Does anyone have any winter performance data from the Heat Geek mini store? Looking for examples during high space heat demand days and under defrost conditions.
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Hi Trystan,
My system should be reasonably good test bed as my heat loss is estimated 6.8
My setup is as follows
arotherm 7kw
28mm primaries
2 heating loops, 1 all standard radiators, 2nd heat pump convector radiator (for cooling aswell)
Mini Store TALL (110L)
Overall Iâve found the DHW to be ok during winter, if you push it high temp to 65 you can have a nice hot shower. However, if set point is any lower for example 55 the shower will run lukewarm after 7-10 minutes when itâs cold weather. In mild weather like now, I essentially have a combi with unlimited hot water which is nice. In terms of COP Iâve been talking to heatgeek as you can see on my monitor itâs not very good for DHW.
Sadly if my winter performance cannot be improved (cop and DHW production)Iâd prefer to have a traditional cylinder. Also losing most of your control of when your DHW runs happen is not ideal, as it essentially decides itself when to run.
Feel free to check my monitor.
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I might regret getting involved here, but here goes. 
Iâm wondering about your flow rate.
12.6 l/min (756 l/h) is more what youâd see on a 5kW, not a 7kW
Look at this Libtek 7kW
You can see they get close to 20 l/m in heating and theyâve cranked the DHW flow rate closer to 21 lm to ensure they keep close to DT5.
Howâs your flow rate configured? Auto on both heating and DHW? Or have they been fixed at a percentage?
Iâm wondering if youâve got some sort of blockage build up?
If I look at this zoomed out view of your system I can see 18.8 (1,128 l/h) back in January, which is closer to like a free flowing 7kW should be doing. (20 l/m, 1200 l/h)
Have anyone tied two mini stores, with one heated to say 40c as a preheater before a 2nd heated to 60c?
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You are quite right, the strainer needs cleaning out I think. 27th of Jan was the last time I cleaned it.
Before strainer clean:
after
after:
They look very similar. So not convinced it is having a big effect of performance.
I wonder what your thoughts are @Ivailo_Stefanov comment from another thread discussing my systems performance.
âThese so called hick ups are due to wrong setting of EEV after compressor modulation drop. I made a query in other thread to @Andre_K I think to monitor and log the EEV position at these points but didn`t recieve any feedbackâŠâ
When you look at the graph for a DHW cycle, could the huge sudden release of heat be the EEV valve opening fully?
No, that appears to be the end of the DHW cycle when the 3-way-valve switches to space heating again. The inrush of cold water from the heating circuit coupled with the hot water still in the pipes from the DHW cycle leads to this high power output. Itâs not like the heatpump compressor actually releases that much energy in this short time, itâs previously created heat that is stored in water that now appears as this power spike because cold water is rushing in and you have an accordingly huge dT.
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Ultimately, 10 minute hot water runs at 50C to 60C flow temps will always have terrible performance.
The Czech tables says a COP of 2 at 60C flow at highest compressor (Normal mode)
So youâre about on spec!
Links to Czech tables can be found here
To get better DHW performance
- Lower compressor (Eco rather than Normal)
- Lower starting temp (bigger hysteresis)
- Lower target temp
- Higher outside temp
Lots of hot water data and info here
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Hi James,
How did you go about getting a Heat Geek Mini Store?
Any further thoughts on its performance?
Hey Dan,
Mine was a heat geek install so they presumably they can order them directly.
Performance is decent in summer, but at the cost of efficiency. By decent I mean showers are hot and seem basically unlimited. Obviously a much easier job for the heat pump in summer with higher mains water input temp and air temp.
I run mine a little strangely in that I set the DHW temp to 65 for an hour at 4.30AM to make use of cheap energy tariff. I then set the DHW temp back to 55 for the rest of the day - occasionally I do the same when I know we will have a string of long showers/baths so that the heat pump starts working immediately instead of waiting for the sometimes slow to update DHW temperature sensor to update. If I donât do this we are often left with a lukewarm shower after about 10-11 minutes.
If I were to do it again, I would probably just get a normal cylinder - in summer its not much of a problem having ârandomâ DHW runs, in fact its quite nice as you have âon demandâ hot water. But in winter it can be sub optimal. Sometimes a DHW run will begin at a âbadâ time. For example, immediately after a defrost immediately followed by another defrost can leave your house without heating for a good amount of time.
Feel free to take a look at my system if you are interested in seeing more data.
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Thanks James.
Is it possible to specify when the heat pump can âdo DHWâ (i.e. heat the store), such as midnight to 4pm and 7pm to midnight? Thus preventing reheats during during the the Octopus Agile peak period. If so, can you programme different target temperatures in each range?