Heating fine, hot water cutting out

[Spiderpig]

Hello, first post, nice to be here!
Got a Potterton Netaheat Profile 80e conventional boiler. The heating works fine with the boiler thermostat turned right up (5 or Max). But if the thermostat is above 2 then the boiler thermal trip activates when heating water for the cylinder. It works ok for both heating and water set to 2 but there's obviously a problem on the hot water side and getting worse 'cos it used to work fine at 3 or 4. Any advice please? I've read around a bit - could an airlock in the cylinder cause this?
Thanks!

 

[bengasman]

... But if the thermostat is above 2 then the boiler thermal trip activates when heating water for the cylinder.

Could you expand on that?

 

[Spiderpig]

Sure, I mean the easily accessible reset-button on the underside of the boiler trips. 'Overheat limit thermostat' as it says in the manual. If the boiler thermostat knob (the only control on the boiler) is set to 2 or less then the reset button doesn't trip. This is only a problem on the hot water side, not heating.
Thanks!

 

[Spiderpig]

Probably should mention I have a water cylinder thermostat fitted which is currently set on low temp as there seems to be an issue with the water overheating at the boiler end. Don't know if that's relevant.

 

[Agile]

Most overheating is associated with a lack of flow rates.

There could be valve or other restriction on the heating circuit.

The first place to check is the header tank is full and the feed pipe from it is not blocked.

See sticky FAQ thread.

Tony

 

[Spiderpig]

Thanks for that.
The header tank is full but a bit murky. The feed pipe doesn't look obviously blocked but could be restricted. Does the hot water require a much stronger flow than heating?
I see in the faq I can rig up a reverse flow to try to blast muck back into the header tank so I could certainly try that. Is it likely that any chemical treatment would help instead? There hasn't been anything added since a dose of Sentinel X100 some years ago.

 

[Agile]

Take out 3 litres and see if the header goes down!

Is there any air bleed vent at the cyl heating coil?

Tony

 

[tomplumb1]

I had a similar problem with a customer a couple of months back – taking some water out done the trick!

 

[Spiderpig]

Um, Do you mean drain from a rad drain point or just scoop out of the header tank? Just did the latter and the header tank refilled ok but realised I'm trying to prove that the water is leaving the header tank (into the system) right?
There was a bleed valve on the cylinder inlet. No obvious air came out, just water. But, but, but.... the boiler does seem to be heating the water now with the thermostat on 5, which it hasn't done for months... hmmm (need crosses fingers smiley).

 

[Agile]

Take three litres out of a heating drain point dowmstairs.

See if header tank goes down!

Tony

 

[Spiderpig]

Thanks Tony, that does make more sense
Boiler did cut out again, set at 5, as expected.
This is where I wish I had any drain points, but sadly not. Guess I can just let water out of a rad bleed valve. Or is that too slow a flow to prove anything?

 

[Spiderpig]

Pretty sure I've found the problem (should check the simple things first!) The Hot water cylinder heating circuit inlet gets hot but the outlet takes 2 hours to warm up (along with the water in cylinder) so I suppose there is a restriction in the cylinder coil.
Any clever ways to clear cylinder coil without (I suppose) disconnecting and pushing a flexi brush through?
Could try the reverse-flow shown in the faq (BLOCKED C H COLD FEED)?
There is still a little flow so is it worth running X400 or X800 through the system?
Thanks a lot!

 

[Agile]

The first place to check is any valves in the HW circuit.

To use X800 close a rad valve on each and then remove most of the water from header tank. Add X800 and add about 3 li of water. Put HW on and drain about 3 li from a rad etc. then run HW for a few hours with cyl stat turned up a bit to keep it on.

If you can find any better way to get the X800 in then even better.

If the cyl coil seems to clear the run for another hour and then open the rad valves an let it circulate through the rads for a few hours before draining and adding inhibitor.

In my experience most coil blockages are at the input and best cleared by opening the connection and using a knife to clear it.

Tony

 

[Spiderpig]

That's great, thanks Tony. I'll let you know how it goes!

 

[Spiderpig]

Well, I to fitted a drain point to downstairs rad (used a Drain Easy kit and builder's rubble bag, both great tips I read on here). Only took four hours That was Saturday and the heating was bled and working again so no grief from the family. My first bit of central heating plumbing, so pleased with myself.
Sunday I drained a few litres and put in two packs of X400 (couldn't get X800). It's circulating now. Hoping the restriction in/near cylinder will clear over the next few days or weeks.