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Air Source Heat Pumps (ASHPs)
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Topic starter
26/05/2026 3:03 pm
Today we start the replacement of our 2-year-old split 7kW Hitachi Heat Pump. Being replaced with a Vaillant aroTHERM plus 3.5kW. The unit is being replaced because it’s over sized for the property and has so many quirks:
- Requires constant management, which a non-technical person could not to manage
- Performs defrost even at OAT 15C
- Poor COP during defrosts and cold periods
- Defrosts every 35mins irrespective of evaporator or outside temperature
- Erratic performance, have to turn off to reset
- Split system not suitable for this property
- HP is operating close to its minimum operating performance characteristics
- Short cycling is managed by a SW fix which monitors temperature Flow Temp and waits for a 2C drop before restoring heat.
As Mars would say “Its like managing a patient on life support”.
Today’s activities:
- Returned R32 refrigerant to HP
- Removed Heat Pump and Internal Water Storage unit (125Kg)
- Hole drilled in slab for defrost drainage
- Vaillant HP and Water Storage placed
2024 build bungalow, Southern England, 179 m2, 14w/m2
Underfloor heating, fully open
Vaillant aroTHERM plus 3.5kW
50 litre volumiser
3.6KW solar panels
Tesla 13.5kw Powerwall
26/05/2026 3:14 pm
Great news that work has started but...is that a 4 port buffer I see before me, its auxiliary pump to the right? If so I strongly recommend to get it replumbed as a volumiser. Your UFH is all the buffer needed and more besides!
This post was modified 3 months ago by JamesPa
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.
Topic starter
26/05/2026 3:17 pm
Great news that work has started but...is that a 4 port buffer I see before me, its auxiliary pump to the right? If so I strongly recommend to get it replumbed as a volumiser. Your UFH is all the buffer needed and more besides!
Yes to be configured as a volumiser. The pump is to be removed.
2024 build bungalow, Southern England, 179 m2, 14w/m2
Underfloor heating, fully open
Vaillant aroTHERM plus 3.5kW
50 litre volumiser
3.6KW solar panels
Tesla 13.5kw Powerwall
1
Topic starter
27/05/2026 9:11 pm
Today. Just the copper pipe work around the cylinder.
2024 build bungalow, Southern England, 179 m2, 14w/m2
Underfloor heating, fully open
Vaillant aroTHERM plus 3.5kW
50 litre volumiser
3.6KW solar panels
Tesla 13.5kw Powerwall
1
28/05/2026 8:38 am
@trebor12345 Nice use of spare ports on the buffer turned volumiser to connect various bits!
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.
1
Topic starter
29/05/2026 6:17 pm
Job completed today. I believe I am happy with the quality of the install.
2024 build bungalow, Southern England, 179 m2, 14w/m2
Underfloor heating, fully open
Vaillant aroTHERM plus 3.5kW
50 litre volumiser
3.6KW solar panels
Tesla 13.5kw Powerwall
1
Topic starter
29/05/2026 7:00 pm
Post deleted, my error.
This post was modified 3 months ago 7 times by trebor12345
2024 build bungalow, Southern England, 179 m2, 14w/m2
Underfloor heating, fully open
Vaillant aroTHERM plus 3.5kW
50 litre volumiser
3.6KW solar panels
Tesla 13.5kw Powerwall
29/05/2026 7:00 pm
Also not bad in terms of duration, showing that in case of a failure of a heatpump, a new one can be swapped into position in a few days..
8kW Solis S6-EH1P8K-L-PLUS hybrid inverter; G99: 8kW export; 16kWh Seplos Fogstar battery; Ohme Home Pro EV charger; Vaillant Arotherm Pro 7kW; 100Amp head, HA lab on mini PC
Topic starter
29/05/2026 7:07 pm
Also not bad in terms of duration, showing that in case of a failure of a heatpump, a new one can be swapped into position in a few days..
4 days, but it was an easy install for them.
2024 build bungalow, Southern England, 179 m2, 14w/m2
Underfloor heating, fully open
Vaillant aroTHERM plus 3.5kW
50 litre volumiser
3.6KW solar panels
Tesla 13.5kw Powerwall
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