Baxi Solo control problem

I could do with some help re a Baxi Solo 50pf. Some mornings we are getting up to a cool house :( The boiler has come on the time clock an obviously run for a while but then stopped working: The "boiler on" light is on and the pump running but nothing more. It almost seems as if it is in "cool down" before going off. Various options to get it going again: Turn off the power to the boiler and then back on - reset the overheat and off it goes. Or turn down the room stat till it clicks off and the change over valve operates sometimes restarts it as does turning down the boiler stat potentiometer until off; pause; back on.
Also secondary simptoms: sometimes when cycling correctly, on the boiler stat, the boiler fires up and immediately shuts off, fires up again,off, fires up and runs.
From my own electrical/electronic experience over many years I suspect a pcb problem, a relay or poor contact somewhere but would welcome others thoughts.
Pete
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In an earlier contribution to this discussion,

What vintage of Solo is it? - there's a Solo (effectively Mk1 but not called that) and subsequently a Solo 2 and Solo 3. They're all different in their PCBs etc. - so it's important to identify the correct model.
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I would be looking at the sensors first and pcb last. Most sensors cos
10 or so. If you can get the data from BaxiPotterton you can prov them. They are thermisters which vary in resistance depending on heat Heat yours in a pan with water and a thermometer and prove it follow the chart. I have the resistance data for just a few but not your Solo A lynx for instance ranges from 0 deg c 33.6kohm through 50 deg 3k5 t 99 deg c 716 ohm. Make your own graph. The Myson Midas otoh ranges 64 at 10 deg through 10k at 50 deg to 3k at 80 deg. The Puma ranges fro 7k (max temp) to 15k (minimum temp). So you see thermioster values ar variable across boilers.
You could empirically test a boiler with a variable resistor an thermometer, see at which temperatures which resistance alters th modureg. Then test your Thermister from that data. With a combi mos manufacturers use the same thermister for heating and hot water, so yo just switch leads to prove that fault (don't forget to switch back o you'll have hot water and heating that doesn't modulate). A friend o mine did leave one in a house with the leads switched for a year unti he next serviced it when he found his mistake. Householder hadn' noticed anything wrong.
Always prove all the little things are working right before you star to blame the pcb
-- Paul Barker
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I should add that it is symptomatic of thermister trouble. Another bi
cause of thermister trouble is sludge and scale . When the thermiste is wet you can clean it directly, but some are in a dry pocket o surface mounted to a pipe. In these case the boiler can need descalin before it modulates properly again. Keep your system clean if you wan your boiler to last.
-- Paul Barker
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... ...

A qualified "well said, that man" to that
However, in certain cases, it's MUCH more likely to be the pcb
I include the Solo 2 in this but not the Solo mk 1
I'd also like to point out that I've come across a couple of cases recently where people have bought new temp sensors which were faulty - blaming my repaired pcb for the problem as "it can't be the sensor as I've replaced it"
--
geoff

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raden Wrote:

Good point Geaff, this is why we have to prove thermisters by getin the test data and proving it functions correctly.
I recently changed a Worcester 240 pcb for a customer because the rela was frequently making and breaking. Discussions with Worcester confirme my choice of part. Now they've phoned me to say it's doing it again Perhaps it was the thermister? I've bought one to take round. If I wa wrong about the pcb I'll refund them in full and fit the correct part I've also taken the precaution of purchasing for 2.50 a suitable rela from RS in case it's the relay. I hadn't learned from my own lesson Relay clattering like mad, must be the relay, customer wants it fixe same day, can't get a relay in time round to hrpc for a pcb.
One bug bear, suppliers won't take pcb's back, so now that's anothe pcb in my stock.
Have you any take on this particular problem
-- Paul Barker
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I had already checked that the thermistor resistance alters with temp on this Solo 2. It certainly does that but I don't know what the range should be. It seemed to give a sensible range over a few test with nothing to indicate any reason for this intermitent fault
Pete
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Yes - relays don't normally chatter because they're faulty, they chatter because there's a fault in what's driving them
--
geoff

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writes

Also ... look at my website www.cetltd.com
--
geoff

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Thanks for all the comments, although I'm not sure I'm much further forward. A few points relating to the comments. I started by cleaning out the insides getting rid of the incinerated bugs etc. There no sign of an electronic flame failure device or any sensor listed for the Solo2. I had already tested the thermistor by immersion in hot water and got a range of values that seemed reasonable (but I don't have any data on the correct values). I orginally suspected the thermistor from the fault finding chart. I must say I am still a bit suspicious of this part of the circuit. As far I can tell there is no obvious relay chatter - nothing audible anyway. The ignition fires fine when called on to do so - the problem is that it isn't being called on (i.e. the pilot warning lamp is off) Now the weather has turned colder there is some evidence of the the boiler going out/switching off soon after firing up i.e. very short cycling times. From the outset it has seemed that the problem was usually seen when the boiler had had to work hard - on stating in the morning, etc.
Pete
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It's the spark electrode, the same lead which conducts the spark to the electrode is used for sensing the flame

So, is the pcb generating a spark ?

If there's no spark, given that the temp sensor is OK, then it is either the pcb, the pressure switch or the fan bearings
to test 2, you need to test the resistance of the microswitch on the APS in each position. You need to test between common and normally closed, then, removing one of the tubes on the APS blow or suck GENTLY to change to the normally open contacts (to common) and test again. You should find the connections in your manual
if 1 or 3, look at my site - www.cetltd.com
--
geoff

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Thanks Geoff
I had already checked the ignition lead to make sure it wasn't loose
If the fan runs, then it sparks - as it should. However when its not working, the fan (and indicator light) is off too.
I will test the pressure switch as you suggest. I can sort out the connections
You supplied me with a fan when the bearings did go! (A year or so ago) But anyway the fan is not being switched on.
Pete
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The Solo 2 and Solo 3 pcbs are the same and interchangeable
The Mk 1 pcb however is completely different
--
geoff

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Pete,
What Solo is it? And have you run through the Installation and servicing guide, especially the faults section at the back? If you haven't got a copy download one from baxi.com. One thing to look at if its a Solo 3 are the small flexible sensing tubes to the pressure switch. They have a tendency to collapse and this can cause cycling. They usually flatten near the pressure switch so a quick repair is to cut off the last 5 - 10 mm and refit.
Andy
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Is it a Solo mk 1 or a Solo 2 (or 3)?
They're completely different animals
--
geoff

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Just noticed my post yesterday to say it was a Solo 2 got lost in the either!
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On Sat, 22 Jan 2005 02:43:49 -0800, Peter Dron wrote:

These would typically be the symptoms of a failure of flame failure sensing.
Either the gas is not really being lit and then the PCB doing its stuff just fine, or the gas is lit but the flame sense probe and/or pcb is not seeing the flames.
Check the ignition and flame sense wiring and electrode positions first the pcb last.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
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By which he means (in layman's terms)
Check that the HT lead is intact and has not broken, especially at one of the connectors (the insulation is also crimped, so it's not always obvious that it's broken
Check that the electrode is actually in the pilot flame and has not corroded and burned away
lastly, I would add, ensure that the earthing is good - no resistance (less than an ohm) from the electrode earth to the pcb and the main earth
--
geoff

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On Sun, 23 Jan 2005 00:02:07 +0000, raden wrote:

There is no cremated bumblebee sitting on the flame sense electrode.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
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I knew there was something I'd forgotten
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geoff

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