Boiler overheat cut-out: Blockage or boiler fault?

Discussion in 'Plumbers' Talk' started by Martyn-F, Dec 17, 2017.

  1. Martyn-F

    Martyn-F New Member

    Hi All,

    I would be very grateful for advice to help troubleshoot a recurring problem we have with our central heating system. It began last winter and has returned in recent weeks as the temperatures have dropped...

    We have an open-vented CH system and hot water cylinder, on a Honeywell Smartfit Y-plan system (T8677A1008 controller) with an Ideal Classic FF (Fanned Flue) boiler. We have 15 radiators in total with all but 3 having TRVs. The system is nearly 20 years old. The main loops are 22mm pipes and the radiators 10mm I believe.

    The symptoms are this:
    • Heating & hot water run fine much of the time, all radiators get hot and no issues with hot water
    • When the CH has been running (from cold) continuously for 1.5 to 2 hours, and the radiators are really hot to the touch, the boiler often trips out (overheat switch pops out).
    A few minutes prior to this cut-out, there's often a gradual change in tone from the pump as it spins faster, accompanied by the slight 'whooshing' sound of bubbles going through the system (water nearing boiling point). Just prior to cut-off, lots of rumbling, clanking and 'clicking' occurs in the pipes - has to be because the water is now boiling. After the boiler cuts out, it all dies down and goes back to normal in a few seconds. If we switch the boiler back on immediately it reoccurs within a few minutes, else we let it cool for 15+ minutes before switching back on. It might then run for a while before happening again, or may not happen at all for a few days.

    Things investigated so far:
    • Radiators which commonly collect air have been bleed
    • All TRVs turned up to full
    • All non-TRV radiators fully open (usually the one in the utility room is off as the boilers gives off enough heat!)
    • Boiler thermostat set to lowest (more on this later)
    • Header tank checked for water - all OK
    • No bypass gate value to adjust as far as I can tell
    • Pump speed changed between speeds '2' and '3' (usually on '3')
    • Bleed point on short vertical pipe above the three-port value opened - nothing has ever come out, neither air nor water - is this normal?
    My two main thoughts about the fault are:
    1. There's a part-blockage in the system, but it doesn't cause any issues until the rooms start to get warm, the TRVs begin to close, and the return feed to the boiler gets hotter. Correct me if I'm wrong, but the boiler should regulate itself if the return feed is hot enough?
    2. The boiler thermostat has a fault meaning it doesn't regulate and goes full pelt until the water boils, once the return feed is very hot.
    Do they sound like reasonable causes, or could it be something else entirely? (Bearing in mind that while I understand the general principles of the CH system, I have no special plumbing or boiler knowledge!)

    Regarding the boiler's thermostat:
    • Sometimes when turning it up/down from 1-6 it will kick in and cut out as you might expect. At other times it doesn't.
    • Sometimes, when set to '1', the radiators get incredibly hot and the fault occurs (as if the thermostat isn't working)
    • Sometimes when set around 1-2, the radiators never get beyond 'quite warm but touchable'. As the thermostat is slowly increased, the radiators get hotter (but never really hot). Eventually the system seems to 'run away' and the radiators get very hot and the boiler trips out.
    The manual (available here: gives the output temps as 54°C at '1' and 82°C at '6', just for reference.

    The Honeywell controller/thermostat in the hall seems to start/stop the boiler of its own accord as temperatures increase, presumably to 'level out' the rising temperature and avoid overshooting the set temperature (compared to older units where the temperature yo-yos above and below the desired temp). As a side note, the LCD has started to fail so the CH and boiler indicator lights never show, but it functions normally otherwise.

    Final note:
    We're well overdue a boiler service so maybe that's a good place to start? For some reason we never got into the habit of regular boiler services; they're not mandatory like a car's MOT so it tends to be forgotten or put off until later. However, the opinion here is that we have a boiler thermostat fault, we may as well get it replaced in parallel with a service (cheaper than two separate visits).

