The backup heater

Some heat pump systems include an electric heating element in the water-filled heating circuit, called a backup heater (in German, Heizstab). It works like the element in a kettle: 1 kWh of electricity gives 1 kWh of heat, a COP of 1. COP, the coefficient of performance, is the heat you get for each unit of electricity, and a heat pump gets well above 1. A heat pump with this element is often called mono-energetic, because both heat sources use electricity.

The element is part of the design on purpose. It is not a sign that the heat pump is failing.

Why it exists

An air-source heat pump delivers less heat as the outdoor air gets colder, just when the house needs the most. The size on the label is roughly what a heat pump delivers on a mild day. In the model, the capacity (how much heat it can deliver) of every air-source size class at -15 °C is roughly two thirds to three quarters of its capacity at 7 °C. Sizing the heat pump to cover the coldest day alone means a larger unit than most hours of the year require. A smaller unit with a backup element can cover the coldest hours at a COP of 1 instead.

The trade-off is cost. If the heat pump runs at a COP of 4, a kWh of heat from the element uses four times the electricity, so the same warmth costs four times as much. A system that leans on the backup heater loses much of the benefit of having a heat pump.

When it switches on in the model

In the model, the heat pump and the backup heater feed the same water circuit, and the heat pump comes first. EnergyPlus, the building simulation program the model runs on, steps through the year in intervals of 15 minutes, or 5 minutes in some houses. In each one, the heat pump takes the heating load up to the capacity it has at that moment, which depends on the outdoor temperature and the supply temperature (the temperature of the water it sends into the heating circuit). If the load is larger, the remaining part goes to the backup heater, up to its own capacity.

That is the whole rule. The backup heater runs in any interval when the heat pump cannot meet the load. Three situations cause that:

A cold spell, when heat pump capacity drops below the house demand.
A short peak, such as the morning recovery after a night setback (the house is let cool overnight, then reheated), when the emitters ask for more heat than the unit can give. Emitters are the floor pipes or radiators that hand heat to the rooms.
A supply temperature that is high for the outdoor conditions, which lowers heat pump capacity (see the article on weather compensation).

Real controllers add rules the model does not have. They may allow the element only below a set outdoor temperature (the bivalence point, where a second heat source joins in), wait for a delay before switching it on, or run it in a comfort mode that fires it sooner. The model has none of these. It has no temperature lock-out (a rule that blocks the element above a set temperature), no hysteresis (a gap between the switch-on and switch-off points that prevents rapid cycling) and no timer.

Default size

The default is 8 kW for air-source and ground-source heat pumps. Ground-source units take heat from a loop buried in the ground instead of from the air. The default does not depend on the size of the house or the heat pump.

The wizard uses this size and does not let you change it. The equipment step shows it next to the heat pump size, for example ”+ 8 kW backup”, so you can set the pair against the peak heating load. A system without the element is called monovalent: the heat pump alone, a single heat source, must cover the whole load. The model does not offer that variant.

Where its energy shows in the results

The backup heater has its own electricity series, and it counts as electricity, not as fuel.

The heat pump electricity figure at the top of the results includes the backup heater.
Beside the heat pump COP, the results show a system figure that includes backup electricity. The heat pump COP leaves the backup heater out entirely, so the gap between the two numbers tells you how much the element is working.
In the monthly electricity chart, backup heater electricity is part of the heating bar.
The Sankey diagram, which draws where energy comes from and where it goes, has a separate backup heater node.

A little backup use on the coldest days is normal. If the system figure is far below the heat pump COP, the element is carrying more than those hours. Check the proposed heat pump size against the peak heating load in the sizing panel, the supply temperature of your emitters, and the setback settings in your schedules. Heat pump size and cycling shows how backup use grows as the heat pump gets smaller, on one house and six sizes.

Limits of the model

The 8 kW default is not sized from the house. Compare it with your design heat load (the heat the house loses on a very cold design day) and the heat pump you are considering.
Control is purely load based, with no bivalence temperature, delay or comfort modes.
The element is modelled as an ideal resistance heater with no standby loss.
Hot water is separate. The hot water tank has its own electric element, described in the hot water article.