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Replacing my 18 month old Hitachi Yutaki ASHP

trebor12345 · 2025-11-23T14:54:01Z

I am perhaps looking at replacing my split level ASHP. What is the best premium brand to replace with (Dakin comes to mind)? Maybe I would like to keep it as a split level system. I am also considering going back to gas, but don’t have a gas supply to the property. So looking into that as well. Why do I want to do this? Oversized HP, too many thermal offs, bungalow not warm on cold days due to defrost cycles, requires constant management to maintain house temperature. My wife would not be able to manage the system if I was not here. The bungalow spec is as the tag line below. (PS, as an example, had a thermal off this morning, was 3 hours before the system came back on. Back on for 1 hour, and not back on after 2 hours.)

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Air Source Heat Pumps (ASHPs)

Last Post by trebor12345 4 months ago

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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 4818

30/11/2025 12:12 pm

Posted by: @trebor12345

↑

@jamespa Can I check something that I done some weeks ago.

My underfloor manifold is set via the 12 flow gauges to a total of 20 l/m. In the UI of the indoor unit I have reduced the primary pump from 28.5 l/m to 24.3 l/m.

Why did I do this. I was seeing a lot of mixing in the buffer tank. Reducing the primary pump flow rate to 24.3 l/m reduced the amount of mixing. Therefore the return temperature to the heat exchanger is reduced slightly (approx 0.5C). Hope this makes sence.

Yes this makes sense. Ideally you match flow either side of the buffer, at which point it is pointless (but that is anyway the case). Since a perfect match isn't possible its better to have slightly more flow in the primary than in the secondary. This should force the cold water to the bottom of the tank thus (hopefully) reducing the flow temperature loss across the buffer to near zero.

Posted by: @trebor12345

↑

Nothing really new, except that these Thermo Off periods are way tooooo long at 1hr 20 mins and 2hrs 20 mins.

Way too long based on what criterion. From what I recall from your earlier post the house wouldn't cool much in this time. So why does it matter?

This post was modified 5 months ago 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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trebor12345

(@trebor12345)

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Posts: 148

Topic starter 30/11/2025 1:57 pm

Posted by: @jamespa

↑

Way too long based on what criterion. From what I recall from your earlier post the house wouldn't cool much in this time. So why does it matter?

You always challenge my thinking! I don't specifically know why I think it's too long. My only thought is do other systems/users have that long time.

Yesterday it did take some time to get heat into building, the off's didn't help.

It was 0C over night, so when I turned the heating off at 11pm it was 22C, this morning it had dropped to 20.5C in the main living area (20sqm of glass), so a little cool for us.

This post was modified 5 months ago by trebor12345

Hitachi Yutaki SCombi Heat Pump
(Indoor Unit ) RWD-3.0RW1E-220S-K
(Outdoor Unit) RAS-3WHVRP1

2024 build bungalow, Southern england, 179 m2, 14w/m2
Underfloor heating all fully open
7KW heat pump
50 litre buffer tank (4 port)
3.6KW solar panels

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JamesPa

(@jamespa)

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Joined: 3 years ago

Posts: 4818

30/11/2025 2:18 pm

Posted by: @trebor12345

↑

You always challenge my thinking! I don't specifically know why I think it's too long. My only thought is do other systems/users have that long time.

Its not so much challenging as seeking to understand. Your house cools very slowly so provided that the right amount of energy is put in over several hours (which, if the WC curve is correctly adjusted it should), you wont notice it. It is quite a long off time but that said my heat pump will do that in one of its modes. My house cools quicker than yours so I do notice it, and therefore dont operate in that mode preferring pure weather compensation instead.

I am reasonably sure that the key to getting this to work is 24x7 operation on WC, with no human intervention other than, not more than once a day and always in the same direction, adjusting the WC settings. Once thats set up then the various behavioural features of the heat pump and your house can be observed and optimised or adapted if necessary. Until it is set up you are operating with a (largely human based) control system which is inherently unstable because of the delayed response of your house and the fact that the control system stops working overnight, and is therefore impossible to diagnose.

