Megaflo 28 system boiler

[slippyr4]

Of course you need to calculate - precisely because the relationship between resistance and temperature is never linear. That's why I mentioned the beta value.

I'll give you a moment to go and
Google that, since you've no idea what it is.

Back? Ok. So the table you somewhat irrelevantly post is in effect the result of someone having done the calculations for you. I've never seen a boiler manual list that sort of table, nor have I ever seen them quote the NTC part number or it's beta.

So testing is a mugs game, as I said.

Plus the reality that NTC tend to just completely stop working. They don't "come out of calibration" or drift.

And if you think about it, the boiler wouldn't know if it was a little bit out. The fact that the boiler has detected a fault on the
NTC indicates a significant fault, not a little drift of temperature.

 

[bernardgreen]

They don't "come out of calibration" or drift.

How do you know that if you do not check them. ?

The fact that the boiler has detected a fault on the NTC indicates a significant fault, not a little drift of temperature.

How far can the temperature drift before the boiler is no lnger working efficiently. ( Controller has false readings so it assumes it is operating the boiler efficiently )

I'll give you a moment to go and Google that, since you've no idea what it is.

Would you allow me instead to look up my old design notes from the 1980's when I was designing temperature monitoring systems ?. There won't be a great deal about thermistors as it was decided that PT100 sensors would be used as they did not suffer the wide spread of values that thermistors had. Swapping a PT100 did not require adjustment of the current drive in the monitoring circuit to compensate for the wide manufacturing spread of thermistors.

 

[Steelmasons]

Fook sake....by the time the maths are done and/or resistance checked @ board etc etc .. I could of removed harness plug from thermistor and plugged it into a spare thermistor I have as van stock.....

 

[hertsboilers]

Fook sake....by the time the maths are done and/or resistance checked @ board etc etc .. I could of removed harness plug from thermistor and plugged it into a spare thermistor I have as van stock.....

5108265 (white), and 5114725 (red).

 

[Agile]

Add to that the fact that these things don't tend to fail other than catastrophically, and that a new part is only a fiver or so, and you see once again why a sensible, professional plumber would just change it and see if the fault goes away, but a google warier would advocate waste far more money in terms of a plumbers time pratting about with a circuit board.

A plumber might swap it.

But a boiler engineer would measure the resistance and see if it was correct. Usually based on the fact that most boilers NTCs are a nominal 10 K which are about 13 K at room temperature of about 21 C.

The better boiler manuals like Biasi and Ideal have a graph of the NTC resistances.

 

[Dan Robinson]

Christ alive.

You wouldn't have gotten very far with yesterday's Atag then Tony.

 

[DP]

A plumber might swap it.

But a boiler engineer would measure the resistance and see if it was correct. Usually based on the fact that most boilers NTCs are a nominal 10 K which are about 13 K at room temperature of about 21 C.

Attended a WB 350 where the heating was coming on, flow pipe goes tepid, burner shuts down

Used meter to check resistance, measured around 11k similar to a new one

Like a plumber , removed lead off existing thermistor and plugged it into new thermistor. Burner comes on. Tracked resistance readings on faulty thermistor. Good readings.

As boiler now getting hot on new sensor, replaced 'faulty' thermistor that you advocate carrying out dynamic resistance checks. Whilst having a cuppa and discussing weather with customer let the system heat up and finish up

Was a waste of time to get the meter out , should have done what steel said in the post from the start

 

[bernardgreen]

I could of removed harness plug from thermistor and plugged it into a spare thermistor I have as van stock.....

I assume there is no way a faulty PCB could ever push enough current through the spare thermistor to degrade or destroy it's element.

Tracked resistance readings on faulty thermistor. Good readings.

Faulty thermistor, but you got good readings from it. Strange to be faulty and give good readings at the same time.

If the boiler was shutting down with only tepid output it suggests the PCB was reading the thermistor as having a low resistance ( falsely indicating high temperature ).

Was there perhaps a short in the cable loom that was removed when you disturbed the loom. Or maybe the thermistor element was shoirting internally to its case and thus to the pipe work and the electrical 0v of the PCB.

 

[slippyr4]

I assume there is no way a faulty PCB could ever push enough current through the spare thermistor to degrade or destroy it's element.

Element? in a thermistor?

