Ideal C28 boiler seeing imaginary frost!?

[NotThatOne]

I have an Ideal mini C28 that has been playing up despite three attempts by the installers to fix the problem. The boiler starts, then stops before ignition, then starts again a few seconds later. They've changed membranes, cleaned valves, but it's still happening.

I took a look at the indicator LEDs on the main PCB. According to these, the boiler keeps starting due to the frost thermostat. BUT, there is no frost thermostat fitted! As soon as it stops, it reports 'faulty CH temperature probe'. This pattern repeats intermittently ad infinitum.

My guess is the control PCB is faulty. Anyone else seen this behaviour?
Matt

 

[doitall]

Can't see why they changed anything yet.

Have they gone through the LED fault finding chart and done all the test, including opening the service mode.

Best to find an engineer that knows the boiler

 

[NotThatOne]

I went through the manual myself and found the service mode procedure. It seemed to be the obvious thing to try. Sadly I haven't been present the previous times they've tried to repair it. I will be next time however.

 

[Agile]

I am intreagued that you have not elected to identify who "they" are.

This is a very simple boiler.

It had a frost stat which doubles as the CH NTC sensor. Clearly the fault is the CH NTC but to confirm that you need to be able to measure its resistance. It should be 11 K-13 K cold ( with the plug detached! ).

That is a very simple deduction and I do hope that as I have told you that you will now tell me who "they" are!

Tony Glazier

 

[NotThatOne]

The CH probe measures 12.36K cold; within the range you stated. Cabling looks to be sound and there is no corrosion or tarnishing on the contacts. I did try exercising all the cable looms to see if it was a wiring issue, but that had no effect.
The manual doesn't make mention of the probe's double duty as frost thermostat, although given that the 'other' error indication is that of a faulty probe then it has credence.
I'm not one to normally name names before issues are resolved; however they begin with 'F' and are based in Cumbernauld.

 

[Agile]

Can you obtain easily a 27 k resistor and put it in parallel across the terminals of the sensor and see what happens?

In reality its probably just as easy to get a new sensor and fit it and that will probably fix the fault. Nothing in this world is certain though and it could still be the PCB but thats unlikely!

Why do you expect the manual to tell you how the boiler works? All it gives is basic information to enable it to be fitted and used and for some parts to be changed.

Let us know the result!

Tony

 

[NotThatOne]

I measured the voltage across the CH temp sensor when cold: 4.33V DC. Placing a 27K (actually 26.61K) resistor across it had no effect.

I would expect the manual to mention if a part has two functions. In fact the boiler brochure (not the manual) does mention that frost protection is inbuilt, so I wasn't entirely correct in my original claim.

The installers' manager is due out along with the engineer who has not been fixing the fault; maybe we shall see some progress at last!

 

[Agile]

Its irrelevant measuring the voltage across the NTC.

Do you mean that LEAVING the 27 K resistor across the NTC still caused the boiler to come on on frost protection?

If so then remove the NTC connector and fit a 12 K resistor onto the connector and see the result. Dont use the boiler yourself though as it will not be temperature controlled. If it still comes on for frost protection then I would suspect the PCB is faulty.

I am afraid that you are overestimating the capability of the average person who fits boilers. The manufacturers just give the most basic information in the manuals and certainly do not give a detailed technical description of how the boiler operates.

The manuals are too complicated for most people already. Over half the people fixing boilers do not understand how to measure a resistance correctly or to select the correct scale on their DVM. Start talking about an ADC and they are totally lost! A neon screwdriver is the only electrical test instrument many possess!

Tony Glazier

 

[NotThatOne]

Sorry, I should have been clearer in my earlier posts. The boiler's spurious frost protection operation is independent of any specific CH or DHW calls. Typically it occurs either when the boiler is otherwise idle (when it starts and stops spasmodically), or during normal CH operation (when the boiler often seems to 'stumble' during it's normal on-off CH cycles). It tends to be most prevalent after periods of delivering DHW, although it can occur at any time.

When I attached the 27K resistor across the NTC, nothing happened. By my very rough calculation, that would pull the effective NTC resistance down to about 8K. The boiler did not start (either in frost protect or otherwise) when the resistor was attached. I was expecting it to, given your suggestion.

Am I correct in thinking that a faulty NTC would go short, rather than open? If my assumption is correct, then I wonder if the problem is on the control PCB, given that dropping the effective NTC resistance (with the resistor) is not sufficient to trigger the boiler's frost protection. I wouldn't have thought an intermittently open-circuit NTC probe would cause false frost triggers.

 

[Agile]

I have edited out the original content as you dont seem too happy with it and now Softus is doing his usual quoting of little bits and making rude comments which dont advance the technical assessment of your problem.

Perhaps he will continue the discussion about your C28 with you.

Tony

 

[Softus]

Oh Dear! I am afraid that you have tried to do thinking and got it wrong. I think that I am going to have to explain it as you dont seem to want to blindly follow my tests.

Maybe he thought you were a condescending fool.

Now everyone who reads these forums will think that they are an expert on boilers and their temperature control and FP systems!

Oh really? What would lead you to make that grandiose and sweeping statement?

 

[NotThatOne]

Tony, I'm not sure whether the tone of your response is meant in jest or not, but I don't think it's appropriate. I can see I made a logical error regarding the type of fault that could cause frost operation, but this is a forum for discussion, not condescension.

Your test suggestions make more sense now that you've described the context in which they should be carried out; you didn't do this earlier.

The relative cost of the parts does concern me, regardless of whether I'm paying up front for it or not. If it's just the NTC that's faulty then great; cheaper all round. If it's the PCB, then too bad; whichever bit is faulty will need replaced in any case.

Few people wish to 'blindly follow tests' without an understanding of the test's aims and an understanding of what they are doing. Patronising comments about the assumed abilities of others are not constructive.

So, I will try your suggestions over the next few days, and I'll let you know if anything interesting happens. I might even have some more ideas.

 

[Agile]

If you employed us to fix your boiler then I would carry out whatever tests I deemed were necessary to determine the fault quickly.

I would do these tests based on my knowledge of how your boiler works but this would not be a discussion with you. I would not assume you knew anything about the boiler and I would certainly not expect you to have better diagnostic skills.

After experiencing an installer who does not seem to have much understanding of boilers or fault finding I can see how the popular perception of an "ignorant plumber" comes about. I dont like to be tarred with that brush.

I specialise in repairing boilers and I like to think that I have quite a good understanding of how they operate including all the components.

Tony

 

[Softus]

If you employed us to fix your boiler...

However, you weren't.

...then I would bla bla bla

I would bla bla bla

I would not bla bla bla...and I would certainly not bla bla bla

I dont like to be tarred with that brush.

I specialise in repairing boilers and I like to think that I have quite a good understanding of how they operate including all the components.

Me, me, me, me, me. Is that all you think about?

This topic isn't about you

 

[Gary59]

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