This post was modified 5 months ago 3 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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bontwoody

(@bontwoody)

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Joined: 4 years ago

Posts: 1009

30/11/2025 4:54 pm

I find having my thinking challenged is always a good thing 🙂

House-3 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60

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Majordennisbloodnok

(@majordennisbloodnok)

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Posts: 1766

30/11/2025 5:32 pm

Posted by: @bontwoody

↑

I find having my thinking challenged is always a good thing 🙂

I’m always pleasantly surprised when I find out I’ve been thinking in the first place. 😎

105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and SPH5000 inverter
1 x Myenergi Zappi
1 x VW ID3
Raised beds for home-grown veg and chickens for eggs

"Semper in excretia; sumus solum profundum variat"

👍

😂

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Toodles

(@toodles)

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Posts: 2733

30/11/2025 5:44 pm

@majordennisbloodnok ‘Sometimes, I sits and thinks, other times, I just sits’. Toodles.

Toodles, heats his home with cold draughts and cooks food with magnets.

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Batpred

(@batpred)

Prominent Member Member

Joined: 1 year ago

Posts: 823

30/11/2025 6:04 pm

@toodles thinking is existing, or did I miss some thinking here?

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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Toodles

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30/11/2025 6:42 pm

@batpred I think I am too! Toodles.

Toodles, heats his home with cold draughts and cooks food with magnets.

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Majordennisbloodnok

(@majordennisbloodnok)

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Posts: 1766

30/11/2025 7:13 pm

Posted by: @toodles

↑

@majordennisbloodnok ‘Sometimes, I sits and thinks, other times, I just sits’. Toodles.

"I think therefore I am" - René Descartes

"To be is to do" - Socrates

"To do is to be" - Jean-Paul Sartre

"Do be do be do" - Frank Sinatra

"Semper in excretia; sumus solum profundum variat"

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Toodles

(@toodles)

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Joined: 4 years ago

Posts: 2733

30/11/2025 7:49 pm

@majordennisbloodnok Tibby or not Tibby - anyone seen my cat?

Toodles, heats his home with cold draughts and cooks food with magnets.

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trebor12345

(@trebor12345)

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Posts: 148

Topic starter 30/11/2025 10:25 pm

Posted by: @jamespa

↑

Yes this makes sense. Ideally you match flow either side of the buffer, at which point it is pointless (but that is anyway the case). Since a perfect match isn't possible its better to have slightly more flow in the primary than in the secondary. This should force the cold water to the bottom of the tank thus (hopefully) reducing the flow temperature loss across the buffer to near zero.

JamesPa. Thats one thing that I do have right. Cross that one off the list.

Hitachi Yutaki SCombi Heat Pump
(Indoor Unit ) RWD-3.0RW1E-220S-K
(Outdoor Unit) RAS-3WHVRP1

2024 build bungalow, Southern england, 179 m2, 14w/m2
Underfloor heating all fully open
7KW heat pump
50 litre buffer tank (4 port)
3.6KW solar panels

👍

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trebor12345

(@trebor12345)

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Joined: 2 years ago

Posts: 148

Topic starter 01/12/2025 10:49 am

JamesPa Below is the plot of yesterdays run (30 Nov). Surprise NO THERMO OFFS. Ran for 10.5 hours. Im afraid it's a fluke!!!

I have always expressed the view that the HP is oversized, as you have found the Night/ECO setting, I turned it down to 50%. This is the result.

It's not a sustainable way to run the heating.

Does it tell us anything else?

I am looking at setting up the WC.

Hitachi Yutaki SCombi Heat Pump
(Indoor Unit ) RWD-3.0RW1E-220S-K
(Outdoor Unit) RAS-3WHVRP1

2024 build bungalow, Southern england, 179 m2, 14w/m2
Underfloor heating all fully open
7KW heat pump
50 litre buffer tank (4 port)
3.6KW solar panels

ReplyQuote