Medium
F.78 on Vaillant ecoFIT sustain 630

Your boiler has lost communication with a hot water sensor linked to an external control, meaning it may struggle to regulate your hot water properly.

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

Interruption to DHW outlet sensor at external control

From this model's manual
Verified against the Vaillant/Vaillant Vaillantpro Ecofit Sustain System Installation And Maintenance I 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.

Engineer required

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

Find a Gas Safe engineer
Search the official register →
Boiler cover options

Avoid the next call-out bill

A cover plan would handle faults like F.78: no call-out fees, no surprise bills.

Approved partner
Domestic Appliance Guard
Unlimited call-outs, 24/7 helpline, no price-hike promise
Get a quote →

Affiliate links. We may earn a commission at no extra cost to you.

Related fault codes

F.77View →F.70View →

All 42 fault codes on the ecoFIT sustain 630

Seeing a different code? Every code this model can display, in one place.

ConnectionF.00F.01F.10F.11F.12F.13F.20F.22F.23F.24F.25F.27F.28F.29F.32F.49F.61F.62F.63F.64F.65F.67F.68F.70F.71F.72F.73F.74F.75F.76F.77F.83F.84F.85F.86F.87F.88F.89F.91F.97

Related guides you might find useful

No hot water but heating works

Almost always a stuck diverter valve on a combi.

Read →

Thermostat not working

Change the batteries first, even if the screen still works.

Read →

Is boiler cover worth it?

Break-even maths on monthly cover.

Read →