Bivalent systems (sometimes spelt as bivailent), featuring ground source heat pumps must be carefully designed to avoid high energy bills and carbon emissions.
Bivalent systems utilise a secondary boiler, which is designed to provide heat into the distribution system when the heat pump isn’t sized for 100% of the peak load. They are generally found in retro-fit applications where insulation levels of the building are not sufficient and a heat pump cannot meet all off the heating load effectively.
Bivalent systems have to be carefully designed to avoid the return temperature of the heating circuit being to high. If this return temperature is above the in-built temperature set point at which the heat pump turns off, the heat pump will never actually turn on and the whole of the load will be taken by the secondary boiler, resulting in higher than expected energy bills and carbon emissions.
The simplest and most effective way off providing the maximum efficient of a bi-valent heating system while retaining the clients comfort is to use an either or control logic. Simply put either the heat pump or the secondary boiler is operating, but not both together.
The system operates by use of an external temperature sensor (TS). This is set at to an external temperature above which the heating load is satisfied by the heat pump alone. If the external ambient temperature drops below this set point then the heat pump is switched off and the secondary boiler switched on and supplies heat into the heating distribution system. Due to the higher output temperature of the secondary boiler it is important that the boilers flow is mixed, via a mixing valve (MV) with the return flow to lower the temperature to a suitable level for the heating distribution system.