Whole System Thinking
Exhaust Air Heat Pumps (EAHPs) are increasingly being promoted in the UK as a Scandinavian solution for new-build flats. But the Danish experience is more nuanced. Exhaust Air Heat Pumps can play a valuable role, but only when applied in the right context and considered as part of a wider building energy system.
In Denmark, EAHPs are more commonly associated with individual homes outside heat network areas, particularly rural properties where each dwelling needs its own heating solution. They should not be confused with the standard approach for multi-residential buildings in heat network zones, where Mechanical Ventilation with passive Heat Recovery is often the cost-effective norm.
Distinction Matters
The issue is not whether EAHPs are useful, it’s whether they are being asked to do the wrong job.
When used as the primary heat source in multi-residential buildings, their performance is constrained by the amount of recoverable heat available in the exhaust air stream.
The reason is that heat recovery capacity is tied to the ventilation airflow rate, meaning that additional electric top-up may be needed, particularly for domestic hot water or during periods of high heat demand.
During periods of low outdoor temperatures and high heating demand, the heat pump increasingly relies on direct electric heating, causing its COP to approach 1. In colder conditions, supplementary heating may be required, often in the form of electric panel heaters.
The below results are taken from technical research undertaken by Danfoss, comparing common EAHPs designs against a communal network served by centralised heat pump.
| Measure | Case 4 (EAHP) | Case 5 (EAHP) | SAV Centralised HP | Units |
Space heating system SCOP | 2.01 | 2.17 | 2.85 | – |
Hot water (DHW) system SCOP | 1.65 | 1.81 | 3.29 | – |
| Whole system SCOP | 1.78 | 1.96 | 3.06 | – |
| Direct electric – space heating | 0.15 | 0.18 | 0.00 | MWh/yr/flat |
| Direct electric – hot water | 0.81 | 0.55 | 0.00 | MWh/yr/flat |
| Total power demand | 2.06 | 2.08 | 1.25 | MWh/yr/flat |
| Peak demand (incl. simultaneity) | 0.9 | 0.97 | 0.51 | kW/flat |
EAHPs and heat networks are complementary solutions when placed in a passive heat recovery mode, an EAHP directly flattens the building’s thermal load profile and safely justifies more aggressive (lower) diversification factors in central heating and heat network designs.
This enables increased use of cost-effective base load generators, reduces utilization of peak load generators and reduces investment demands. An EAHP in cooling mode offers relief cooling to dwelling tenants during warm and hot days. In turn, the district heating network provides cost-effective low-carbon heat to buildings, which removes the need for direct electric panel heaters and cylinders with electric immersion, which are operationally expensive for consumers and place additional pressure on the electricity system.
The better Danish lesson, therefore, is not “install EAHPs everywhere”. It is to design the whole system properly. Where heat networks are available, EAHPs should be considered as complementary technology, rather than replacing efficient networked heat.
The UK Risk
That is where the UK debate risks going wrong. Denmark’s success has never come from copying isolated products. It has come from matching the right technology to the right setting: individual heat pumps for individual homes where networks are not available, and efficient shared infrastructure where buildings can be connected.
In that sense, Exhaust Air Heat Pumps are not the problem. The problem is when Danish technology is imported without Danish system thinking.
