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).

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 

Posted by: @sim0n0

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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?

@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

It cost me just under £3000 to fix and a near nervous breakdown! I will almost certainly switch heat pump brands sooner rather than later but that’s not down to the kit itself it’s down to not being able to rely on support IF it breaks down

@jamespa Yes it cycles a lot. In fact I smile when it’s -3 outside knowing that my heat pump is working efficiently - a couple of days a year LOL. I don’t look at SCOP as it’s as good as it can be. We’ll use nearly 27000kWh this year…

@jamespa Why does he need any hysteresis? Just set the heat pump to 20 and it will go as low as it can. I’d have to check but I think mine will get down to 25-26 ish. There’s also a setting to turn the heating off completely at a set outside temp with a hysteresis setting. I do have sympathy as it should have all been set up by his installer. And he says he’s had Hitachi out to update the firmware - why didn’t they set up the basics for him? They updated firmware but didn’t turn on weather comp. I’m sorry sound cynical and I’m sure there are ‘good guys’ out there but I haven’t yet found them.

My developer pocketed the £7500 grant and I end up spending another £3000 fixing it. My installer hadn’t got a clue and is now Heat Geek registered. And don’t start me off with the joke that is MCS! I had to educate myself

Posted by: @sim0n0

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Why does he need any hysteresis? Just set the heat pump to 20 and it will go as low as it can

I have wondered the same.  It seems a Hitachi engineer set up the thermal off and it's hysteresis to prevent short cycling which was apparently occurring.  Personally I doubt this was the right decision, but I have no way to test or prove.

@jamespa The new sw was to prevent rapid short cycles which we were seeing every 3 to 5 minutes.

BTW.  Heating ticking away at 50% Night Setting.  No thermal offs and a good COP.

Posted by: @trebor12345

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@jamespa The new sw was to prevent rapid short cycles which we were seeing every 3 to 5 minutes

Fair enough.  There seem to be ways to achieve the same without flipping dip switches which personally I would have preferred because they are more accessible.

Never mind, there are always multiple ways to do things!

@jamespa I’ve re read through the instructions a I can’t see anything in the DIP switch settings relating to Thermo off or hysteresis. My system doesn’t require any DIP switch changes to alter the config. Is your installer referring to the setting that turns the heating completely off (Thermo off in Hitachi speak)? There is a setting that allows you to set an on/off temp with hysteresis.

Also looking at your controller setting you’ve got your pump speeds set to fixed rather than automatic delta T. I run fixed 100% for DHW as that’s what it needs in my system and auto for heating as that maintains the delta between flow and return much better than fixed.

For clarity its not my heat pump.  The heat pump (and house) belongs to @trebor12345  My only role is to tray to assist!

@jamespa Apologies I did know that but I seem to have replied to the wrong post