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.

@editor Just a thought really but, if the heat pump is not used to heat the DHW so only used during the cooler months of the year, (ours is still being used for short periods of heating in this ‘spring’!) will there still be similar refrigerant loss when the pump is on vacation? There should of course be less fouling (shouldn’t there?). Regards, Toodles.

@toodles very good question and honestly I don't know... but I think @ashp-bobba will have a far better idea here. 

Posted by: @editor

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

I think the laws of thermodynamics pretty much guarantee anything that starts of ordered gets less ordered, and this in turn pretty much guarantees it works less well.  Of course thats only a direction of travel not a velocity!

I dislike the laws of thermodynamics intensely.

Posted by: @temperature_gradient

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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 ?

The issue with over sizing or sizing correctly is equally related to it minimum capacity as well as max req capacity, cost difference between capacity model is modest, physical sizes of the systems in some brands until they jump to a much larger capacity is minimal or has no change. The main issues is when designed to work on weather curve compensation the systems will need to live most of it life between 20% and 50% operation as your home will only ever need 20% to 50% heating. England sits around a mean temperature 10Deg C across the year. 10 Deg C is the average for the Spring and Autumn, if your home only needs a low capacity for 70% of the heating seasons it is already oversized for the most part. Its harder to add an oversize of 2kW and make that run correctly than to worry about the system becoming only 85% efficient in 15 years time, the cost of short cycling for the 1st 10-12 years would cost way more than preparing for what happens at the end of life. 

A quick fix to limp the system for its last legs at the end would be just to add another very small heater in the centre of the home, if it really came to needing a little more heat, but it is possible to over clock these systems in their last years to squeeze a bit more out. Most systems would be idling in the dead of winter, they just run for longer and at higher capacity, there are ways to set the system to make it work a bit harder to get a little more out of them until they are finished. 

Posted by: @toodles

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@editor Just a thought really but, if the heat pump is not used to heat the DHW so only used during the cooler months of the year, (ours is still being used for short periods of heating in this ‘spring’!) will there still be similar refrigerant loss when the pump is on vacation? There should of course be less fouling (shouldn’t there?). Regards, Toodles.

Technically these monoblocks do not loose refrigerant unless they are failing, they are hermetically sealed, refrigerant top ups are not a service item they are a failed repair item. we have systems over 15 years old that have never been touched for refrigerants. Now there are EEV's and pressure transducers that are thread connected so these are the weaker points, the service should identify if these are leaking. If they have leaked this would be a repair needed to remove, rethread connect and or change the part. 

Posted by: @toodles

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@editor Just a thought really but, if the heat pump is not used to heat the DHW so only used during the cooler months of the year, (ours is still being used for short periods of heating in this ‘spring’!) will there still be similar refrigerant loss when the pump is on vacation? There should of course be less fouling (shouldn’t there?). Regards, Toodles.

I see where you were also going with this, its is likely where you use your system less (for not heating water) your system is less likely to need servicing as often and less likely to fail, leak, ware and so on. This would just be based on run hrs, I think it is safe to say we can base everything on the running hrs and the hard work it needs to do, I think it is also safe to say it can be very much like a car millage, if your ASHP has 100,000 miles compared to a same aged ASHP that has 20,000 miles I would expect the lower to be in better condition.