Heat pump trouble shooter Bob Long focuses on cost effective methods of providing domestic hot water (DHW) from a heat pump
Heat pump installations are steadily on the rise I am pleased to say and, provided our government sticks to its promise of delivering the RHI clearly in this coming year, the continuing rise in heat pump installations will continue.
In previous issues of REI, I have discussed a number of heat pump topics and, hopefully, how to avoid some fundamental problems. But domestic hot water production is perhaps the most difficult commodity to provide economically from a heat pump.
Although there are a small number of ‘high temp’ heat pumps available in the market, the most economical output is always achieved at the lowest operating temperature.
Utilising low temperature water for heating a property isn’t generally an issue, as a suitable choice of emitter will ensure adequate delivery of energy.
Domestic hot water is quite different, however, requiring higher temperatures at the point of use and even higher temperatures when stored, to eliminate the possibility of legionella, made safe through pasteurisation.
DHW supplying showers is generally needed at a flow rate of somewhere between 10 and 15 litres a minute, although power-showers are significantly more, and perhaps should be considered ungreen due to their high water and energy usage.
To create perspective with regard to the quantity of energy consumed in the production of DHW, a flow rate of only 10 litres/min will require an energy input of around 25kW. This is equal to the energy consumed by twenty five, single bar electric radiators!
At 15 litres/min, the energy requirement would be 37kW (Assuming incoming water at 10⁰ C).
This level of energy exchange is easily accommodated by a conventional type of boiler, but not so easily with a small domestic heat pump that is often dependant on a single phase electrical supply.
To meet periodic on-demand needs, DHW is heated by the heat pump over an extended period of time and stored in a stratification cylinder. This type of DHW storage employs the principal of the warmest water occupying the top section of the cylinder, from which the DHW is drawn as required. As hot water is drawn off the top of the cylinder, cold water enters at the bottom, and is heated by energy from the heat pump through an internal heat exchanger.
In this type of system it is usual for heat pumps to operate at two different output water temperatures, differentiating between heating the property and heating DHW, and accomplished by a motorised valve, directing the output from the heat-pump as required, between the heating system and the DHW cylinder.
The water stored in the DHW cylinder can be maintained at a sufficiently high temperature for normal usage but, periodically, the contents of the DHW cylinder will require pasteurisation – usually accomplished by an electrical powered immersion heater capable of raising the temperature of the whole cylinder during the pasteurisation period.
The energy required to perform the pasteurisation process is by comparison expensive, and should be used sparingly.
With the advent of modulating-flame boilers, DHW production in a heat pump system can be made simpler and in some instances more cost effective. By employing a secondary energy source the required water temperature can be produced on demand.
The incoming water to the secondary boiler is of course economically pre-heated by the heat pump reducing the amount of energy required to reach the target temperature.
This method of DHW production combines the economics of a heat pump with the flexibility of a conventional boiler, eliminating the necessity for pasteurisation, as no DHW is actually stored.