Boiler Help

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

Reply to
PhilÅ
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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.

Reply to
BigWallop

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??

Reply to
PhilÅ

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.

Reply to
BigWallop

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

Reply to
Skip

In message , PhilÅ writes

Do you not have an installation manual with a diagram showing location and method of replacement?

Reply to
geoff

In message , PhilÅ writes

I could help, but I ignore top posters

Reply to
geoff

In message , Skip writes

500 ohms cold? I don't think so
Reply to
geoff

Sorry geoff

Didn't mean to offend.

Phil

Reply to
PhilÅ

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

Reply to
PhilÅ

In message , PhilÅ writes

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.

Reply to
geoff

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

Reply to
PhilÅ

I have had a good look at the boiler tonight, there are two thermistors at the top of the boiler, on the pipes by the top of the heater thing by the flames, behind the big cover.

The only other one is attached to the hot water exit pipe at the bottom, this is what I think Boshe are referring to.

I have measured the resistance across this component, out of circuit. When cold attached to the pipe, it reads 14K, when it is hot it reads 4K. Is this normal?? The markings on it are 6655 9003 3-5

Any ideas appreciated Phil

Reply to
PhilÅ

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.

skip

Reply to
Skip

In message , PhilÅ writes

Prolly not because the ucontroller on the pcb monitors the change of 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

Reply to
geoff

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}

Reply to
Gnube

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.

Reply to
Ed Sirett

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??

Reply to
PhilÅ

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.

Reply to
BigWallop

fast........you

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??

Reply to
PhilÅ

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