Shower runs hot then cold then hot......

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Chemical clean is just upsetting the apple cart. Powerflush is the way to go in most cases (not all)
 
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I think new boiler warranty demands a power flush/chemical clean before installation, from (horror) posts I've read on here, it has to be properly carried out with multiple flushings and water testing so a good idea to do it before removing the old boiler, but will the new boiler warranty be valid if you then run the system with the old boiler for a few days/weeks before new boiler installation?. I presume that the normal method is to do the whole operation in one, ie, remove old boiler, clean the system, then install new boiler. Chemical cleaning is probably "worse".
I was going to use a Worcester accredited installer (the same one my daughter used) and I'm pretty sure he power flushed the system prior to installing the new boiler. On a different point (and as I'm still paying for Worcester Bosch boiler insurance) I've arranged for one of there engineers to attend. Will be interesting to see if he says that its just the poor turndown ratio that's causing the issue and if so if there is any solution.
 
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Your Hot Tap tests will be interesting, measure your cold mains temp as well.
Hi Jonntheo5 - I never got around to doing the additional tests, however a Worcester Bosch engineer came today and changed the flow switch (thankfully I have a maintenance contract) - problem cured. I ran the shower before he changed the flow switch and you could see that it wasn't operating at low flow rates. New switch operated perfectly.
 
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I'd say Nigel probably just the unusual combination of the poor turndown to 12.8kw plus the high mains temperature plus the poorish shower flowrate. There doesn't seem to be any filter on the boiler mains but I thought there may be hot and cold ones somewhere on the shower inlets.
It was also good to get an engineer that really knew his stuff. Every question I asked him he was able to answer fully. Just looked up a new flow switch and its around £135. I think its time for a new boiler........ (that was his recommendation as well).
 
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Very good to hear that Nigel.

Are you saying that you can now get a showering temperature as low as 35C without burner cut out?, if so, with a (my) present mains temp of 19C then shower (mixed) flowrate must be 11.38LPM to satisfy a minimum boiler output of 12.7kw, you were getting a shower flowrate of ~ 9LPM so the boiler minimum output must now only be 10.05kw or else your flowrate has improved.
 
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Very good to hear that Nigel.

Are you saying that you can now get a showering temperature as low as 35C without burner cut out?, if so, with a (my) present mains temp of 19C then shower (mixed) flowrate must be 11.38LPM to satisfy a minimum boiler output of 12.7kw, you were getting a shower flowrate of ~ 9LPM so the boiler minimum output must now only be 10.05kw or else your flowrate has improved.
So Its interesting. We ran the shower for nearly 10 minutes and the boiler kept running without cutting out but was modulated to (presumably) its lowest setting. I haven't rechecked the flow rate but it doesn't seem any better. However after my last post my wife took a (longgggg) shower and she said that at the end it still ran cooler for about 30 seconds. So maybe not totally solved but definitely much better. I am going to go ahead and order a new boiler. I will contact Wessex water to see if there is anything they can do to improve flow rates (suspect not) and consider getting one of the mains pumps although think they are limited to 12 l/m so maybe not worth it,
 
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Your new boiler will be able to modulate far more than your old one, if you are happy enough with your present shower flowrates then no need to spend more money, from now on anyway as mains temperature falls your old one should do the job fine as each 2deg fall in mains temp means 1.25kw more boiler output required at 9LPM.
 

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