But you assume correctly. If our suspect thermistor think's it hotter than it is, and has a resistance of say about 10k, then it would need 10,000 volts across it to induce a current of 1 amp. Which still isn't that much.

If you consider also that a thermistor is usually put in a voltage divider circuit, and R2 would be of a similar magnitude to the thermistor, then you'd need to double that voltage to 20,000 volts for that 1 amp of current.

Most boilers are supplied from the 230V mains though, rather than high tension pylons.

At 230V, a 10k resistance could have as much as 23 mA through it.


(as an aside, you wouldn't really want to connect the 0 V / DC ground of the PCB to the boiler casing, so the short theory is unlikely to be the cause)

 

[bernardgreen]

Element? in a thermistor?

Not all elements are heating elements.

then it would need 10,000 volts across it to induce a current of 1 amp. Which still isn't that much.

Why would you need 1 amp to destroy the element. ( it would of course destroy it ) A far lower current can destroy a small thermistor element.

If you consider also that a thermistor is usually put in a voltage divider circuit, and R2 would be of a similar magnitude to the thermistor,

With highly integrated analogue circuity available the voltage divider method is being overtaken by other methods but voltage dividers are accurate enough for domestic heating boilers

0 V / DC ground of the PCB to the boiler casing

Doesn't have to be a zero Ohm connection, protection of the circuitry from the EHT spark ignition requires some connection between electronic 0v and the metalwork of the boiler to dissipate stray energy from the ignition. Therefor a short between thermistor lead asn thermistor case will affect the temperature sensing.

 

[DP]

Faulty thermistor, but you got good readings from it. Strange to be faulty and give good readings at the same time.

Reported as seen, not googled. System works. Could have looked deeper, but it was not necessary when thermistor does not require kings ransom to purchase. Have been times when one does delve deeper to investigate cause of malfunction as it is useful in future

 

[Steelmasons]

Hey DP , the guys that actually do this job for a living sure stand out from the Google wannabes.

 

[DP]

Hey DP , the guys that actually do this job for a living sure stand out from the Google wannabes.

I await Tony's dynamic temperature response during heating cycle of uv cylinder but I think I am going to be disappointed due to lack of replies he berates others of

 

[slippyr4]

Not all elements are heating elements.

Of course not. But thermistors are not, and do not have elements.

Why would you need 1 amp to destroy the element. ( it would of course destroy it ) A far lower current can destroy a small thermistor element.

Merely a worked example to show what an idiotic statement you made about current frying it.

With highly integrated analogue circuity available the voltage divider method is being overtaken by other methods but voltage dividers are accurate enough for domestic heating boilers

What other methods are overtaking the voltage divider? you talk absolute rubbish. The de facto standard way of using a thermistor is in a voltage divider, from a reference voltage to ground, with the centre tap being fed into an ADC input on a microcontroller. Without a voltage divider you can only sense current, which is impracticable to do.

Doesn't have to be a zero Ohm connection, protection of the circuitry from the EHT spark ignition requires some connection between electronic 0v and the metalwork of the boiler to dissipate stray energy from the ignition.

no it doesn't require that connection.one end of the secondary of the ignition coil needs to be grounded but the primary usually would not be.

 

[bernardgreen]

What other methods are overtaking the voltage divider? you talk absolute rubbish.

Constant current fed into the thermistor, voltage developed varies inversely with resistance.

Hobbyists do it http://www.bristolwatch.com/ccs/LM334_sensors.htm ( found by Google for you )

From http://www.ni.com/white-paper/7112/en/ this extract mentions sourcing accurate current and refers to the sensing element.

Considerations

Because the nominal resistance of a thermistor is very high, you need a source that can output low currents accurately.

If you cannot dissipate extra heat, heating caused by the excitation current can raise the temperature of the sensing element above that of the ambient temperature, causing an error in the reading of the ambient temperature. You can minimize the effects of self-heating by lowering the excitation current.

Signals emitted by thermistors are typically in the millivolt range, making them susceptible to noise. Lowpass filters are commonly used in thermistor data acquisition systems to effectively eliminate high-frequency noise in thermistor measurements. For instance, lowpass filters are useful for removing the 60 Hz power line noise that is prevalent in most laboratory and plant settings.

you talk absolute rubbish.

To you maybe it does seem like rubbish, but then do you have skills and experience in electronics design ?