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Vaillant Arotherm Pro vs new Plus

 
Air Source Heat Pumps (ASHPs)
Last Post by Batpred 4 weeks ago
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Batpred
 Batpred
(@batpred)
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Joined: 2 years ago
Posts: 1066
Topic starter 09/06/2026 6:37 pm   [#3051]

We are considering installing a Vaillant Arotherm 7kw. 

Heat Geek are now providing the Pro model, which has some advantages. We were lead to consider the new Plus, that apparently can be used with a wider range of cylinders. The trouble is that it still has no clear availability date. 

I had the impression that any Heat pump type of cylinders would be compatible with any heatpump...


8kW Solis S6-EH1P8K-L-PLUS hybrid inverter; G99: 8kw export; 16kWh Seplos Fogstar battery; Ohme Home Pro EV charger; 100Amp head, HA lab on mini PC


   
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JamesPa
 JamesPa
(@jamespa)
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Joined: 3 years ago
Posts: 5123
09/06/2026 7:32 pm  

Posted by: @batpred
↑

I had the impression that any Heat pump type of cylinders would be compatible with any heatpump...

Generally yes although some heat pump manufacturers recommend specific ones.

There is no fundamental thermodynamic reason why a heat pump that will go to 75C (which R290 pumps do) shouldn't be used with a non heat pump cylinder with a typical 0.6sq m coil, albeit efficiency will be lower (but the business case for replacing a perfectly good cylinder is unlikely to pay back in short time!).  However there apparently some issues with the flow temp algorithm which can cause problems with some heat pumps, particularly larger capacity ones. 

This whole 'heat pump cylinder' thing is in essence a hang over from older refrigerants which couldn't reach sufficiently high flow temperature, it is really time that it went away and became an efficiency/reheat time consideration not a 'must do'.  In fact I believe Heat Geek now do not replace cylinders by default.

For some while Vaillants own 'heat pump' cylinders have had a coil that is smaller than the 3sq m normally specified.  Maybe Vaillant have tweaked the FT algorithm further in their new model and so can now comfortably specify even smaller coil sizes.    


4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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Batpred
 Batpred
(@batpred)
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Joined: 2 years ago
Posts: 1066
Topic starter 10/06/2026 11:25 am  

That makes sense. If the heat transfer into a cylinder is better, the pump can be run at a lower FT, helping efficiency. 

In our case, the heat geek calculator was generally providing a low guaranteed scop around 3.5. Potentially this is why it was excluding Vaillant tanks when in combination with the Pro 🤔. 


8kW Solis S6-EH1P8K-L-PLUS hybrid inverter; G99: 8kw export; 16kWh Seplos Fogstar battery; Ohme Home Pro EV charger; 100Amp head, HA lab on mini PC


   
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