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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)

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JamesPa

(@jamespa)

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02/12/2025 11:51 am

Posted by: @sim0n0

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It’s a long time since I’ve looked at the manual but I thought I remember Thermo off just controlling any secondary circulation pumps?

I’ve spent hundreds of hours in those Hitachi config menus - happy to help if I can

Simon thanks. If you have the time perhaps you could look at the past 2 pages of posts.

Essentially OP is operating his heat pump on a fixed flow temp and turning it on and off manually to control the house temperature, which is not sustainable. His house has UFH, a loss of only about 3kW, and its temperature falls very slowly taking 18 hours to change by a couple of degrees when there is no heating at all and its near zero outside (lucky person!). He has a buffer tank and is operating open loop.

Not surprisingly, manual control based on room temperature, which is in essence 'rear view mirror' control, doesn't work, because once the energy is in the slab is going to come out come what may (like a storage heater but without the louvre on top to control the output). So my advice is to switch primarily to weather compensation, so that the energy stored in the slab is a closer match to the heat loss and you get the advantage that the OAT changes before the house responds, so you deal with the lag. To use the storage heater analogy, by basing the energy sent to the slab on OAT (trather than sending a constant amount of energy) you are looking ahead to see what's going to happen indoors and only charging it to the amount needed, thus it doesn't matter if there is no louvre on top. OP hasn't yet tried that, he is (sensibly IMHO) trying to prepare the ground and understand before diving in and changing variables.

The challenge is the low flow temp combined with the 4C thermo off hysteresis. Even at 0C OP needs a FT of only about 31C, so at 12C he is going to need an FT in the 23-27C region and the 4C hysteresis is likely to muck things up.

It seems an engineer is scheduled to reduce the hysteresis to 2C, but its unclear what he has to change to do that (a dip switch?!) and its also unclear what exactly it measures to determine when to switch on/off

Any clarity you could add would be welcome. Im not really sure what the point of 'thermo off' is, given that the UI has plenty of parameters by which you could more easily control short cycling (which is why it was switched on in the first instance).

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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Sim0n0

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02/12/2025 11:53 am

My issue is an oversize 11kW heat pump and a poorly designed UFH. My house gets too hot made worse by solar gain from large south facing windows

My weather comp is set to 20 (min) at 12 degrees and 38 at -3 degrees. Eco mode 2 degrees setback overnight. Runs 24x7

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JamesPa

(@jamespa)

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02/12/2025 12:00 pm

Posted by: @sim0n0

↑

My issue is an oversize 11kW heat pump and a poorly designed UFH. My house gets too hot made worse by solar gain from large south facing windows

My weather comp is set to 20 (min) at 12 degrees and 38 at -3 degrees. Eco mode 2 degrees setback overnight. Runs 24x7

Apart from the oversized heat pump and the solar gain, do you have any other problems in terms of comfort or running cost? Im guessing it cycles a fair amount but does that materially compromise SCOP for you? In other words, is there anything we can attempt to help with or do you think its as good as its going to get?

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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Sim0n0

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02/12/2025 12:03 pm

@jamespa I’d need to read back through the manual but I don’t use the 4 degree Thermo off. If I’m honest I don’t even remember what that is. I doubt it’s something that should be on.

He needs to set up weather comp before worrying about anything else. Set min to 20 and max to either design parameters or more likely lower. I also had to reduce my upper threshold down to 12 with a flow temp of 38. There is also setting to adjust the shape of the curve which allowed me to ramp up the temp quicker as the temps dropped and vice versa

It took tens of hours to get right and a degree here and there makes a massive difference. I also find that wind affects the weather curve as my external unit is very exposed and weather comp alone doesn’t account for internal solar gain but even with our badly designed UFH I have 19.5-21.5 between hottest and coldest rooms 24x7

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