When your gas water heater suddenly won't ignite, your wall-hung boiler's heating cuts in and out, or your shower water goes hot and cold... many people's first reaction is to dig out the repair hotline.

But after the technician arrives, more often than not, you find out: a seal replacement cost over a hundred yuan, a clogged filter was diagnosed as a faulty motherboard, or simply dead batteries led to hours of needless fuss.

The reality is often: small problems get exaggerated, a tiny part replacement can set you back several hundred yuan, or it's just a dead battery or a clog, but they tell you the motherboard is broken.

We've put together a common fault self-diagnosis checklist covering forced-draft/constant-temperature/zero-circulation water heaters, standard wall-hung boilers, and condensing boilers. Run through the steps first — you'll likely avoid an unnecessary service call fee.

Safety Golden Rules (Must-Read Before You Do Anything)

Before you start, always take these three steps:

Let the unit cool down before operating — don't take shortcuts.