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Cascade System or larger single unit

Air Source Heat Pumps (ASHPs)

Last Post by Aventus Eco 3 weeks ago

5 Posts

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Renewables345

(@renewables345)

Eminent Member Member

Joined: 1 year ago

Posts: 14

Topic starter 18/09/2026 3:01 pm [#3166]

Hi all,

I’m looking for some advice on an upcoming domestic ASHP project.

The design heat loss is around 28kW, and I’m currently looking at the options for supplying this with either:

1 × approximately 30kW ASHP Panasonic
2 × approximately 16kW ASHPs Panasonic

There will also be a 500L DHW cylinder.

I’d be interested to hear from anyone with experience of systems around this size and what you would consider when deciding between a single larger unit and two smaller units.

In particular, I’d like to understand the practical pros and cons of each approach, such as hot water recovery, efficiency and modulation.

If you've installed or designed both types of systems, I'd really appreciate hearing about your experience and which factors you would look at before deciding.

Thanks in advance.

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Cascade

JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 4 years ago

Posts: 5520

18/09/2026 3:51 pm

How sure are you of 28kW, thats absolutely massive. I presume either your house is absolutely massive (Im thinking well over 500sq m) or it has zero insulation or both? Whats your current fuel consumption?

If you arent sure then I would be going for 2x16kW on the basis that this will give greater flexibility to deal with possible oversizing.

This post was modified 3 weeks ago 2 times by JamesPa

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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Renewables345

(@renewables345)

Eminent Member Member

Joined: 1 year ago

Posts: 14

Topic starter 18/09/2026 3:54 pm

@jamespa Very confident with the heat loss. The house is 700m2, all up to new build spec, 3 floors, very tall windows and very high ceilings.

This post was modified 3 weeks ago by Renewables345

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Carpenterstation

(@carpenterstation)

Trusted Member Contributor

Joined: 2 years ago

Posts: 50

18/09/2026 4:25 pm

Hmmm. We have 500sq metres here (including some very high ceilings) and 16kw cascade (2 x 8 kw) Ecodan units. House is (apparently - according to our EIC) very poorly insulated - even though half of it was built in 2018 to modern building standards.

anyway… as 2 years of experience has shown us, 16kw is way too big. I’ve never seen both units running at the same time, let alone at full chat. The house is permanently comfortable.

I know that the vendors overstated our heat loss and issued a useless EIC to justify it - resulting in us buying more than we needed.

a modern house, well insulated - even at 700 sqmetres should not need a 28kw system.

Just to add, in each of the 3 years prior to installation we used 27GW of energy to heat the house (3800 litres of LPG) per annum. This might help with your comparison?

we do have 250sqm of UFH, and also a 500 litre hot water tank

This post was modified 3 weeks ago 2 times by Carpenterstation

👍

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Aventus Eco

(@aventus-eco)

Active Member Member Professional+

Joined: 2 years ago

Posts: 8

19/09/2026 1:38 pm

@renewables345 I would say if it a new build it’s no more than 16-17kw heat loss, I’m doing a 600sqm one just now and it’s 13kw

the big M series is the commercial unit , you could defo do the whole house from one heat pump. All the M series are single phase supply up to 12kw then 3 phase 16kw upwards. Just need to make sure you have correct size of pipework coming from it and feeding all the underfloor heating manifolds, 12kw need 35lM

👍

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