    Thanks in advance for any advice. Apologies for the post length. I thought the more info the better. If I've missed anything let me know.


  2. Nice comprehensive post, Martyn!

    I don't think you need to look any further than the fact the thermostat control - which should provide a mere 54oC (that's pretty cool) at position '1' - often allows the water to virtually boil.

    Why look further than this?! Your rads all heat up nicely, so there's clearly no blockage. No air comes out of rad bleed screws or the air vent, so no obvious signs of internal corrosion or air being drawn in.

    That air vent - it sounds like an 'auto' type, albeit one that still needs a little thingy opened to release any air. Ie - only air should come out and not any following water (except perhaps a drip). So chances are it's working fine. However, if it's a simple sir 'cock' - a simple 'tap' - then water should also come out. If in doubt, post a photo for us to ID.

    But I'd be looking at a new control thermostat, although your long-overdue service guy should confirm.
  3. Hmm, looks like the control on that model is a potentiometer mounted on the PCB. If so, that's good and bad news.

    Usually PCBs are dead easy to replace. But, they can be costly. If you do need a new PCB, I would recommend a reconditioned unit from a reliable source on eBay, for example. Confirm with them that the therm pot has been renewed as part of the refurb.

    But, see what your GasSafe says.
  4. Martyn-F

    Martyn-F New Member

    Thanks for the quick reply!

    Well, it did seem likely that the boiler thermostat was an issue from its seemingly intermittent behaviour, but always worth second opinions. We had to have the CH power-flushed in our last house after some hard-to-diagnose issues which also sometimes tripped out that boiler, so it's always in the back of my mind. Plus, I seem to recall these issues started roughly around the time I'd finished painting my son's room, and after re-fitting the TRV head (removed to paint the wall) it was stuck shut. A few taps around the valve with a hammer sorted it but it made me wonder if I'd dislodged some rust or gunk.

    OK, will look for a suitable person and organise a service ASAP and go from there. With a view to source the parts myself if they're quoted as being pricey. Sounds like a good plan. I did consider squirting a little WD-40 onto the shaft of the thermostat knob, in case it might work its way in and clean a dirty contact (it seems like it's a potentiometer), but ... in case it was a daft thing to do (electrics & all), I didn't want to risk ending up with no heating for Christmas!

    I really do appreciate the help, thank you.

    By the way, the air vent is indeed the 'auto' type based on a quick search. It looks like this one:
  5. Crowsfoot

    Crowsfoot Screwfix Select

    I agree with Dev's.
    It's most likely a lower limit thermostat that's failed and the boiler is shutting off on the upper limit thermostat which is obviously working.
    Main fault #2 is what I'm basing my reply on.
    It's a common fault on boilers of that age.
  6. The Teach

    The Teach Screwfix Select

    Its knowing which part to source,the ideal classic has 3 parts in the variable heat control chain that could be intermittently faulty (potentiometer,pcb or thermostat sensor) Or it could be a completely different fault activating the overheat.

    Only onsite testing will narrow down the faulty part or other fault,there a very good boiler and with some tlc will last an age.

    If you can post the actual boiler model ie FF340 it narrows down what could be faulty,the classics do vary.

  7. Mike83

    Mike83 Screwfix Select

    Don’t rule out a blockage also.
    If no,air or water from vent pipe this could point to a lack of water in the system.
    Pcb, potentiometer, and thermistor all control the boiler temperature.
  8. I doubt WD40 would get past the pot's shaft (tho' no harm in trying), but if you are 'happy' to gain access to the PCB then - absolutely - try a squirt of proper contact cleaner (cheap stuff) in through a wee hole that almost always exists on pots - I think it's where the case has a piece stamped in to act as a limit stop for the turning shaft. Don't overdo it - a half-second burst will sort it if it's sortable.

