Heat pump trouble shooter Bob Long continues his series of articles focussing on issues for installers working with this technology.
Sticking to the subject of bivalency, it is always advisable to consider alternatives when deciding upon a fuel source for the delivery of supplementary energy.
In general, a well balanced heating system, designed to operate with the lowest effective water flow temperature will optimise the efficiency of any heat pump, and deliver the best financial and low-carbon result possible.
However, domestic hot water production is an entirely different matter and requires a much higher water temperature, often beyond the economically viable limits of a heat pump.
Systems are sometimes offered with “Y plan” strategies, where the heat pump has the ability to operate in two different temperature ranges. Installations are usually equipped with a motorised diverting valve to periodically apply energy to the hot water cylinder at higher temperature, and then revert back to heating where low temperature water production is resumed.
This solution is not ideal for two fundamental reasons. 1) Cost of hot water production, 2) Minimum safe temperature required for elimination of the legionella virus.
Attaining a “safe temperature” is usually achieved by periodically raising the temperature of the DHW cylinder. This is achieved by employing an electrical immersion heater, and a timer fitted with an auto-reset and limiting thermostat.
A pre-determined pasteurisation cycle is initiated by a timer which is organised to re-set as soon as a defined target temperature is reached, usually somewhere around 65 to 70°C.
Although this method of DHW production is used widely in heat pump system design, it is not without potential problems, and usually relies upon expensive direct electrical energy, which by comparison is probably the most costly of all available forms.
Natural gas, LPG, Diesel oil, Bio fuel, or Log burner with back-boiler, are all potential heat sources capable of generating high grade energy for DHW, and further, maintaining the DHW cylinder above the minimum safe holding temperature, eliminating specific pasteurisation cycles.
The possibility of using one of these heat-sources should not be dismissed for dedicated production of domestic hot water, as the advantages can be multiple.
A heat pump, dedicated to space heating purposes, and designed for low temperature operation, i.e. under floor or fan/coil, can and will deliver its most economic output under these conditions.
To optimise the economics of DHW production, a heat exchanger can be incorporated into the system’s buffer cylinder. The heat exchanger is used to pre-heat the incoming domestic water supply prior to feeding the DHW cylinder. This method provides a significant temperature lift to the mains water supply very economically, reducing the amount of energy required for the DHW to reach its target temperature.
Numerous fuel sources
Assuming DHW is held in a dedicated cylinder, and heated by one of the numerous fuel sources available, the energy contained within the DHW can also be a source of bivalent energy. The energy can be used to supplement the heating circuit when outdoor temperatures fall into minus figures, and bivalent energy input is permitted.
Integration of the cylinder to provide bivalent support requires nothing more than a heat exchanger coil embedded in the DHW cylinder and a thermostatically controlled diverting valve to control the flow path of low temperature water from the heating circuit as required.
In this way, bivalent support for the heating circuit can be supplied from the alternate fuel source, primarily installed for the production of DHW.
The methods discussed here, are an illustration of the wide range of possibilities and solutions available to engineers, when considering designs for specific locations and customer needs.