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ASHP Capacity loss with aging

Air Source Heat Pumps (ASHPs)

Last Post by Mars 48 minutes ago

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Temperature_Gradient

(@temperature_gradient)

Trusted Member Member

Joined: 9 months ago

Posts: 42

Topic starter 18/05/2026 10:49 pm [#3008]

ASHP are being promoted on the basis they will be quite long lived, lasting 10 to 15 years or so between replacement. That's a long time, and in that time a lot of air, dirt and moisture will be going through the fine finned outdoor heat exchanger, likewise the compressor internals will see a lot of operation.

So over that 10 to 15 years of operation, how much does the power capacity of the heat pump reduce as the components age, degrade and get fouled?

Most articles I've read emphasize the important of sizing the heat-pump as a close as possible to the design heat demand, but presumably that ASHP capacity is when it's brand new, when the fine finned heat exchanger is perfectly clean and the compressor is as tight and well sealed as it will ever be, that 8kW heat pump is 8kW brand new out of the box, but what's it actually capable at year 10 when the heat exchanger is somewhat fouled , fins are coated with a layer of dirt or some areas even somewhat clogged and when the compressor internals are loosening up.

I'm wondering if there could be an argument that having a bit of a safety margin (how much?) between the design demand and the heat pump capacity, to ensure it's still capable of meeting demand in years 10 or 15 ?

This topic was modified 1 hour ago by Temperature_Gradient

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Mars

(@editor)

Illustrious Member Admin

Joined: 5 years ago

Posts: 4612

18/05/2026 11:07 pm

This is a good question. The short answer IMO is yes, capacity does degrade over time.

On the degradation evidence, the most relevant research I found is that annual performance degradation in air conditioning and heat pump systems runs at around 3.7% per year, with coil fouling and refrigerant loss being the primary mechanisms… and that beyond twelve years many systems have lost close to half of their cooling capacity. That’s a striking figure. It’s worth noting that research was conducted primarily on US residential systems that were poorly maintained, but the physics of coil fouling and refrigerant migration are universal.

More specifically on heat pumps in heating mode, research modelling a residential unit over a 12-year operational lifetime found that 40% condenser fouling caused a 16% performance degradation and a 30% refrigerant leakage caused a 12% drop, with the effects compounding when both occur simultaneously.

Evaporator fouling (your dirty outdoor finned coil) was actually the lesser concern at around 3%, which is perhaps counterintuitive. The bigger culprits are refrigerant charge loss and internal fouling.

So a well-maintained unit, serviced annually with refrigerant charge checked and coils cleaned, should hold its capacity far better than those degradation curves suggest for poorly maintained systems.

An interesting question. Curious to hear what others think.

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