Bivalent point and hybrid switchover

The bivalent point is an operating condition at which a heat pump no longer covers the whole building load on its own and another heat source begins to contribute or takes over. In an air-source system it is often expressed as an outdoor temperature, but it also depends on the required flow temperature and the unit’s capacity at that condition.

A hybrid switchover should be a design output. Choosing a generic cold-weather temperature in the controller can waste fuel, increase direct-electric use or leave the home short of heat.

Capacity bivalence

Building heat loss rises as outdoor temperature falls. Heat-pump capacity can also change with outdoor and leaving-water temperature. The capacity bivalent point is where those two curves meet.

Above it, the heat pump can meet the calculated load. Below it, the design needs one of the following:

• the heat pump and supplementary source run together
• the supplementary source takes over
• the indoor temperature is allowed to fall, which is normally not the design intent

The comparison must use the exact heat-pump output map and the system’s weather-compensated flow temperature, not the model nameplate.

Common operating modes

Parallel bivalence keeps the heat pump running and adds the other source when needed. This can maximise heat-pump contribution if the emitters and controls can accept both sources together.

Alternative bivalence stops the heat pump and transfers the load to the other source. This may suit hydraulic, temperature or product constraints, but gives up heat-pump output below the changeover point.

Part-parallel operation adds the other source for a band and may later stop the heat pump at a lower condition. The controller needs clear start, stop and priority rules so the sources do not hunt against one another.

The current MCS hybrid route

As at 22 July 2026, MIS 3005-D Issue 3.0 requires all heat sources in an MCS hybrid design to provide at least 100% of the calculated design heat load together.

The heat pump itself must provide at least 55% of that load. For this test its output is rated at 55°C flow and the location’s design external temperature, even where the emitter design uses another flow temperature.

The standard also requires:

• heat-pump use to be prioritised in an agreed control philosophy
• all sources to be integrated under one master control system
• the contract to state the proportions of space heating and hot water designed to come from the heat pump, excluding supplementary electric heat

These percentages and temperatures are requirements of the dated standard. They are not a universal optimum switchover for every hybrid.

Economic and emissions bivalence

A controller can switch sources for cost or emissions before the heat pump reaches its capacity limit. That is a different bivalent point.

For a cost comparison, convert both fuels to cost per delivered unit of heat. The heat-pump side depends on electricity price and COP at the current outdoor and flow temperatures. The boiler side depends on fuel price and actual efficiency.

The comparison can be written as:

heat-pump cost per kWh of heat = electricity price per kWh ÷ operating COP

The alternative source needs an equivalent delivered-heat calculation. Standing charges already paid for other reasons and tariff time bands may affect a household decision, but they should not be hidden in the controller setting.

An emissions-led strategy uses current emissions factors and operating efficiency instead of price. Cost and carbon changeover points need not match.

Automatic optimisation that uses tariff or carbon data needs a defined data source and update route.

Flow temperature affects the answer

A heat pump may cover the building load at a low emitter temperature but fall short if the control raises the flow unnecessarily. Poor emitter sizing can therefore create an earlier bivalent point and more boiler use.

Set the weather curve from the room heat-loss and emitter design. The hybrid controller should use the actual leaving-water target when evaluating heat-pump capacity and efficiency.

If a boiler shares the circuit, its own minimum temperature and return requirements may conflict with low-temperature heat-pump operation. Hydraulic and control design must address that conflict rather than simply raising the whole system temperature.

Control stability

A single temperature threshold can cause rapid switching when outdoor temperature moves around it. Good control uses appropriate hysteresis, minimum run times or a calculated load condition.

It should also decide:

• whether sources may run together
• how pumps and valves change state
• which source serves domestic hot water
• what happens during defrost
• how a source fault is reported
• whether manual override expires automatically

Two independent thermostats controlling two heat sources are not necessarily an integrated hybrid system.

Supplementary electric heat is not automatically a hybrid

An immersion or electric backup heater built into a heat-pump hydraulic unit is usually a supplementary heater, not a separately optimised hybrid heat source. For a full-load hydronic MCS design, its output does not count towards the heat pump’s required 100% capacity.

Its enable temperature and fault fallback should be documented. An owner should be able to see when direct-electric heat has run because it can materially change electricity use.

Commissioning the switchover

The design record should show:

• building load versus outdoor condition
• heat-pump output at the relevant flow temperatures
• capacity, economic or emissions reason for the chosen control
• parallel or alternative operating sequence
• boiler or supplementary-source output and efficiency basis
• hysteresis, minimum run and failure behaviour
• space-heating and hot-water proportions

Commissioning can force the relevant states through the manufacturer’s test mode rather than waiting for rare weather. Check valve and pump operation, source priority, alarms and energy metering.

Monitoring in use

Review how often the alternative source runs and under what conditions. Unexpected use can result from an incorrect outdoor sensor, high weather curve, disabled heat pump, hot-water call, defrost support or manual override.

Energy totals by source are more informative than a controller icon. Compare them with outdoor temperature and flow temperature before changing the bivalent point.

Related entries

• Hybrid heat pump systems
• Heat pump sizing margin
• Design flow temperature
• Weather compensation controls
• Backup immersion heater