Potterton Profile Netaheat overheat stat tripping only on hot water setting?

[retrogeezer]

Evening folks, hopefully someone can help me with this issue. I have searched already for overheat stat problems but can't see anything the same as the one I have.

We fitted a new pump last weekend as the old one was noisy and had started to leak, system was drained down and new gate valves also fitted. System all started up fine, rads bled etc and all working as they should. The system all worked perfectly last week until yesterday morning when the water was set to come on with the timer.

We got up to warm water a tripped overheat stat but with the water and heating then put on for an hour there were no problems. Fine in the evening as well then again, this morning we had the same issue.

So it seems if I have hot water and heating on at the same time we have no problems but with just the water on the overheat stat keeps tripping. Boiler stat set on 3, cylinder stat on 60.

What could be the problem? Thanks in advance.

 

[picasso]

Sounds like the cylinder coil is air locked, is there any way of bleeding it ?

 

[retrogeezer]

I've no idea? What should I look for? There are 2 bleed heads on pipes near the tank and I've bled those.

 

[picasso]

post a pic of the cylinder pipework.

 

[retrogeezer]

 

[retrogeezer]

 

[retrogeezer]

Hope that worked? Also, should that bypass valve be open or closed? It's a vented system with TRV's on all but one radiator.
*edit* After a bit more googling I think the bypass valve should be almost fully closed? It was probably half open so would that possibly cause the problem? i.e, sending the just heated water from the boiler straight back into it instead of going into the cylinder coil?

 

[picasso]

Did you get any air out of the bleed valves ? they can get blocked .

 

[retrogeezer]

Which bleed valves? Rads or those 2 tank ones? I've bled the rad ones a couple of times, 2 had a fair bit of air in still the 2nd time. Only a tiny bit of air from those tank ones though.
Did you see my question above about the bypass valve? Thanks for your help so far.

 

[picasso]

The bypass is not needed if you have a rad without a trv, close it completely and see how the system runs, lack of circulation is causing the boiler to trip so most likely culprit is air in the cylinder coill.

 

[retrogeezer]

The bypass is not needed if you have a rad without a trv, close it completely and see how the system runs, lack of circulation is causing the boiler to trip so most likely culprit is air in the cylinder coill.

Thanks Picasso, a bit of improvement. I wound the bypass valve shut last night and then backed it off about 1 turn. This morning we have much hotter water, probably as hot as it should be but the boiler had still tripped when I checked it. Obviously it had been running for longer before tripping though. I will shut the bypass valve completely and keep my fingers crossed.
So how can I bleed air from the coil? Thanks again for your advice/help.

 

[picasso]

Leave it for the time being and see if closing the bypass has shifted the air.

 

[ianmcd]

When bleeding everything make sure the pump is not running as you bleed, is the replacement pump set at the same speed as the older one ?

 

[retrogeezer]

When bleeding everything make sure the pump is not running as you bleed, is the replacement pump set at the same speed as the older one ?

Will do. Pump on speed 2 like the old one but the old one was really knackered.

 

[Alec_t]

Is that an air-bleed point just behind the Honeywell valve?