Baxi 105e unstable heat on DHW

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Small issue with DHW on Baxi 105e combi. Recently hot water has been hot to like warm. Seems flame heat is dropping on and off. HEX was cleaned 18 months ago and after reading through many forum posts seemed seemed likely cause. New HEX fitted and diaphragm service kit as was letting by. After all was changed still same issue, water heats to what I assume is temp and then flame cuts , temp drops and flame starts. Resistance across DHW thermistor at cold 11K and when hot at 70 degrees 5.5K. Diverter flow pipe for CH not getting hot so that side seems ok and CH is working fine. The issue happens on all taps showers etc so assume not related to shower valve . Could main HEX be an issue as system has been in many years and to my knowledge never had any inhibitor added . ? Any additional suggestions would be appreciated. Thx
 
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The min output of that boiler is ~ 11kw so if the DHW thermistor is reading the temperature correctly at 70C then the HW flow rate would need to be as low as 3.2LPM from cold at 20C (now). If the primary heat Hx is OK and it does as the CH is OK then its more than likely that the primary side of the plate Hx is also OK, clean. So that leaves the plate Hx, and if the flow rate from that is > say 5LPM then no problems there?., maybe points to diverter valve or DHW thermister faulty?. or PCB board?.
 
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get someone to check the high and especially the low gas pressures on the gas valve if the low is set to high then the boiler will not modulate down correctly allowing too much heat into the hot water thus it cuts outs out
 
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Open every hot tap in the place + shower to max temperature and see if the boiler stays firing.
 
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JOHN, So opened 4 sinks , shower and bath on max flow. Boiler fired and stayed fired .. Think you have something in your method ..
 
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Very good, do the same again, then shut off the shower first, then progressively shut off one sink at the time and see how many are required to keep it running, whatever that number is, say 2, measure their combined flowrate by using a container and letting the tap(s) run into it for exactly 30 secs, measure this with a one or two litre milk bottle/container, X2 to give the flowrate in LPM. If the boiler is firing flat out at 30kw then the flowrates to give 30, 40, 50 & 60C DHW temps are 43LPM, 21.5LPM, 14.33LPM & 10.75LPM, ensure the DHW temp control is turned to max. Keep a eye on the LED temperature indications while doing the above, they might just show the primary (and not the DHW) temperatures but will be of some help.
 
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Strangely I have never tested with heater control at max . Here's the thing , with heat control at max I tested kitchen sink which was 4ltr at 70 degrees indicated over 30 seconds. Also did cloakroom which is farthest 5ltr again at 70degrees over 30secs . Lastly tried shower with thermostatic valve on sold hot. Difficult as large rain shower head, but same result or slightly over to kitchen as couldn't quite capture all water ..
 
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4LPM will require a boiler output of 14.51 kw and 5LPM will require 18.14 kw, to give 70C from (my) mains of 18C.
Not sure if boiler continues to fire after this 30secs? If not then boiler not modulating properly or not fast enough after ignition.
If it is continuing to fire then the lowest achievable DHW temp is 57.4C at 4LPM and at its minimum output of 11 kw.

If you run the rain shower at a normal showering temp of say 38C it will need to flow a minimum 7.9LPM to give 38C when water at 70C is mixed with cold mains at 18C, boiler at minimum output of 11 kw.
 
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John, a little lost and trying to get my head round your comments, however you mention LPM . My figures are over 30 seconds , so assume X2 which for most part gives 8 LPM .. Apologies if I'm being a tad ignorant and not fully understanding , but assuming LPM is x 2 from my earlier post , how does that work for my situation and what if any steps would be recommended .??? To add please don't think I am being ignorant to your comments , just trying to get my head around it , and do indeed appreciate your time on this ..
 
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Apologies, I assumed wrongly that the numbers were LPM was wondering alright how a rain shower was only piddling 4LPM!!

OK, 8LPM at 70C requires 29 kw almost full boiler output.
Is the boiler continuing to run??, if so, then assuming it can modulate to 11 kw then the minimum temperature achievable is 37.7C. at 8LPM which means that you will have to open any HW tap fully otherwise the temp will start rising and eventually trip the burner, assuming DHW temp set to 38C.
Would suggest setting the DHW temp to 50/60C, 50C will then require 17.9kw at 8LPM and the flow rate may be reduced to 4.9 LPM at a minimum boiler output of 11 kw.
 
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Ok now I sort of understand where we are. Here is my dilemma. Boiler has been working 100% for last 2 years since last issue. Temp control has been set half way over that period (2 years) . Taps in general and shower are always opened fully , yet now all of a sudden shower goes hot/cold and same for any tap opened. With temp control now at 50% water gets extremely hot and then flame cuts. Working on your calcs seems I have to run temp very hot as 60 degrees displayed will easily scold hands .. in short not sure what's changed and now when I try and run your settings (37.7c or above up to full tilt) flame still cuts .. just a tad bewildered .. seems anything below 75% on heat control and flame cuts . If I run 75% heat control or above I'm running on excessive heat , more gas , more expense . Not sure if correct, but that's my logic ..
 
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Thx gaz much appreciated. New plate hex , thermistor seems to check ok, but maybe failing under load.. very frustrating I must say ..as was working fine up until a couple weeks back..
 
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Well, at least its narrowed down a bit?, IMO. unlikely blocked PHE as hot water flow seems OK and boiler output based on one tap measured flow and temperature 29kw.
If one accepts that the minimum achievable HW temperature is 60C at a flowrate of 8LPM then the boiler output has to be 23.4kw, this can be checked quite accurately by taking the gas consumption from the meter over say a 3 min period, once this is ruled in or out then time to call in the experts?.
Gas consumption over a 3 min period should be about 0.125m3.
 
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John , again thx for your input , learnt a thing or two , strangely interesting .. either way experts or other , I will update as things hopefully progress .
 

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