Intermittent hot water combi boiler

[robbo83]

Hi,

I've got a Worcester Greenstart 36CDi combi boiler and we've got intermittent hot water at the moment. It happens when filling the bath in the shower and taps when run for long enough so looks to the boiler.

The day before it started we drained the system and removed 2 radiators as we are decorating and going to replace them, could this have caused iT somehow or is it just a coincidence?

Cheers!

 

[oldbutnotdead]

Err yes you could well have caused a problem.
Check the manual on your boiler- some combi boilers will operate dhw only with heating circuit drained but not all.

 

[robbo83]

Thanks for the reply. Sorry just to clarify the rad pipes are capped off and the CH system was repressurised, we haven't left it drained. We've done the same process a couple of months when replacing a rad in a different room.

 

[oldbutnotdead]

Ok. Have you bled the air out of the heating circuit (especially the boiler). Is there any circulation path available on the heating circuit from the boiler?

 

[robbo83]

I've bled all the radiators but not the boiler as the manual says it has as an Automatic Air Vent to release trapped air in the boiler.

I am not sure about the circulation path to be honest as I haven't got access to all pipework. If there isn't could it impact the domestic hot water?

 

[oldbutnotdead]

It could do. What idiot light indications are you getting on the boiler when you are running hot water and it stops heating the water?

 

[kidgreen61]

My bet is that your plate heat exchanger is blocked, and needs cleaning/replacing.

 

[terryplumb]

What exactly do you mean by intermittent ? Does hot water run hot / cold/ hot/ cold continually ,or is it the case that it runs hot and then goes cold and will not get hot again at all?
Or is it sometimes not getting hot whatsoever when tap is turned on ?

 

[CBW]

Does the boiler fire when you have no hot water?

 

[robbo83]

What I meant by intermittent is that if we turn tap/shower on it will get hot as normal and then after a minute or two it will go cold for roughly 30 to 60 seconds and then get hot again and then repeat. So if having a 10 min shower it will have gone cold 4-5 times during it.

Strangely we tried running the bath as a test this evening as I wanted to check if the boiler displayed any messages and the problem didn't happen. WTF.

We'll see how the showers are tomorrow morning.

Thanks for your help so far.

 

[foxhole]

Shower head Limescale ? Back pressure could slow flow to point boiler stops heating . Try running with shower head off to test .

 

[terryplumb]

Shower head Limescale ? Back pressure could slow flow to point boiler stops heating . Try running with shower head off to test .

Apparently the same issue arose whilst running taps ,which would rule out the shower head.

 

[robbo83]

That's correct the issue was occurring on taps also so ruled out shower head.

Had a shower this morning and was fine, didn't go cold at any point. Completely baffled now.

Is there a chance it could have been a temporary gas supply issue, maybe low gas flow from the provider?

Would you guys get an engineer to look at the boiler anyway and clean/replace the heat exchanger or would you wait to see if it happens again before doing so?

 

[CBW]

Could be a few things, diverter motor or paddle sticking, plate heat exchanger, flow turbine could have debris in it.

 

[Johntheo5]

I've asked this on a separate thread, on a combi boiler with DHW contro, if you set the DHW to say 40C and if the flow rate is reduced so that the temperature rises to say 50 C, does the burner trip out? I know they have a low flow switch but even if this is set to 3LPM, a 36kw combi will still flow ~ 14.7LPM from mains at 15C.