    Since it's an electronic thermostat (as opposed to a 'bellows' type) there will also be a thermistor on the outlet pipe or the exchanger to inform the PCB of the temp. Thermistors are generally very reliable, but it's also something that an engineer should test - hopefully they'll know the expected resistance of it at whatever temp it's at. And then it could be a poor contact between the thermistor and the PCB - I'm guessing that's plug-in.
  9. Is the vent shown in #4 the type that allows air out when opened but not water?
  10. Martyn-F

    Martyn-F New Member

    Thanks all for the latest thoughts. I'm amazed at the helpful and insightful people popping up in such a short time. What a great community :)

    Good point - it's an FF240. I recently collected all the info I could and went hunting online for manuals etc. - always good to have in an emergency and useful in case of problems, but I missed that detail.

    I don't mind taking things apart (carefully) provided I've some confidence in what I'm doing. I figured the boiler isn't really something to muck around with, but I do trust myself not to do anything stupid. That said, I don't know the first thing about boilers, besides the obvious. But leaving the gas piping well alone, there's little harm to do by isolating the electrical supply and only undoing the clearly accessible screws which look like they're there for access (in conjunction with the manual). We've not had a service in 4 years actually, since we moved in, so it is probably best letting a professional look first. I'm always happy to learn and help myself though (as you might have noticed).

    About the vent in post 4, the websites I found didn't specify its function. No description to be found anywhere!{47}8"-BSP.html
    And so on...

    Either way, the vent pipe is about 4-5 inches long, directly above a T-joint where the supply from the 3-port value branches off to the hot water cylinder, and it gets very hot, so I assume it's not full of air. The pump runs nice and quietly at all times except when everything goes into 'meltdown' mode.
    KIAB likes this.
  11. Since that vent is referred to as an 'auto' air vent, I presume it allows air out but not water - so no worries that neither air nor water came out.

    Also, if the pipe going to that vent is hot all the way up, that too - as you've sussed - is a good indication that it's full of water, and not air.


    Please keep us posted about how things go - it's always good to hear :)
  12. Martyn-F

    Martyn-F New Member

    Hello All,

    Just a quick update on this thread from last year, where you kindly helped me troubleshoot the issues with the boiler cutting out.

    We had a technician out to diagnose the problem (friend of a friend). I wasn't present, my wife was, but he confirmed that the outlet water pipe was at around 95°C when the boiler tripped out, so it was over the limit it should be allowed to reach and the water was starting to boil. Although he was not overly confident about doing it (with a warning he might have to drain the system) or if it would solve the issue, we had the thermistor changed on a subsequent visit. Problem immediately solved!

    Attached is a photo of the old one. I'm not sure what they are supposed to look like, in terms of the condition of the sensor at the end, but it's clear that the sheath is damaged near the other end exposing the bare wire. Whether this is from removal or was part of the problem I have no idea, but thought I'd post it up in case it's of interest.

    We also had the boiler serviced during the same visit. £180 (ish) later and no more than an hour's labour, we can warm the house without having to 'coax' the heating by nudging the thermostat up a bit at a time. I'm always amazed (read: disappointed) at the cost of plumbers/electricians etc. in this country which is why I try to avoid calling them out wherever possible. :D Don't get me wrong, they are essential jobs and some experience & knowledge is required especially with regard to changing safety guidelines, but it's not rocket science. Anyway, I'm conscious of my surroundings and the patronage here, so I'll stop grumbling ;)

    Attached Files:

    Deleted member 33931 likes this.
  13. Many thanks for the update - it's frustrating to never hear the outcomes from interesting threads :)

    Excellent result, and the cheapest possible cause too :D

    I can't really comment on the cost at £180 - it might appear high for 'just' an hour's work, but I guess plumbers also modify their charges according to what they achieve - eg a full repair. Ie, they've put their training in to fixing the issue, and it's their accumulated experience that allows them to check and replace a part so quickly. Added to that is the fact they are effectively 'guaranteeing' the repair; if it goes faulty again, most customers would expect them to come out FOC and sort it.

    I think, overall, that's a decent result :)

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