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In the past there used to be a degree of flexibility regarding heat pump sizing; if the heat pump could not cover all the heat load at low outdoor temperatures then direct electric heating picked up the shortfall. However, with tighter control now in place, we discuss what is meant by a ‘supplementary boiler’ (rather than a back-up boiler) in relation to heat pump systems.

The microgeneration standard, MIS3005, requires heat pumps to be sized to at least 100% for 99% of the heating season for any property, meaning that the heat pump should be able to entirely heat the house without electric or other back up under all but the most extreme conditions. In practice, this translates to systems being designed to a target external design temperature of between -0.2°C and -3.9°C, depending where the property is located in the UK.

Nu-Heat always supplies a heat pump that will achieve the 99% target, unless the size of the heat pump is restricted, in which case a supplementary boiler will be needed. This is a possibility where a larger heat pump using a 3-phase electricity supply is required but the local DNO either does not offer 3-phase or it is prohibitively expensive, for example.

A system that incorporates a heat pump and boiler is termed ‘bivalent’, and there are two possible modes of operation.

Bivalent parallel

Down to a certain outdoor temperature the heat pump will be able to cover the whole load on its own. This temperature is known as the bivalent point. If the temperature drops below this in bivalent parallel mode, the heat pump keeps working but the boiler will work alongside it. This is the mode used by Nu-Heat.

Bivalent alternative

When operating in bivalent alternative mode, below the bivalent point the heat pump switches off and the boiler does everything. This is really only likely for systems that require a higher flow temperature (70°C, for example), in a poorly insulated house.

The heat loss parameter of a property needing a supplementary boiler should be as good as (or better than) where a heat pump is used on its own, usually around 60W/m²; this is because the heat loss parameter is directly related to the flow temperature required. The boiler exists to extend the power coverage of the heat pump, but NOT the temperature range.

Nu-Heat believes that a heat pump should be able to supply at least 50% of the power needed at -3°C (so in a property sized for 24kW, the heat pump will cover 12kW or more at -3°C). This is because the bivalent parallel system works by the blending valve taking water from the heat pump and boiler and mixing them proportionally to get the right temperature output. Just below the bivalent point, the heat pump will do most of the work and the boiler will contribute a little (to make up the difference). Therefore, it is important that the boiler isn’t any bigger than it needs to be, or it will cycle on and off under these conditions, adversely affecting energy efficiency. In theory the boiler will never be required to ‘take over’ the heating system.

With the bivalent parallel theory, and in order to maintain the integrity of the system efficiency, boiler sizing is as important as the sizing of the heat pump. However, the choice of boilers on the market dictates that it may well be 12kW or more, so the model must be a modulating version, which means that it can trickle heat as required rather than give 100% heat supply.

As with other systems, heat pumps give priority to domestic hot water over hot water for heating. Therefore, if the DHW calls, the heat pump will answer, and if the heating suffers, the boiler will pick up the difference.

Whichever fuel source is used for the boiler, energy figures should be provided by the fuel supplier or boiler installer before it is fitted, so the homeowner can base their decision on actual costs.

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