Definition
The U-value is the rate of heat transfer through a building element per square metre per degree of temperature difference, in W/m2.K. A low-U wall or window loses little heat; a high-U one loses a lot.
U-value vs the exchanger k-value
Do not confuse them. The exchanger overall conductance (UxA in the epsilon-NTU method) describes the recovery core; the building U-value describes the envelope. They meet at the load calculation: better envelope (lower U) means less heating demand, which both cuts energy and changes how much recovery is worth recovering.
Why recovery still matters with a good envelope
Even a Passive House with very low U-value still ventilates (see Passive House recovery), and that ventilation air is the dominant remaining load. Decarbonisation strategies pair a tight envelope with high-effectiveness recovery so the ventilation load – not the walls – is what gets addressed.
Frequently asked questions
Does better insulation remove the need for recovery? No – it shifts the largest load to ventilation air, which only recovery addresses.
Is lower U always better? Up to a point; beyond code, the marginal cost often beats the marginal energy saving.
Need help specifying the right air-to-air heat recovery for your project? Contact the QIYU engineering team – WhatsApp +86 157 5335 5505, or email kuns913@gmail.com.
Frequently Asked Questions
What is U-value?
U-value is the rate of heat transfer through a building element per square metre per degree of temperature difference, in W per m2 per K; lower means better insulation.
How does recovery relate to U-value?
Recovery reduces the ventilation load that the building envelope U-value would otherwise have to compensate for; together they set the total heating and cooling demand.