Cooling upgrade
Two-circuit heating and cooling systems
If your home is heated by radiators, the robust way to add cooling is not to chill the radiators — it is to keep them for heating and add a separate fan-coil branch for cooling. This is the two-circuit approach.
The two-circuit idea
A two-circuit system keeps your existing radiators on a heating-only branch, and adds a second branch of cooling-capable fan coil units. The heat pump feeds both. In winter it sends warm water to whichever emitters are heating; in summer it sends chilled water only to the fan-coil branch.
This keeps each emitter doing what it is good at: radiators heat, fan coils heat and cool. Nothing is asked to work outside its design.
Why you don't cool the radiator circuit
Sending chilled water to ordinary radiators causes two problems. First, radiators are ineffective at cooling, so the room barely changes. Second — and more seriously — their cold surfaces, and the cold pipework feeding them, drop below the dew point and collect condensation. That means damp walls, floors and pipe runs.
Isolating the radiator circuit when cooling is active avoids all of this. The radiators simply sit idle in summer, as they would anyway.
How the cooling branch is controlled
The cooling branch has its own pump and valves so it can run independently of the heating circuit. Isolation valves keep cooling out of rooms where it is not wanted — a bathroom, for example. The Vaillant controls decide when to call for cooling and hold the flow temperature safely above the dew point.
A competent installer balances the system so each fan coil receives the flow it needs, and so the heat pump always has a clear path for its minimum flow.
Retrofitting a two-circuit system
On an existing home this is real plumbing and electrical work: new pipe runs to the fan coils, condensate drains, valves, wiring and controls. It is well within the scope of a competent heating engineer, but it is a designed installation — not a plug-in accessory. A Vaillant-approved installer should size and commission it.
The cooling resistor authorises the heat pump itself to run in cooling mode.