Dual service heat pumps: ensure correct sizing

Dual service heat pumps heat and produce domestic hot water. They can be installed in new builds as well as in renovated homes, replacing a more conventional heating appliance. It must be installed by a qualified professional because the correct size of the appliance is key to the system’s performance.

With their inclusion in the RT 2012 since 2013, dual service heat pumps are being more and more developped. They have several advantages: they save space and have a reduced footprint thanks to the compactness of the appliance which provides two functions; a limited investment compared to using two solutions ; and only one maintenance contract.

For the selection of the equipment, the NF PAC certification guarantees the conformity of the appliance with the standards in force, as well as meeting minimum performance standards. In addition, since 2015, all heating equipment is subject to energy labelling. In the case of a dual-service heat pump, this has two columns: one for the energy efficiency of the heating mode and the other for the energy efficiency of the hot water production mode.

A professional installer, ideally RGE * qualified, will also be able to help you in this choice, taking into account the specificities of your accommodation. In June 2014, the RAGE programme (Grenelle Environment Guidelines) published professional recommendations for designing, sizing, implementing, maintaining and looking after this type of appliance.

A solution adapted to each case

If these appliances can be simple to use, they do however rely on the heating and hot water requirements being balanced. The calculation of the hot water tank volume must be carried out in relation to the real needs of the dwelling in order to satisfy demand. The solution will therefore be adapted to each case. For example, if the new build is particularly well insulated, the hot water requirements will be higher than the heating requirements: a relay for the supply of hot water can then be studied. Conversely, in a renovated dwelling, the dual service heat pump can be installed with a boiler for alternative heating and production of hot water. In regions with a severe climate, the effectiveness of the dual service heat pump can be reduced because, by giving priority to the production of hot water, it can reduce the thermal comfort inside the dwelling. A backup heater may be necessary.

The importance of regulation

The most common dual service heat pump system circulates the water leaving the condenser from the heat pump either to the heating circuit or to the Domestic Hot Water (DHW) cylinder. This switching between the two functions is controlled by the regulation, via a three-way directional valve. In this way, the dissociated mode of the CAP gives priority to the production of hot water at night, when the heating needs are reduced and the price of kWh is lower. This mode of operation is advantageous because it is easier to interrupt the heating periodically during the night but does, however, require a large hot water storage capacity. The production of hot water can also answer two different instructions: for example, it can be programmed at 55°C at night and at 40°C the rest of the time. Some appliances operate in an "all-or-nothing" way and others with variable speed. The latter are, for the most part, equipped with a compressor with electronic speed variation (inverter technology). It is the installer who decides what is the best solution to manage the peaks of hot water consumption in the morning, noon and evening.

Two large families of dual service heat pumps

There are two main types of Dual Service heat pumps: outdoor air / water heat pumps as a single unit or with separate elements; heat pumps on vertical thermal probes, on horizontal buried sensors or on ground water.

Separating the thermodynamic machine and the hot water tank may allow greater latitude in placement and sizing of the latter. It is recommended that it is installed as close to the drainage points as possible as, by minimizing the lengths of the pipes between the hot water storage and the points of drawdown, one limits the thermal losses, the risks of development of legionary bacteria, as well as the waiting time at the taps. It is also recommended that no loops are created in the distribution of domestic hot water: not only does it consume a lot of energy, this technique also destroys the stratification in the hot water tank.

Limiting the risks of thermal discomfort

As for the power of the heat pump, it will be determined according to the heating requirements and the losses of the dwelling, taking care that this power does not induce thermal discomfort for the occupants when the heating is interrupted during the production of hot water

The rules of the RAGE programme contain tables that are useful for evaluating the required water reheat times based on the average heat output of the pump and the capacity of the storage tank under different control scenarios with or without supplemental resistance.

For example, some outdoor air / water heat pumps will only start heating the domestic hot water via the electric heater built into the balloon on the coldest days, in order to limit the risk of thermal discomfort. The rest of the time, when heating the domestic hot water, an automatic restart of the heating can be carried out if necessary, depending on the ambient temperature.

*RGE is the abbreviation for "recognised environmental guarantor". The RGE label certifies the ability of a craftsman to carry out work in accordance with the rules of the art to improve the energy performance of a building and to achieve the current objectives set by energy and environmental standards.