Controls and electrical design for a hybrid heat pump and boiler setup
A customer wants a heat pump as primary heat source with the existing gas boiler retained as backup for very cold days. The property has a 100 A single-phase supply. The heat pump is rated 9 kW electrical input 2.6 kW, the boiler 24 kW. There is an existing 3 kW immersion, a 7 kW EV charger planned, and a consumer unit with spare ways but no dedicated heat pump circuit. The customer asks for the heat pump to run on a cheap overnight tariff and for the boiler to cut in automatically when the heat pump cannot meet demand. The installer proposes a simple changeover switch between heat pump and boiler on the heating circuit, controlled by a single room thermostat.
1. Evaluate the electrical supply and diversity for this proposal.
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Do a maximum-demand assessment. Heat pump 2.6 kW ≈ 11.3 A. Boiler (pump, fan, controls) small, say 1 A. Immersion 3 kW ≈ 13 A. EV charger 7 kW ≈ 30 A. Total connected load ≈ 55 A plus general lighting and sockets. With diversity applied per the usual guidance for domestic installations, the EV charger and immersion are unlikely to run simultaneously with everything else, but a 100 A supply is tight and a proper load calculation and DNO notification are required.
Key points: the heat pump needs its own dedicated circuit with correct protective device and RCD/RCBO selection per manufacturer instructions (some specify Type A or Type B RCD depending on inverter). The EV charger must have load management or the supply will be overloaded. Do not rely on a single changeover switch for a 9 kW heat pump - it is a fixed appliance and switching it off at the contactor while running can cause compressor damage; use proper control interlocks, not a manual changeover.
2. Critique the proposed single-thermostat changeover control and describe a better control strategy.
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A single room thermostat switching between heat pump and boiler is inadequate. It gives no weather compensation, no bivalent setpoint, and risks short-cycling and simultaneous operation.
Better strategy: a bivalent controller that monitors outdoor temperature and heat pump capacity. Set a bivalent switchover point (e.g. -2 °C) below which the boiler assists. The heat pump runs as lead with weather compensation on its own curve; the boiler is enabled only when the heat pump cannot maintain flow temperature or when outdoor temp falls below the switchover. Use a buffer/volumetric vessel and a low-loss header so the two sources do not fight each other. Ensure DHW priority logic and legionella cycle are handled. Provide a lockout so the boiler cannot run when the heat pump is capable, and an interlock so both cannot fire into the same circuit uncontrolled. Document the control philosophy for the customer.