Fault finding
Boiler fault finding practice: the call-outs
In short
Every call-out starts with the customer’s words. Confirm the complaint yourself, then follow the live from point to point with the meter: the fault lies between the last point with 230 V and the first without. Each page gives the symptom, what it already proves and the method. The simulator hides the fault until you prove it.
If this is your home: this page is written for heating engineers. Mains wiring and gas appliances are not DIY jobs; call a Gas Safe registered engineer. If you smell gas, leave the building and call the National Gas Emergency Service on 0800 111 999.
By symptom, in the customer’s words
No heating: hot water fine, radiators cold
“Hot water's fine but the radiators are stone cold, whatever I do with the thermostat.”
Hot water works; central heating does not.
S-Plan · Y-Plan · Combi
No hot water: heating fine, cylinder cold
“Heating's lovely and warm but there's no hot water - the cylinder's cold.”
Central heating works; hot water does not.
S-Plan · Y-Plan
Everything dead: no heating, no hot water, the controls dark
“Nothing works. No heating, no hot water, and the controls are dead.”
The whole system is dead - no supply is reaching the controls.
S-Plan · Y-Plan · Combi
Boiler will not fire: the controls work, nothing heats
“The programmer's on and I can hear things whirring, but the boiler never fires - no heating and no hot water.”
The controls respond but the boiler never fires, for heating or hot water.
S-Plan · Y-Plan
Radiators hot with everything off at the timer
“The radiators get warm when nothing's asking for heat - it's all off at the timer and they're still hot.”
The heating runs with no demand: something is opening the heating path when nothing asks for it.
S-Plan · Y-Plan
Boiler fires with everything off, radiators cold
“The boiler keeps firing with everything off at the timer, but the radiators stay cold.”
The boiler fires with no demand at all, and no zone is open: something is feeding its switched live directly.
S-Plan · Y-Plan
Hot water scalding: the cylinder keeps on heating
“The hot water's scalding - far hotter than it used to be, whatever the dial on the cylinder says.”
The hot water keeps heating after the cylinder is up to temperature: the hot water path never switches off.
S-Plan · Y-Plan
Heating stops once the cylinder is hot (Y-Plan)
“The heating's fine on its own. With the hot water on as well, the radiators go cold once the cylinder's hot.”
The mid-position valve does not sit where the demands say it should: with the hot water on and the cylinder hot, it stays at MID and nothing fires.
Y-Plan
Boiler fires, kettles and cuts out; radiators barely warm
“The boiler fires, makes a terrible kettling noise, then cuts out. The radiators hardly get warm.”
The boiler fires but the water is not being circulated: it overheats and locks out.
S-Plan · Y-Plan
Boiler overheats and cuts out; the pump hums, radiators cold
“The boiler fires, gets really hot and cuts out. I can hear the pump humming but the radiators stay cold.”
The pump runs but no water moves: the boiler overheats and locks out.
S-Plan · Y-Plan
Hot water only when the heating is on too (Y-Plan)
“We only get hot water when the heating is on as well.”
Hot water only heats while the heating is also on.
Y-Plan
Boiler cuts in and out on its own
“It keeps cutting in and out. Works for a bit, the boiler drops off, then it comes back on its own.”
The boiler drops out and comes back while the demand is steady.
S-Plan · Y-Plan
Two ways to practise
Fault Vault jobs
A fault set in a wired system and walked check by check, the way the DrawWarm Fault Vault walks it: see the fault for yourself, follow the checks with the meter, isolate and repair, then prove the repair.
Random Fault
One hidden fault at a time, at Easy, Medium or Hard. Find it with the meter, report it, score. Short, and a little gamey.
Confirm the complaint before you pick up the meter. Hints cost points and never name the part. Report the wire or part you have proven faulty, answer the question, and the round goes on your record.
When the customer is no help
Some call-outs come with words that are true and tell you nothing. Then you establish for yourself what works and what does not, before you pick up the meter.
- “It's just not right. My husband says it's the boiler.”
- “The radiators get warm when the heating is on.”
- “It was fine until the weekend. Can you just have a look at it?”
- “The house feels cold, and the smart meter says we're using loads.”
- Nobody home - the key was left next door. The note says: 'Heating engineer: please sort it.' Find out what does and does not work before anything else.
Read the explanation
- Last 230 V, first 0 V: the rule that finds a fault with three readings
Heating fault finding with a meter: follow the demand point to point, probe to a neutral, and the fault lies between the last 230 V and the first 0 V.
Practise it in the simulator.
Runs in the browser on a laptop or desktop. Cancel any time.