Issues with boiler heating water in cylinder

[fartingspartan]

Hello,

Appreciate any thoughts on an issue I am having as I've run out of ideas.

Firstly have an oil boiler which runs heating through radiators and heats water in a unvented indirect cylinder.

Cylinder also has immersion option which i am getting by on at the min.

Essentially went for a shower few days ago and noticed water was a bit cool.

Boiler is set to heat water for hour in morning and hour at night.

Went to boiler and boosted water through hive app.

Boiler fired up, port valve opens and hot water starts to flow to cylinder (feel pipes) - same with return from cylinder.

But boiler only stays on for 3-4mins then Boiler turns off - almost like it thinks water is at temp.

At this point Boiler will not fire up again.

I thought it might be faulty thermostat on cylinder so replaced this including probes - no difference in behaviour.

I did notice a drip coming out of tundra but not like flowing and prob only once every 5 mins you get a single drip.

Heating works fine.

Could HW issue be anything to do with drip? Should I check expansion vessel pressure?

Immersion side works fine.

Appreciate any advice.

 

[Participant]

Probably a depressurised expansion vessel. Any water going past the tundish is abnormal and points to a fault.

What I can't understand though, and I'm no expert, is why the boiler stops after a few minutes, as with our unvented, the boiler kept firing until hot water was absolutely spewing past the PRVs and tundish. Maybe it's just hitting the required temperature really quickly as there's no expansion take up so the temperature increases rapidly.

What happens if you increase the set water temperature in the cylinder? Does the boiler fire for longer?

But yes my guess is faulty and / or depressurised expansion vessel.

 

[fartingspartan]

Probably a depressurised expansion vessel. Any water going past the tundish is abnormal and points to a fault.

What I can't understand though, and I'm no expert, is why the boiler stops after a few minutes, as with our unvented, the boiler kept firing until hot water was absolutely spewing past the PRVs and tundish. Maybe it's just hitting the required temperature really quickly as there's no expansion take up so the temperature increases rapidly.

What happens if you increase the set water temperature in the cylinder? Does the boiler fire for longer?

But yes my guess is faulty and / or depressurised expansion vessel.

Thanks for the reply, yeh like you I thought a pressure issue wouldn't create the behaviours I'm seeing in the boiler.

I checked expansion cylinder and it was a little lower than it should be so pumped it up but still same behaviour.

Immersion heater works fine so again it's odd as if there was anything outwith the boiler you would think it would have same effect in Immersion operation.

Beginning to wonder if it's something in the boiler related to hot water... but my basic understanding is the heating and hot water operations around the boiler is the same (flow/return and 2 port diverter) so I'm stumped as heating works fine.

Going to maybe have to admit defeat and call my boiler engineer, he is due March to service boiler but might need to bring it forward.

 

[henry hoov3r]

What boiler is it? Is the heating working ok?

 

[fartingspartan]

What boiler is it? Is the heating working ok?

Can't remember the name of the boiler(will check when I get in) and yes heating is working fine.

 

[Calpol]

Possible that the thermostat on the cyclinder is faulty?
Immersion is basically on/off AFAIK but normally the temp of the HW is set by a stat.