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Design Of Cascade System Replacing Undersized ASHP

 
Air Source Heat Pumps (ASHPs)
Last Post by pash44pump 4 months ago
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 pash44pump
(@pash44pump)
Trusted Member Member
Joined: 1 year ago
Posts: 70
Topic starter 27/06/2026 8:22 am   [#3078]

Hi folks

 

I have posted before about my ASHP problems which eventually led to my installer admitting that the original ASHP was undersized. We are now getting close to the time where they will replace the system and I am waiting for them to confirm the system design.

To summarise - original heat loss calculation was 8kW and 12kW Midea was installed. System could not get house to desired temps when OAT was below -1C. I ran my own calculations, collected data and had a new, independent heat loss calculation done. My calcs came out at 13kW and the independent one came out at 14kW. Lots of discussion and help on here and we don't need to revisit that element of the discussion.

The options for replacement were a 16kW single unit or cascade of 2 x 10kW Clivet Evo Edge units, which is what my installer is going with. After 2 years of wrangling with them, I'm not going to fight this.

House has UFH downstairs and rads upstairs. Upstairs not used much and no problem running single flow temp as all upstairs rooms are bedrooms and they can hit desired IAT with same flow temp as the UFH.

They are doing the refit at their cost and the units will be new with new guarantees. My current ASHP is nearly 5 years old so there is upside in this.

My questions are therefore related to how the system should be configured. In other forum posts, there are suggestions that with a cascade, a buffer can actually be a good idea although my previous conclusion from extensive reading here on single ASHP systems, was that a system volumiser would be a better idea. What do I need to know about cascade systems to make sure the installation is correct?

Specifically

Do I need a buffer or would system volumiser be better?

How should the cascade be set up?

Should 1 unit generally run until more power is needed when unit 2 kicks in or should they both run equally?

Should 1 unit do heating while the other does HW and additional power for heating when needed?

Will I have any control over what each unit does after installation?

What else do I need to consider?

All help welcome

Many thanks



   
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Cascade ASHP undersized
 Temperature_Gradient
(@temperature_gradient)
Estimable Member Member
Joined: 1 year ago
Posts: 89
27/06/2026 9:06 am  

I don't know the full details of your house or installation so this is just thinking out aloud, but going through this current heat-wave, I'd be thinking about whether there's a solution which could provide the additional top-up heating and some cooling.

For example, would it be feasible to have an air-to-air / aircon type unit in a room, or a multi-split to provide additional heating to a few rooms with the summer cooling thrown in?

Or, if it has to be an additional heat-pump, can it be installed in a way that allows some fan-coils to be added to provide additional heating/cooling.



   
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 pash44pump
(@pash44pump)
Trusted Member Member
Joined: 1 year ago
Posts: 70
Topic starter 28/06/2026 3:18 pm  

@temperature_gradient thanks but my focus is just getting best ASHP set-up



   
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 alincork
(@alincork)
New Member Member
Joined: 5 months ago
Posts: 6
30/06/2026 7:52 pm  

How does the installer propose to control it? Do Clivet offer a controller capable of modulating 2 outdoor units? Will both outdoors have their own pumps, how will these be controlled? Would one unit be master, start on demand, ramp to 80% and if PI&D is increasing then ramp down and bring on second unit and modulate both until demand is hit? In theory easily done, in practice especially with pumps involved it could take some tuning, I've no great experience with domestic controlling but have worked on numerous commercial installs with multiple outdoors. Hat tip to Graham Hendra, this isn't a cascade:) https://www.linkedin.com/pulse/is2-heat-pumps-cascade-graham-hendra?trk=public_post_feed-article-content



   
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 pash44pump
(@pash44pump)
Trusted Member Member
Joined: 1 year ago
Posts: 70
Topic starter 08/07/2026 11:38 am  

Thanks for your reply. Answers to the questions you raised is exactly the kind of info I am looking for. I have asked the installer for a full PI&D and will revert when I have this

 

Pash



   
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Topic Tags:  Cascade (3) , ASHP (88) , undersized (3) ,
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