What are the specification drivers of monobloc and split system air source heat pumps for residential dwellings?

Architectural drawings of buildings woth Dimplex heat pumps

The main specification driver of monobloc air source heat pumps and split system air source heat pumps for residential dwellings is focussed on selecting a heat pump with the right performance output to match the heating and hot water demands of the dwelling. Other key considerations include potential project restraints that may exist on available internal and external space, as well as acoustic or aesthetic requirements.

Both monobloc and split system air source heat pumps offer very similar performance outputs. Once a heating engineer has analysed the dwellings thermal performance and calculated the required kW output of a heat pump, the specifier could select either type of air source heat pump to supply the energy required.

It is the differences between the design and operation of monobloc and split system air source heat pumps that may make one more suitable than the other for specification to meet specific project parameters.

What are the key differences between monobloc and split system air source heat pumps to consider during their specification?

The key difference to consider during specification of either a monobloc air source heat pump or a split system air source heat pump is in the size of the space that they need to operate and the position of that space requirement: either inside or outside the dwelling.

In a monobloc air source heat pump, all the components needed to draw energy from the outside air and pass it on to the dwelling’s heating system, such as the evaporator, compressor, condenser, expansion vessel and refrigerant in a closed loop circuit, are contained in one large unit. This is positioned on the outside of the dwelling and connects directly through insulated pipes to the hot and cold-water connections of the heating system in a similar way to a conventional gas boiler.

The monobloc air source heat pump is usually positioned in an area that gives sufficient airflow to the unit, whilst being as close to the connections for the heating system as possible, usually a maximum of 6m, to limit heat loss from the pipes.

By contrast, a split system air source heat pump has one unit on the outside and another unit on the inside of the dwelling. The latter contains the condenser and expansion vessel. The outside fan unit contains an evaporator and compressor to draw heat from the air and transfers that energy via refrigerant pipes to the indoor unit.

How do the size and positioning options between monobloc and split system air source heat pumps generate flexibility for specifiers?

The choice of placing one large external monobloc unit close to the property or a smaller external split unit further away from the dwelling if required, allows specifiers to take a more flexible approach when considering project space limitations.

The split system offers more flexibility to the specifier due to the smaller footprint requirement for the outside fan unit and, as it is linked to the internal unit by pipes carrying refrigerant, not water, it can be positioned up to 15m away. This gives options to the specifier who may wish to site the external unit away from the house to reduce noise levels for the occupants, for aesthetic reasons, or to maximise external access around the property.

How do noise considerations factor into the choice between monobloc and air source heat pumps?

The noise ratings of monobloc and split system air source heat pumps available from a range of manufacturers are broadly similar. If a monobloc unit is compared to the external fan unit of a split system, where both are placed in the same position on the outside of a property, the effect of the external noise on occupants of the dwelling is unlikely to be vastly different.

However, the ability of the split system to move the external fan unit further away from the dwelling will clearly reduce noise levels and, if acoustic comfort is a key factor in the design considerations for a dwelling, then specifiers may choose this a preferred option.

Are differences in the installation of monobloc and split system air source heat pumps a factor during their specification?

The installation of monobloc and split-system air source heat pumps varies due to the way that each outdoor unit is linked to the system inside the dwelling.

As the connecting pipes of a split system that run between the external fan unit and the internal unit contain refrigerant, the installation must be carried out by an F-Gas qualified installer.

In the case of the monobloc unit, the connecting pipes contain water, so installation is more straightforward and there is no requirement for F-Gas accreditation.

This factor is worth considering when specifying air source heat pumps to ensure suitably skilled installers are available for the size and scale of your residential project.

With regards to the refrigerants used by air source heat pumps, it is worth highlighting that there are several that can be used, and their selection will depend on the choice made by the individual manufacturer that best suits their system. Refrigerants vary in their impact on the environment and details on health and safety and global warming potential (GWP) should always be sought from the manufacturer and included in the appropriate documents supporting any specification.

How can specifiers get the key information to inform the selection of monobloc or air source heat pumps?

There is a wide choice of both monobloc and air source heat pumps on the market and from a kW output perspective, both types will be available that meet the heating demands of residential projects.

The key information about output, size, noise levels, installation requirements and allowable distances between the external unit and the dwelling will all be available directly from individual manufacturers.

Dimplex has a range of both monobloc air source heat pumps  and split system air source heat pumps. Please contact one of our pre-sales team specialists who are on hand to offer guidance on which Dimplex air source heat pump system is best suited to deliver your residential project specification requirements.