Boiler Problem

[anarchy]

Hi, is there anyone that can help me with my boiler problem please?

I have a Worcester Green Star 38 CDI classic boiler with hot water issues. When I turn on the shower the water fluctuates between hot and cold. The temperature goes up to 110 and then it cuts out - there are no error codes. The kitchen sink tap gets to 88, but it never cuts out. It's not the shower thermostat, as I've taken the thermostat out of the shower and ran the hot water but it still over heats. The heat exchanger and divertervalve are new; also a new flow sensor. It's not the pump, as the heating is fine.

As an after thought, I reduced the water pressure at the stopcock to nearly off - the temperature stays at 65 on number 4 dial.

Thank you for your time and I look forward to reading your answers.

 

[Madrab]

Sounds like there is a flow issue through the plate HEX. The boiler is overheating as it can't get rid of the heat in the system water quickly enough as it passes through the plate HEX and then it shut down. Once it cools it starts again and that cycle continues. Was it the plate HEX that was changed?

Whoever changed the plate (if it was the plate that was changed) and diverter needs to come back and fault find.

 

[anarchy]

Sounds like there is a flow issue through the plate HEX. The boiler is overheating as it can't get rid of the heat in the system water quickly enough as it passes through the plate HEX and then it shut down. Once it cools it starts again and that cycle continues. Was it the plate HEX that was changed?

Whoever changed the plate (if it was the plate that was changed) and diverter needs to come back and fault find.

Hi Madrab, thanks for getting back to me.

Yes the HEX is new. The new divertervalve came about because it was noticed that the central heating pipe was getting hot whilst running the hot water, so renewed at the same time as the HEX. Parts have been flushed, as we've gone on, just can't see or think where a blockage might be if one or what part to check next.

What would you recommend?

 

[Madrab]

Ideally, the way these work is that if the boiler cant transfer the heat into the water for4 whatever reason - usually the cold flow isn't enough - then the boiler will modulate down to maintain the HW set temp output.

The trouble starts when, as normal the boiler will fire on full rate when there's a HW demand - but if the system temp overshoots too fast then the boiler has to shut down. There is 1 fundamental reason for that - the flow through the plate HEX isn't correct and the system temp overshoots, causing the boiler to shut down.

The questions is then - what can cause the flow through the plate to slow down? The answer to that is normally 1 of 2 things - the Plate is restricted on the system side of things due to it being clogged up or the flow through the CW side is restricted. The latter is easy to see as that would be seen as a reduction in flow at the outlets. The former is normally down to either - blocked plate or pipework by dirty system water or a lazy pump. That is borne out by the system climbing to 110deg or even 88deg - it shouldn't be getting anywhere near that.

I would be tempted to put a cleaner through the system but firstly draining and inspecting a few bit'n'pieces involved with the HW delivery. I presume that the system was bad/black if the plate had to be changed? Does it have a filter fitted? BTW just because the CH is fine doesn't mean that the pump isn't to blame. How large is the system/rads?