@jamespa 

I have been back into settings.  There is nothing set.  

Sorry but I dont know.  There must be something causing this timing so, if it matters, which I think it does, it needs to be found.  If you think it doesnt matter can proceed without tracking it down, but something is causing it and its unlikely to be a fault

Posted by: @jamespa

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There must be something causing this timing.

Is this the Themo Off timing that your referring to?  Just need to clarify.  Just had another power cut, 3rd in just over a week.

Posted by: @trebor12345

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Is this the Themo Off timing that your referring to?  Just need to clarify.  Just had another power cut, 3rd in just over a week.

No, the fact that flow temperature reduces by ~3-4 C during nighttime hours (I havent yet worked out the exact hours) regardless of whether defrosting is occurring.  Until you posted your latest graph it was at least plausible that this was caused by defrost, even though the evidence wasn't quite consistent.  Your latest graph shows the reduction  but no defrosts.

This one, from about 11pm on.  Thats not thermo off so far as I can see, its a deliberate reduction in FT.  Thermo off occurs earlier, at around 7pm.  Look at the difference between the curves.  As a secondary point I would also question whats happening at 12.30, a very sudden rise followed by a plateau for no apparent reason.

@jamespa 

I have annotated the image to show what was going on.  This was a manual run because we had 13C outside.  Hope this helps.

Posted by: @trebor12345

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I have annotated the image to show what was going on.  This was a manual run because we had 13C outside.  Hope this helps.

Ah that makes sense.  I think 'off' is actually 'setback' rather than truly off, which is clearly what has happened, a 3C setback in the target flow temperature.

So are we back to suspecting defrosts causing a drop in temperature, or do you usually turn the heating off for bed?

Posted by: @jamespa

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Posted by: @trebor12345

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I have annotated the image to show what was going on.  This was a manual run because we had 13C outside.  Hope this helps.

Ah that makes sense.  I think 'off' is actually 'setback' rather than truly off, which is clearly what has happened, a 3C setback in the target flow temperature.

So are we back to suspecting defrosts causing a drop in temperature, or do you usually turn the heating off for bed?

In mild conditions, yes I turn the heating off.  This is done by putting the Heatmiser thermostat on the wall into the off position.  So there is absolutely no demand for any heat. 

I have even taken to turn the indoor unit into the OFF position at bed time.  Which it is now. Circuit 1 is the heating.

When its cold (below 6 ish) I leave the heating on all night. 

I have no setbacks.  

Yes I am 100% positive that the defrosts cause the drop in temperature. 

Posted by: @trebor12345

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Yes I am 100% positive that the defrosts cause the drop in temperature. 

OK noted, although I dont think any of the charts posted show this.  This one definitely seems like human intervention not defrost because of when the rise and fall in flow temp occurs relative to onset of defrost.

However I dont think it is now crucial.  I think there is enough evidence that nothing is critically broken to proceed to try to get it working on Weather compensation, which should, if you can get the parameters right, mean you can mostly just forget about it.

I have reproduced below (with a couple of amendments, my previous suggestion.  The key bit of this is the weather compensation, the rest is to ensure that you have continuous call for heat/continuous pumping through the secondary and there are no distractions from night shift etc.  I reckon it will take a couple of weeks to get it right because you are going to have to make changes slowly given the slow response time of the house.  The objective however is to exactly balance the house loss with the useful energy put into the slab.  The house loss is proportional to the difference between outside air temp and inside air temp.  The (useful) energy put into the slab is (approximately) proportional to the difference between flow temperature and inside air temperature.  Once the two are in balance the house temperature will stay constant.

Subject to any new revelations I would suggest you start with parameters like this:

Night shift OFF or, if noise is actually a problem, timed for the night time hours only.
Secondary pump on 24x7 until we find a way to sync it (I think its already like that), thermostats etc to max (ditto)
Heat pump on 24*7, no setback/eco etc
Pure weather compensation, all room influence disabled
Thermal off hysteresis 1-2C (we need to work out how to adjust that parameter or indeed whether its the best one to use to control short cycling).  If you feel more comfortable then leave this for now, but be ready to change when it becomes obvious its a problem (which I am pretty sure it will)
Weather compensation.  There seem to be multiple ways to program the weather compensation which, without having the unit in front of me, are impossible to decipher exactly.  From the manual I would go for 

• Water calculation mode: Gradient, set up so that initially
• FT 20C at 20C. (see note 1)
• FT 35C at -2 or whatever your design temperature is. (see note 2)
• Vertex offset 0C
• I think you can then just adjust one variable - gradient, to change the curve.  This makes things simpler.

My suggestion is that you read the relevant bits of the manual, note the current settings and then see how what it does on the UI relates to the manual.  Hopefully the graph, shown on page 77, is displayed in the UI so you can see whats going on.

The weather comp settings are pure guesses based on what you have said.  

Adjust the gradient to get the house temp right, but do so very slowly  no more than 1C increase in the cold end FT every 24 hrs (which is probably a change in gradient of 0.05 - but the UI will have a minimum increment so probably use that)

Monitor iat and ft continually during setup period if you can

If you make any changes or switch off, by whatever means, make notes and be sure to pass them on when you post some results

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Note 1:   It is possible that you would be better off with FT20C at just above the temperature at which you stop heating, to account for the 'free' heat from people and electrical items.  Only experiment will tell!

Note 2:  This is a guess and maybe too high.  Feel free to start at 33 or even 31 @-2 based on what you already know about what FT is needed to keep the house warm at the coldest temps

----------------

If you start with this you can then look at cycling etc and decide whether a further tweak is required.  For example it may be best to set it to operate say 3 equally spaced periods each day to deal with the oversizing.  However I would personally worry about this only after you have got the house to the point where it is comfortable without continually adjusting everything.

Do take some time to understand the relevant bits of the manual before you change anything and ask if anything is unclear.  Go slowly and methodically.

If you make the initial changes and then leave for a couple of days, posting some results may enable a 'fast track' route to be worked out to the correct WC parameters, otherwise its just a process of iteration.

If you get it right you should hardly even need to touch the controls.

I should caution that I am not a heat pump installer, just an ordinary homeowner (albeit one with a background in physics and engineering) who has spent a couple of years studying this stuff to inform his own purchase, and has since optimised the setting of his own heat pump.  Anything you do is therefore at your risk.  Take notes!

JamesPa Thank you for your very detailed reply.  It's going to take me sometime to get to grips with this.  I have started to keep a record of whats going on.  Yesterday was a waste because of the power cuts.  Will be intouch.

Posted by: @trebor12345

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JamesPa Thank you for your very detailed reply.  It's going to take me sometime to get to grips with this.  I have started to keep a record of whats going on.  Yesterday was a waste because of the power cuts.  Will be intouch.

Understood, its your house and your heat pump so take all the time you need.  I will respond to queries but otherwise wont post again until I hear from you.  The key, I think, is to understand weather compensation and why it (generally) works, then try to relate it to your house.  If I need to try an alternative explanation please ask! 

BTW Weather compensation exists also for boilers and in some EU countries has been compulsory for decades.  By and large the UK didnt take it up which has meant that most of us have paid 10% more for our heating than we needed to, and enjoyed a lower level of comfort!  Madness.

@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.

JamesPa Just for information 29th Nov run.  Nothing really new, except that these Thermo Off periods are way tooooo long at 1hr 20 mins and 2hrs 20 mins.  I am looking into this in more detail.