That’s a really interesting question and I think the 20,000 starts figure Viessmann quoted is quite conservative and probably specific to their internal testing or warranty calculations, it’s definitely not a universal threshold for compressor lifespan.

To my understanding, most modern compressors (especially the scroll or inverter types used in heat pumps) are rated to handle significantly more than 20,000 starts. Industry-wide, it’s not uncommon to see scroll compressors designed for 100,000+ starts under normal conditions.

In fact, I recall @simon-w saying on a recent podcast that he’d heard that some compressors have a life of 160,000 starts.

And inverter-driven systems (which I believe the Viessmann is) don’t really “start” in the traditional sense… they modulate up and down, so wear and tear is much less than with old-school on/off cycling.

Also, what’s more damaging than the number of starts is how those starts happen. Frequent short cycling (caused by oversizing, poor weather comp settings or lack of thermal mass) can really stress components. Interestingly, warm restarts (when the compressor’s already been running and turned off for 10 minutes, for example) are generally easier on the system than cold starts from rest, so a system that cycles a few times a day in long runs is much better than one that’s stop-starting every 30 minutes.

Just checked mine.  543 starts from mid November to now.  So, generously, 1000 per year unless I use if for cooling in summer.  20 years seems a reasonable lifetime, certainly not something Im going to sweat about.

Ours is 1000 starts in 3200 hours running time over 6months. So twice the rate of James, but again nothing I will worry over.

Nice discussion from Mars about the factors which dominate it.

My fridge and freezer seems to do at least one per hour, 8000 per year.  Just saying!

The number of start-stop cycles a heat pump compressor goes through can vary depending on several factors, including system size, climate, thermostat(s) used, and how well the system is sized for the home. But here’s a general idea:

 Typical Range:

• 2 to 3 starts per hour is ideal for most systems.
• Up to 6 starts per hour can still be considered acceptable, especially during extreme temperatures or in systems without variable-speed capabilities.

  Daily Estimates:

• If a system runs 2–6 cycles per hour and operates for 10–16 hours a day (common during heating/cooling seasons), that’s 20 to 96 cycles per day.
• Annual total could range from 7,000 to 35,000 cycles per year.

  Too Many Cycles?

Frequent cycling (short cycling) can:

• Increase wear and tear on the compressor.
• Reduce efficiency.
• Indicate the system is oversized, has poor airflow, or has a boiler-type stop/start thermostat.

  Ways to Reduce Start-Stops:

• Use an inverter heat pump.
• Don't use a stop/start boiler-type thermostat to run your heat pump.
• Ensure the system is properly sized and maintained.

It might be worth checking in with a heat pump engineer if you’re seeing a high number of cycles, especially short ones (like <5 minutes per cycle).

At work we have a big freezer with a number of cooling units for redundancy, that turn on and off again once or twice an hour.  By my calculations if it is twice an hour, for the last 15 years that makes 262,800 starts. 

By contrast our heat pump spends 4 or 5 months of the year coming on twice a day to heat water and the rest of the year up to 24 times a day, but more usually maybe 15 or so. 

@jamespa, your fridge is not doing hot water, so it will be considerably less.

@simon-w we have a Samsung fridge freezer and I wish it could do hot water to help de-ice it. Worst fridge ever!

That is a lot of ice! Has the heating element in the freezer packed up? It sounds counter intuitive and I was surprised to find out that it had one.  I too have had a Samsung fridge freezer with ice maker for many years and the only issue I have had was the heating element in the freezer gave up causing it to ice up severely. Replaced by a local engineer for £30 and no further issues.

@editor This model only has one compressor, cost saving, to do fridge and freezer; but it was touted as frost free, likely for the freezer only 🙂

The one i had turned the fridge into a freezer every year. It was a simple fix to remove the lower back panel in the fridge, there is a condenser behind it that would ice up if the drain holes got blocked. There is a temp sensor on that pipework but it likely wasn't doing a good job.

Good excuse to empty the fridge and deep clean it every winter

Had the FridgeFreezer for 13 years till the compressor started clonking; bearings were worn out.

@abernyte We’ve spent a small fortune on engineer call-outs for this fridge. Early on, we even had a major component replaced (details here). The ice maker is a mess… it constantly looks like a cave full of stalactites, and there’s clearly water leaking down from somewhere that end up beneath the fridge tray and the freezes. It’s an appallingly designed appliance, and Samsung’s customer service has been equally terrible, as we’ve outlined in the post. Will never buy anything Samsung again.