Boiler Help

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Ok next problem ...
I have a Worcester 24 CDi boiler combi
The hot water is a problem, for example, if I run a bath, and put the hot tap on full, it used to restrict the flow depending on the temperature set on the front panel of the boiler.
But ohh no it doesn't, it used to, but now I get a mega blast and a bath full of warm water. The knob on the front does not change the flow rate/temperature.
I have been onto bosch about it and they recommend replacing "Domestic Hot water thermister part number 8716142302" Now I am not an expert, but this I believe is the component strapped to the hot water exit pipe, it has some heat transfer compound and two yellow wires connected to it. I have taken it off and had a look, and reconnected it. Now assuming I have the correct component, what are the odds of it failing????, can I measure the resistance across is as the hot water flows and heats up the sensor/pipe.
Any advice would be most grateful
Many Thanks Phil
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The part on the exit pipe is only the sensor for the thermyster in the boiler and some have this on the main circuit board. There should be two sensors connected across the thermyster, one checks the internal temperature of the water entering and one checks the exit pipe temperature. Between the two points there should be the set difference. If this is how your boiler works, then it is the whole lot you'll need as they are normally factory calibrated for each boiler model.
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There are two sets of wires going into to the bottom of the boiler bit, these are the boiler in and out sensors, these, I think, are ok as the boiler for central heating is fine. The thermistor connector to the hot water exit pipe, is different and I think this is what boshe are on about??

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But the part on the pipework should only be the sensor and not the actual activator. Try removing the sensor from the pipe and switching the boiler on to fire up. Place a lit match or cigarette lighter under the sensor, mind the wires and your fingers, and see if it shuts down the boiler due to overheat, or if it keeps the boiler burning due to sensing the heat. If it keeps the boiler burning, then it is doing its job OK.
Now leave the sensor off the pipe and get the boiler to fire up again. If it doesn't fire up, then the sensor is working OK and the problem is somewhere else on the boiler.
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I could help, but I ignore top posters
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geoff

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writes

Sorry geoff
Didn't mean to offend.
Phil
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No probs - I think you'll find that the thermistors are about 10k at room temperature. The resistance reduces with increasing temperature, normally down to about 3-5 k at max boiler temperature.
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Thanks geoff
I will remove the thermistor and check it with a hairdryer or something, thing I am wondering at the moment will this reduce the flow of hot water??, if so how does it do it?? If I connect a 3-5K resistor to the wires, this will fool the boiler in thinking very hot water is flowing, I will have a look again.
Any other ideas, please let me know
Many Thanks Phil
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resistance. I've not tried testing a pcb with a thermistor working in a discontinuous way, but I think that it would recognise the step function and decide that something's wrong.
What you want to do is first test the resistance of the thermistor over temperature (at the point where it comes into the pcb, so you are testing the wiring too).
If the resistance drops from 10 k to <about 5k, the odds are that the thermistor is OK and your next point of call is the pcb...
This is where I have to do a bit of self advertising
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geoff wrote:

Does this boiler actually control the flow rate of DHW from the PCB or is the flow [no longer] restricted by some other valve which has moved?
Sounds like the thermistor is OK. One of the ones at at the top of the boiler is the overheat cutout and is likely to be a simple fixed temp normally closed thermostat in a 'tin'. Butthe problem does not lie there.
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As far as I can remember, there is no mechanism for controlling the flow rate

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geoff wrote:

I can only think that there is some external reason why the boiler flow has suddenly increased. If you can slow the HW down to a reasonable flow rate and it then comes out hot enough then: I suggest you add some sort of restriction on the flow to brings things back under control. Perhaps the Water Co. have improved the pressure in the area?
Note that some boilers have an adjustable restrictor (usually on the the CW inlet) to do exactly what you require.
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In that case I'll lend you a tenner, but then since you ignore top posters........ you're short on luck I guess! ;O)
No, I don't like them much either, but we can all forget once in a while!

Take Care, Gnube {too thick for linux}
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Phil
Temp. set on front panel only alters max. temp.of hot water not flow rate.
Remember as winter comes the water inlet temp. will fall. To maintain high outlet temp. you may have to reduce the flow rate.
Hot water thermistor cannot be tested for resistance with the power on to it. Run water through boiler with boiler turned off - take resistance reading cold.
Reconnect wire/wires removed for above test and run appliance hot as normal - turn off power to boiler again, remove wire(s) and re-test while still hot before boiler cools - note resistance reading.
From memory expect somewhere around 3K ohms hot, down to 500 ohms cold. Generally a faulty one will be miles out.
Hope this helps
Skip
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500 ohms cold? I don't think so
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Thanks for your time Skip,
I hear what you are saying, but the first two years of the boiler, the hot water used to come out of the hot bath tap, as a slow to normal speed, piping hot. Now it blasts out, just like the cold tap, and is luke warm. I did have a closer look at the boiler gas jets(is that the term) and they do alter with the dial on the front of the boiler, but it is coming out of the hot tap as the same force as the mains in.
I will disconnect the thermistor and connect a ohm meter to it, then heat it up with a hair dryer and check resistance levels etc.. Maybe this is at fault after all. Only thing is, it may be the Overheat sensor that shuts down the boiler, now I am confused??
Any other help anybody
Thanks in anticipation Phil
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Imagine passing your hand through a candle flame really fast........you won't burn. Now pass your hand through the flame slowly........it's red hot.
Your boiler works the same........if water is going through the boiler too fast it's not spending long enough over the flame to get hot.
You say the water now comes out of the tap much faster than it used to.......this looks like you problem & answer.
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I understand why the water is warm, as you say it is passing through the boiler too quickly, but why is this, it never used to, I thought the boiler restricted the flow depending on the thermisitor, and temp setting on the front??
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But the water entering the boiler might be colder than it was in the summer months, and now that winter is setting the boiler is having to work harder at heating the water with the same faster flow rate. If you slow down the flow entering the boiler, then you might get hotter water from the outlet pipe.
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It has been like this for about 12 months, but a combi mixer shower is soon to be installed and I think this problem may cause problems??
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