Because of the return monitoring? I have never come across that before, it seems a barmy idea, as you imply due to the mass flow dependency on return/flow temperature rise.
I will ask BG tomorrow if they have checked with Chaffoteaux.
You and me both, Ed. This is my first tangle with them and must say it was good to be forewarned by previous d-i-y tales. I looked at the ventilation issue today, and it doesn't seem too bad. There's a louvred window at least 1ft wide x 2ft high from memory, didn't measure it, practically opposite the boiler in a 4ft wide box room. Obviously doesn't meet the manual spec of some high level and some low level air, but apart from 5 or 6 slats its wide open to the outside (on the 4th floor), so I reckon it was a bit naughty to put the frightners on with a warning notice - which they followed up with a formal "switch it off" letter. Must remember to measure it tomorrow.
and it does have a PCB, part number 2201, GC No. 366475, Manf Part No. 58100. But on the evidence so far no reason to suspect it, is there?
There is a sketch of it in the pdf. Search for "2100 ELECTRICAL BOX". It sports quite a number of discretes, and what looks like a transformer (for the spark?)
Yep, plus unnecessarily swapping out overheat stat and thermocouple
I hadn't mentioned this, but the office manager panicked and called in the original installers at the outset, because their label was still stuck on the boiler. THEY did suck in through their teeth and say "Can't do anything its too old; it needs replacing" and sent an 80 quid invoice. I doubt that will get paid! Its 12 years old actually. Mind you, it might be cost effective to upgrade to a decent condenser, but not right now.
Boiler shutdown during the night so office cold this a.m. Also shutdown couple more times during day. Turned stat to min - this keeps it going most of the time at expense of decent heating, so it seems it is performing to Ed's predictions vis-a-viz TRVs bolloxing up the mass flow rate. Presumably things are ok while the rads are cold and boiler delta is around 10C, but as the TRVs close down the boiler delta goes up relative to the stat controlled return, and blows the overheat stat. Result - boiler runs from cold but shuts down as soon as the place reaches reasonable air temperatures.
What is puzzling me is how it has managed to work satisfactorily for the past 2 years.
Could it be that it has been cycling on the overheat stat, but now a PCB fault has developed that converts an overheat into a shutdown?
Is this related to the 50 seconds delay I am seeing before the pilot goes out after the o/h stat trips? Maybe this used to be several minutes which would allow the boiler to cool and hence relight?
Has anyone ever seen such a control system where the main stat is on the return?
No sign of the BrGas engineer today, and no notification of what they plan to do. I spent abortive 20 mins talking to call centre "very sorry sir, the technical people don't have a phone number, I will pass your message on [that I believe they fitted wrong stat, and please call me]. I will book the engineer to call tomorrow 9-1pm".
If, as I imagine, this boiler simply has the O/H stat in series with the thermocouple then, no.
This would be a serious cause for concern if breaking the thermocouple circuit does not cause the immediate drop of the thermoelectric gas valve and pilot light. Is it possible the main gas was being cut on the normal (return) thermostat and then the O/H cut it out about 50 seconds later due to residual heat?
Um... Maybe we have a pump over run issue here ?
Not on it's own without the flow sensor to work as well. Modern state of the art kit will have return sensors to help decide the pump and gas settings.
Yes that seems to be the configuration, so no problems with the PCB.
The main gas is going out straight away, its just the pilot flame that lingers on.
The BG engineer came back today, to his credit was in conscientious conversation with his technical dept by phone and then Chaffoteaux's tech people when I arrived. He established that the main stat is the correct one - Chaffoteaux had modded it to make it (sort of!!) fit several boilers hence the mechanical differences. It is rated at 45 to
75C so does seem to be designed for return temperature monitoring. He checked out the pumps (there are 2) with a clever attachment to a laptop which had a current probe to clip round the supply lead and displayed and measured the waveform. This confirmed the pumps were running under load at a reasonable speed and hence OK. He also measured flow and return temps and found almost 20C delta. This latter he said is the problem and, I had to concede, indicated a restricted heat exchanger flow. I have to take his word that the pump measurement indicates a satisfactory mass flow, though the piping hot rads do tend to confirm that. His explanation is that it only takes a slight h/e clogging to cause that sort of temperature delta.
