Upgrading your heating system for active coolingWhy active cooling is a system upgrade, not a single part
Fitting the coding resistor authorises a Vaillant aroTHERM or VWL heat pump to run in cooling mode — but authorising cooling and delivering it are two different things. To get useful, condensation-free cooling, the heating system itself has to be adapted: a dedicated fan-coil cooling circuit, condensate handling at every indoor unit, and controls that monitor the dew point. This page explains what that upgrade involves and why ordinary radiators are not the answer.
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The resistor authorises cooling — the system delivers it
The cooling resistor is the token that tells the heat pump it is permitted to cool. Once it is fitted and an installer enables cooling, the heat pump can chill the water it sends indoors. But chilled water on its own does not cool a home comfortably — it needs the right emitters, the right controls, and somewhere for condensate to go.
In practice, adding cooling to an existing heat pump is best treated as a system upgrade. The strongest general approach, per the research behind this site, is not to try to cool through conventional radiators, but to keep the heat pump as the hydronic plant and add a dedicated cooling-capable branch with proper condensate management.
Why radiators cannot cool well
Radiators are sized to give off heat by convection and radiation when filled with hot water. Run cold water through them and two problems appear. First, their surface area is simply too small to absorb much heat from a room, so the cooling effect is weak. Second, a cold radiator surface can fall below the dew point, and moisture from the air condenses on it — dripping water and risking damage.
Vaillant’s own documentation reflects this: for ground-source flexoTHERM systems, cooling mode with radiator heating systems is not permitted because radiators do not provide adequate heat-transfer area. The practical conclusion for any cooling retrofit is the same — radiators are for heating, not cooling.
A two-circuit hydronic arrangement
The robust answer is a two-circuit layout. Keep any standard radiators on a heating-only branch, and create a separate cooling-capable branch served by emitters designed to cool. When cooling runs, the radiator branch is isolated; when heating runs, the whole system can be used as normal.
Vaillant’s controls and indoor PCBs are built around this pattern — there are signal outputs that can drive a cooling-circuit pump or close an isolation valve for circuits that should not be cooled, such as a bathroom. Confirm the exact wiring and zoning approach with a Vaillant-approved installer against your installer manual.
- Heating-only branch — existing radiators, left out of the cooling circuit.
- Cooling-capable branch — hydronic fan-coil units sized for cooling duty.
- Isolation valves so non-cooled rooms are excluded when cooling is active.
- A control strategy that meets minimum-flow requirements in each mode.
Fan-coil units and condensate handling
The emitters that make cooling work are hydronic fan coil units. A fan coil passes room air across a chilled water coil, so it can move real heat out of a room and is comfortable to run. Good units publish both low-temperature heating outputs and cooling outputs, so they suit a reversible heat pump year-round.
Because a fan coil chills air, moisture condenses on its coil. Every indoor cooling unit therefore needs a condensate tray and a trapped drain to carry that water away safely. Condensate management is an engineering requirement, not an optional extra — it is the single most common thing a poorly planned cooling retrofit gets wrong.
Controls with dew-point monitoring
A Vaillant system control such as sensoCOMFORT (or the legacy VRC 700) manages cooling properly. It supports automatic and time-controlled cooling, and — importantly — it includes dew-point monitoring: it compares the minimum cooling flow temperature against the current dew point plus an offset, so the water is never chilled cold enough to cause widespread condensation.
Where rooms need separate zoning, Vaillant adds modules such as VR71 or VR70. The exact controls and minimum-flow rules for your home should be set by a competent installer; the principle is that cooling is only safe when the controls actively prevent condensation.
The layers of an active-cooling upgrade. The resistor is only the first line — the rest is what actually delivers cooling.
| Layer | What it does | Why it matters |
|---|---|---|
| Coding resistor | Authorises the heat pump to permit cooling mode | Without it, cooling cannot be enabled at all |
| Cooling-capable emitters | Fan coil units that chill room air | Radiators cannot cool effectively or safely |
| Condensate handling | Trays and trapped drains at each indoor unit | Chilled coils produce water that must drain away |
| Vaillant controls | sensoCOMFORT with dew-point monitoring | Prevents surfaces dropping below the dew point |
| Two-circuit hydronics | Separate heating-only and cooling branches | Lets radiators stay on heating, fan coils on cooling |
Frequently asked questions
Can I just fit the resistor and get cooling?
No. The resistor authorises cooling, but the system still has to be able to deliver it. Without cooling-capable emitters, condensate drainage and suitable controls, fitting the resistor alone will not give you comfortable, safe cooling.
Do I have to remove my radiators?
No. The usual approach keeps radiators on a heating-only branch and adds a separate cooling-capable branch with fan coils. Your radiators continue to heat the home in winter exactly as before.
Can underfloor heating be used for cooling?
Underfloor systems can sometimes provide gentle cooling with careful dew-point control, but it is a specialist design. Fan coil units remain the most reliable cooling emitter. Confirm what suits your home with a Vaillant-approved installer.
How big a job is the upgrade?
It varies with the number of rooms you want cooled and how easy it is to route pipework and drains. It is a proper installation project — expect emitter units, valves, drainage and controls work, not a one-hour visit.
Who should carry out the work?
A Vaillant-approved installer. They can confirm your model is cooling-capable, fit and commission the resistor, design the two-circuit layout and set the controls so cooling runs without condensation problems.