Medium
D.28 on Vaillant ecoTEC System

Your boiler is having trouble communicating with an external control or smart home accessory, which might prevent your heating or hot water from turning on correctly.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

What the manual says

Manufacturer manual

Switching accessory relay 2

From this model's manual
Verified against the Vaillant Ecotec installation & service manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

What causes this fault?

This fault occurs when the boiler's main internal computer loses communication with an external control box or smart home wiring module. The most common reason is a loose connection in the wiring or a technical failure within the accessory module itself that tells the boiler when to turn on and off.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

D.27View →F.27View →F.70View →

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