Why won't my Vaillant heat pump cool?

Reason 1 — no cooling plug fitted

On an aroTHERM Plus supplied without cooling, the most common reason is simply that the cooling coding plug is not fitted. Without it, the heat pump runs in heating and hot-water mode only, and the cooling option stays unavailable in the installer menu.

What to check: confirm with your installer whether a cooling plug is present. If it is not, that is almost certainly why cooling is unavailable. The genuine Vaillant part is 0020269259, boxed and labelled “Coding plug, cooling function”, or the identical 0020266328.

Reason 2 — cooling not enabled in the installer menu

Fitting the plug authorises cooling, but it does not switch it on. After the plug is present, an installer still has to enable the cooling function at installer level and commission it.

What to check: if the plug is fitted but cooling still does not run, the cooling function may never have been activated in the settings. This is an installer-level setting, not a user setting — ask a Vaillant-approved installer to confirm cooling is enabled and commissioned.

Reason 3 — the system has only radiators

A heat pump can authorise and even run cooling, but if the home has only standard radiators there is nothing that can deliver useful cooling. Radiators have too little surface area to absorb heat from a room, and a cold radiator can cause condensation.

What to check: look at what your indoor emitters are. If they are all conventional radiators, the system needs cooling-capable emitters — typically hydronic fan coil units on a dedicated cooling branch — before cooling will do anything useful. This is a system upgrade rather than a fault.

Reason 4 — controls not set to request cooling

Even with a cooling-capable system, the controls have to actually call for cooling. If the Vaillant system control is not in a cooling mode, or no cooling schedule or set point is active, the heat pump will not start cooling.

What to check: confirm the control — for example sensoCOMFORT — is set to allow cooling, that a cooling mode or schedule is active, and that the room set point is below the current temperature. Dew-point and minimum-flow limits can also hold cooling back in humid conditions, which is normal protective behaviour.

Reason 5 — a model not set up for cooling

Not every Vaillant heat pump uses the cooling plug. The plug is listed for the aroTHERM Plus range, VWL 35/6 to VWL 125/6; other models, especially ground-source units, may enable cooling differently or not at all.

What to check: identify your exact model from the data plate. A Vaillant-approved installer can verify how cooling works on your specific unit and serial number.

When to call an installer

Several of these checks — enabling cooling in the installer menu, confirming model capability, and designing a cooling-capable circuit — are installer-level work. If a quick look does not resolve it, a Vaillant-approved installer is the right next step.

They can confirm whether the plug is fitted, whether cooling is enabled, whether your emitters can cool, and whether your model supports cooling at all — and put right whatever is missing.

Frequently asked questions