Medium
30 on Alpha Cd50s Cd70s

Your boiler has detected that the water leaving it is significantly hotter than the water returning, suggesting a circulation problem in your heating system.

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

Temperature difference between system flow and return too high

From this model's manual
Verified against the Alpha Cd50s Cd70s installation & service manual, page 30.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

DIY fix possible

Check that all radiator valves throughout your home are fully openEnsure your central heating pump is operating and not seizedCheck for any blockages or airlocks in your system that might be restricting flow

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 30: 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

28View →

Related guides you might find useful

Banging / kettling / whistling noise

Each noise points to a different fault.

Read →

Radiators cold at the bottom

Sludge build-up — needs flushing, not bleeding.

Read →

Is boiler cover worth it?

Break-even maths on monthly cover.

Read →