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Compressor and Evaporator Stratification

 
Air Source Heat Pumps (ASHPs)
Last Post by trebor12345 23 hours ago
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trebor12345
 trebor12345
(@trebor12345)
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Topic starter 24/07/2025 9:24 am  

I have had a few issues with my Hitachi spilt heatpump over the last winter.  Hitachi have now been to site and installed new software to fix some of the issues.

I have one issue that I would like to ask.  I appreciate that you will not know the specific Hitachi unit, but I think this could be generalised across different brands.

So the issue. On occasions following a short cycle or off, I see a 100m high area on the evaporator coil that is very cold and wet to the touch and the rest of the evaporator is cold and dry.  I have been given the following explanation as to why this is:

The lowest section of the evaporator being cooler during first start up.

The reason is that during compressor stop the liquid refrigerant settles at the lowest point, (in this case the evaporator lower section) it won’t settle in the compressor because the crank case heater is designed to keep the compressor warm to avoid this. So when the system first starts up, until the refrigerant has started circulating around, the bottom section of the evaporator would be observed to be cooler than the top.

Does this sound a reasonable and valid explanation?

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

2024 build bungalow
Southern england
179 m2
High level of insulation
Underfloor heating
All 12 circuits are fully open all the time
1 thermostat in family room
7KW heat pump
50 litre buffer tank (4 port)
3.6KW solar panels
Energy used by heating 2527 KWh - 7527 KWh (SCOP 3.5 approx)


   
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