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How To Use CO2 Heat Pumps

For Heating & Hot Water


CO2 delivers high temperatures, high efficiencies with low climate impact, and its cheap too! However the key to unlocking all these amazing benefits is to have a low return temperature.

The heating system return temperature should be between 30 and 40 degrees C

Periods of operation above 35, up to 45C, are acceptable but lower efficiency as shown in the graph here.


Our Tips For An Efficient System

-Check and size the heat emitters for a high DT – there are many suppliers who manufacture high DT emitters of all types. Sometimes its not necessary to change but its important to check

-Control the emitters well – use two port control so there is no bypass from flow to return and flow rates are optimised

-Sequence the heating circuits so the high temperature circuits take their heat first

-Control pumps using remote sensors at the index point or as a minimum using their onboard pressure VSD intelligence

-Use a buffer or thermal store to separate the HP from the heating circuits. This shares some benefits of a low loss header but the increased volume prevents mixing. N.B. LLHs should never be used.

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Domestic Hot Water Generation

CO2 is ideal for DHW generation because the incoming cold water guarantees the low return temperature.

The recommended method is to use a plate heat exchanger to load a thermal store or calorifier. The store of heat provides a quicker reaction time to demand and will also meet peak demand in excess of the heat pump capacity.

Here is an example arrangement with a loading plate heat exchanger and pump. The calorifier boundary layer moves up and down as the hot water is drawn off and recharged.

It is not recommended to use coil heat exchangers to heat calorifiers because the primary DT reduces as the calorifier reaches target temperature.



You can find a full schematic here which shows the primary arrangement that we’d suggest for a full system.

Of course every building is different and so our team are always happy to talk through a project and share insights that will help.

The end user benefits of spending a bit more in design, install and commissioning are very high, especially as utility costs are rising. Our estimates show at least a 30% improvement in total cost of ownership when using CO2 over other refrigerants.

Northumbria Uni PSDS Lift 3 April 2022


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