Thorn Apollo boiler problem , drivin me mad !!

Discussion in 'Plumbers' Talk' started by wontbebeaten, Nov 9, 2009.

  1. wontbebeaten

    wontbebeaten New Member

    Desperate for help on this one please ??!!

    I have a Thorn Apollo boiler , model 30/50 , propane version.
    This thing has been a pile of junk ever since new when I moved into the property.
    Problem is the pilot light won't stay on. I've replaced endless thermo couples and they tend to work for a while then fail. This season I relit the boiler and couldn't believe it actually worked for a few weeks without any problem !! Then it started again ! Replaced the TC , ok for a few days then off again. Went inside again trying to find anything obvious without result. Did clean heat exchanger as that has worked in the past , removed and replaced the connectors on the hi and lo stats etc.
    Then it did work for about 2 weeks , then this morning off again !!
    Checked high temp cut out stat , ok.
    The pump overun stat is working ok.
    Switching hi or lo boiler temp makes no difference. Pump is fine.
    The problem this time is , I can relight the pilot , it will stay lit. Then after the boiler goes through the first working cycle after relighting pilot , boiler turns off , then the gas valve closes about 30 seconds later even though the pilot is still on !!

    Now , I have read a post on here about the thermo couple voltages , didn't quite understand where to measure these , but have measured at a terminal where the TC connection enters the gas valve body , it's about 16mv there , then another terminal on the gas valve body just where the push button is , about 2mv there.

    Can anyone help and stop me going insane ???!!!

    Reason I can't get an engineer in is that I just cannot afford it !!
     
  2. palavaman

    palavaman Well-Known Member

    glad to know its UR hse.
    U realy shouldn't interfere with gas???
     
  3. wontbebeaten

    wontbebeaten New Member

    It's no problem , I have experience with industrial pipe work and connections with all sorts of gases at much higher pressures than domestic gas installations.
    Also , this is a private lpg supply anyway.
     
  4. picasso.

    picasso. New Member

    the thermocouples need to be nickel plated to cope with the extra heat, as already said you shouldent be messing with gas especially LPG.
     
  5. tomplum

    tomplum Active Member

    these thermacouples have an interupter, overheat cut out,that is reducing the voltage to the gas valve, replace that, they're about £30, DONT bypass it or you will need a new boiler :)
     
  6. wontbebeaten

    wontbebeaten New Member

    The correct high temperature TC is fitted , I realised this from the parts list on the boiler outer cover.

    I'll replace the high temp cutout even though it measures ok , it's not the interrupter type though , the TC goes straight to the gas valve , the cut out goes across the 2 terminals on the gas valve.

    Could someone please explain how the gas valve operates with only 25 mv or so from the TC ?
     
  7. tomplum

    tomplum Active Member

    the interupter is the overheat, the pilot creates a small voltage, 18 millie volts, this voltage is carried along the t/c to the terminal, this terminal is just a connecting point from the t/c to the overheat. then up through the overheat and back into the bisness end of the gasvalve, where the voltage should be no less than 10 m.v, this holds an electro magnet that keeps the gas valve active, if the voltage drops the valve shuts down, it can be possible for the magnet to go wrong, the only cure for that is a new gas valve,
     
  8. wontbebeaten

    wontbebeaten New Member

    Ah ha !! I think this is where the problem is !! The voltage on the gas valve terminal is only just above 4 mV. This is after bypassing the overheat cutout. I strongly suspect the wires to and from the overheat cutout have gone high resistance as they were very stiff , especially at the cutout ends. The wiring is the only thing that can be reducing the voltage from the TC to the gas valve. Can it really be after all this hassle that I have finally sorted this out ??!! I'll order an overheat cut out tomorrow and replace that and the wiring , I'll use high temperature aircraft wiring !!
     
  9. wontbebeaten

    wontbebeaten New Member

    Ok , need a source to obtain the parts I need , I'm getting some very silly prices by just googling !!

    Looked on the interparts database and have determined I need the following :-

    Lo Stat , GC# 382373

    Overheat cutout , GC# 377271

    Thermo couple , only have the original p/no 402S1747

    Can anyone recommend a company that doesn't want a second mortgage for these bits please ??!!
     
  10. tomplum

    tomplum Active Member

    false econemy to go buying parts before you know defo what to buy, you could buy all them bits then it turns out to be the gas valve at about £130,

    get a pro in its gonna cost either way.
     
  11. wontbebeaten

    wontbebeaten New Member

    Pretty certain it's the overheat cutout and or wires to it. I removed the overheat cutout , stripped the wires and joined them and the boiler has worked ok since last night.
    Replacing the TC and low stat as a precaution.
     
  12. นิà¸

    นิภNew Member

    can i ask if the pump overun is wired correctly and that the pump does continue to run after a demand. had a few of these in the past that were not wired in correctly and the latent heat will cause the overheat to trip.
     
  13. Dick Puller

    Dick Puller New Member

    Have you tried Curzons for your spares 'clearlybeaten'.
    That's our buyers first call for any Myson parts.
     
  14. wontbebeaten

    wontbebeaten New Member

    I replaced the pump overun stat last year as I found it had fallen apart ! Works fine now.
    I'm 99% sure it's sorted now , it's run fine since I bypassed the overheat trip. The trip itself measured ok until I looked at the actual resistance and it was about 3 ohms !! That combined with the cables going high resistance was enough to drop the TC voltage below an adequate level to keep the gas valve activated. Until I read another thread somewhere on here I didn't know that the TC voltage was so low and therefore so dependant on all the parts in such a deceptively simple circuit being in perfect condition.
    I've ordered the parts from a local supplier , but the overheat stat was almost £50 !!! For something that looks identical to the hi and lo stats which cost about £4 thats outrageous !!
     
  15. silversurfer$$

    silversurfer$$ New Member

  16. wontbebeaten

    wontbebeaten New Member

    Thats not the same part number as the one I checked on interparts database.
    Checked a few places on the web and they all seemed to be high for this part. It's 402A2608 , GC 377271
     
  17. SANDY

    SANDY Guest

    Hi
    I've also got an Apollo 30/50 boiler. I designed and fitted  my 3 zone central heating system in 1983 and have to say that it has been great and still going strong. The only problem I have is that I need a new spark electrode. I ordered one from interpart  (Part No 308S116) which did not fit. I don't have the manual for my boiler any more. CAN ANYONE HELP ME OUT??
     
  18. tom.plum

    tom.plum Screwfix Select

    hi sandy,  its part number 402S189, but no longer available from the stockists i deal with, good luck finding one
     
  19. HOTDOG ø

    HOTDOG ø Active Member

    Get on the blower and make some calls, it is obsolete but some merchant somewhere probably has one gathering dust.
     
  20. Weedrip

    Weedrip New Member

    Have you tried the obvious and cleaned the pilot injector and shroud. Then correctly adjusted the pilot flame to evelope the T/c tip correctly. This is, in my humble experiance the main reason for low voltages on the T/c -  these are especialy vulnerable due the interupt. [lots of resistance]
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice