What it does
Connects the buffer to the heating expansion vessel so system-water volume changes can be absorbed without excessive pressure change.
Geothermal heating + cooling + DHW
Understand how the system works
The System map connects the ground loops, heat pump, buffer, domestic hot water and underfloor circuits. Select anything in the diagram for a plain-language explanation, operating context and live readings.
Animation
Select a component, pipe or connector to understand its purpose, operation and live readings.
Selected system element
Buffer port 1/top · expansion vessel
Home Assistant
What we can observe
Live readings show the current operating picture. Select a value for its physical meaning, related signals and history.
Buffer temperature
The main and top entities had the same 91-value sequence throughout retained history from 7-14 August 2026 and are treated as aliases. The wired probe beside BUF-3 remains the sole visible installed buffer sensor.
- Canonical buffer temperature used by the schematic; physical pocket mapping still needs a controlled check.Physical association unverified
sensor.cu401b_g_buffer_main_temperature - Mirrored API alias retained for transparency; not evidence of a second probe.Physical association unverified
sensor.cu401b_g_buffer_top_temperature
Technical provenanceConfidence, source evidence and remaining checks
Confidence
ConfirmedEvidence
Household physical recheck and IMG_20260814_113200926.jpg; quote 45 independently confirms a 40 litre heating expansion vessel.
Remaining checks
Confirm the tank manufacturer's nozzle function and vessel service/pre-charge arrangement. No pipe direction is implied.
Home Assistant · operating stories
System insights
Choose a question, then inspect the complete hydraulic and room response instead of one arbitrary time range.
Finding cooling performance episodes…
The useful mental model
Three separate fluids move heat through the system
“Primary” and “secondary” name the two sides of a heat exchanger. They do not mean first priority and second priority.
Primary / source loop
Glycol brine circulates through both boreholes. In winter it brings ground heat to the evaporator; in summer it carries room heat from the NC-Box back into the ground. The Vitocal primary-pump percentage is a source-pump command, not measured flow or available capacity.
Sealed refrigerant loop
When the compressor runs, refrigerant moves heat from the colder brine side to the warmer building-water side. This loop is internal to the heat pump and never mixes with either water circuit. Passive cooling bypasses it.
Secondary / building-water loop
The external Wilo moves clean water through the mixer, buffer and floor manifolds. It can keep delivering stored buffer heat with the compressor off. During passive cooling it moves floor water through the NC-Box instead.
NC-Box bridge
The NC-Box connects the two liquid loops thermally, but not hydraulically. Its heat exchanger lets emitter water give room heat to the brine while its pumps, mixer and safeties control the passive-cooling path.