So his bottom line is that the h/e needs replacing and is going to send a quote for that - I think I already mentioned it is excluded from the contract. Interestingly all the parts will be itemised and costed so we can check if we can get it cheaper elsewhere - he seemed relaxed about DIYing it or getting someone else in to do it, this would not invalidate the contract so long as it was done properly, i.e. so long as any future faults can not be pinned on a bad h/e replacement. He also volunteered that they used to clean them out with something akin to brick-acid, but health and safety put a stop to that, but was relaxed about me doing it.
So am I convinced? Not entirely. Temperature rise across the h/e is proportional to flow rate for a non-modulating boiler. So if several of the rads were switched off and/or TRVs shut down this could give the same effect. I notice around 1/2 the rads do have TRVs so I think my next step is to open all the valves up and check what the h/e delta then is. If it comes down to around 10C or less that will show the h/e must be ok. But that will leave the question of how has it been working for the last 2 years, and the harder question of what will happen as rad valves, TRVs, are turned back down. I found this in the manual at the bottom of page 4... "Where thermostatic radiator valves are used a system by pass will be necessary to maintain a flow rate in the monotube of not less than 8.1 g.p.m. required to prevent recirculation in the monotube due to the action of the boiler pump."
and this at the bottom of page 5... "The thermostat is adjustable and on its maximum setting gives a nominal
82C (180F) flow +/- 4C (7F) depending on flow through monotube."
There does not seem to be a bypass fitted, unless its elsewhere on the circuit - the boiler is on the top floor and the rads over 4 floors below.
Below is the water circuit. MMMMMMMMM is what Chaffoteaux call a monotube. It is a 54mm dia copper tube. As far as I can tell it is just an open tube.
First the BrGas gripes... after several calls to the call centre we still don't have a quote for replacing the heat exchanger. 1st engineer promised to drop it in on his way back to the office last Friday. on Monday the call centre said they would send another engineer next day to quote, yesterday they said would send one today... still waiting. Meanwhile I have an independent one coming Monday. BrGas don't seem to want the work or/and are just incompetent!!
Now the problem itself. Having copied out the ASCII art above I gave a bit more thought to how this system actually works, and hadn't given much thought before to the fact it has two pumps.
What I now realise is happening is that P2 is the main pump circulating water through the loop consisting of the rads and the monotube MMMM. The boiler pump is drawing some of this water into the heat-exchanger and delivering it heated into the flow loop. I have checked temperatures and with the IR sensor can follow the temp rise along the monotube. it stays at the return temp for most of its 2 ft length, quickly rising to the flow temperature where the boiler flow tees into it. This shows the flow is left to right in the tube confirming the main pump is creating enough flow to carry all the heat into the rads circuit.
Significantly the temperature rise across the monotube is 20 to 21C. Now with the enormous 58mm bore monotube being effectively a bypass (if not a complete short-circuit) across the boiler, there is no way that boiler flow is being restricted by any other than restrictions within the heat-exchanger itself, given that boiler pump has been checked out for correct speed under load by the BrGas guys mains current sensor. So I am now convinced that we have a clogged heat-exchanger, and given BrGas exclude sludge and scale from their 'peace of mind' (duh) contract I'm looking to get an independent quote for changing it.
Given the age of the appliance just cleaning it through might be more economic.
This boiler works differently to any other (non-combi) model I've encountered so far. That probably means it needs outside the envelope of experience for a proportion of the technicians who look at it.
Thats a good point Ed. I noticed it has some kind of top-up or dosing reservoir at the high point of the system. Maybe I can dose it with some fernox type cleaner via that, or failing that I'll look up Andy's patent method of injecting it into a radiator. Luckily the boiler is on the top floor of a 4 floor building so refilling would be easy if the h/exchanger has to be taken out. That is, if there is a drain valve somewhere, haven't noticed one so far, but then I haven't been looking for it.
I'm going down again wednesday so will investigate.
Yes, it certainly threw me off the scent for quite a while, but it seems like quite a sensible scheme, not unlike the way underfloor heating works with two pumps. By pumping the boiler water and the c/h water separately, they have don't need a bypass valve (though I think the manual says there should be one?).
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.