, due to the length of pipe work from the boiler or thermal store. Measurements in three properties showed delays of up to 46 s. A recirculation system could maintain the water in the pipe at a suitable temperature and for a 15 m pipe is expected to use 30 W less electrical power than a local thermal store at the delivery point. Time-steady eigenvector solutions to the linked temperature equations allow optimisation of the recirculation flow rate, pipe diameters and insulation thickness. The pumping power is <1 mW and a pumpless thermo-syphon system should be possible in some installations. Heat loss coefficients have been calculated for a pair of pipes in a common insulation sleeve to minimise the losses. A plug flow transient advection model for water and pipe temperature based on a sliding 1-D grid has been developed and validated for a single pipe, then extended to also model recirculating systems. Heat losses in a pumped system may be reduced using a timer so the system can cool overnight. The transient model predicts the relationship between flow and warm-up rate to optimise pump size and timing.