Johnson and Starley Warm air boiler

Discussion in 'Plumbers' Talk' started by JBN, Oct 11, 2019.

  1. JBN

    JBN New Member

    We have just purchased a house with Warm Air heading using a J&S boiler unit. We seem to have a problem with the fan running all the time when the mains electricity is on. The unit otherwise runs fine, heats the house great, but when you turn the thermostat down, even after cooling the fan doesn’t turn off.

    I have replaced the FCD & Limit switch and the capacitor as recommended by a engineer however this hasn’t seemed to solve the problem. Any thoughts? Cheers JBN
  2. Mike83

    Mike83 Screwfix Select

    Does it have a summer mode?
    If it does the fan will run to provide cool air
    teabreak likes this.
  3. JBN

    JBN New Member

    I can’t find anything on the boiler itself or in the handbook that refers to ‘summer mode’. Perhaps when I get home I could play around with the timer and see if that has any affect. However I would hope the engineer who visited would have thought of these things and the problem is a bit more obscure! Could the wiring between the FCD and limit switch be at fault? The LED screen on the front doesn’t show any fault code when operating. Cheers
  4. Mike83

    Mike83 Screwfix Select

    What model is it?
  5. JBN

    JBN New Member

    Economaire 50/intejan
  6. Mike83

    Mike83 Screwfix Select

    What thermostat is fitted.
    Is it a regular stat or a Johnston and starley thermista-stat.
  7. Mike83

    Mike83 Screwfix Select

    If it’s the thermista-stat then turning it all the way down will activate the summer mode.
    It won’t if the unit has a summer switch on the front though.
  8. JBN

    JBN New Member

    ADF13FD1-62D6-4549-B31F-A9AE7A47B118.jpeg image.jpg
    It’s a Johnson and Starley stat. I’ve just noticed it has a summer airflow setting, I’ve had a little play and nothing seems to be stopping the fan from spinning.
  9. Mike83

    Mike83 Screwfix Select

    With the heater switch at off and the stat set at 5 does it go off.
    It’s not that easy to test if it’s the stat at fault. I’m sure the stat is low voltage
  10. JBN

    JBN New Member

    The fan is still running with the heater off and stat set to 5. Dodgy thermostat?

    Thanks for your help on this
  11. Mike83

    Mike83 Screwfix Select

    That’s what I’m thinking. Not easy to test though as it will be a simple 2 wire install that controls both the heating and cooling aswell as powering the stat.
  12. The Teach

    The Teach Screwfix Select

    Mike83 likes this.
  13. Mike83

    Mike83 Screwfix Select

  14. Mike83

    Mike83 Screwfix Select

    So basically link between 8 and test.

    good little guide that is.
  15. JBN

    JBN New Member

    Thanks for all this info chaps. Looks like I’ll get a new thermostat ordered!

  16. The Teach

    The Teach Screwfix Select

    Not many of them j&s fan flue (newest) heating systems around,often fitted on housing developments or retro fitted inlieu of previous j&s open flue warm air units.

    Its a bit cheeky :D have you got a neighbour who has same system,maybe ask to borrow their Thermista-stat for a very quick test ;)

    although there are some fault finding procedures,some are to be used when in phone contact with the manufacture they will be asking for multi meter readings from different terminals.

    tbh the new j&s wau + circulator are fairly good products but they need specialist repair/service people,getting an expert to maintain them is the idea.
    many companies will send out a gas service technician but they may have never seen your appliance before :( believe me that happens.

    Good luck:)
  17. Dave bevan

    Dave bevan New Member

    JBN’s old man here. The issue is fixed. It was this (failed to be picked up by a professional heating engineer):

    1) FDC thermistor was fine - tested with multimeter and the resistance of it changed then its temp was adjusted by my warm finger touching it (disconnected from the boiler of course!).

    2) Assumed Limit Switch was ok.

    3) Original motor run capacitor (10uF) was replaced using the part number in the manual. However, J&S have subsequently updated their design specs and online service manual for the Econoaire 50, changing the cap value to 4uF. A replacement was sourced online (same model/range) for £4.24.

    4) The specific symptom was “Main airflow fan powers up immediately you supply mains to the system, and runs at max speed.”

    So, a close inspection of the control module was conducted. No blown can capacitors (tell-tail is the silver top bulging up). Removed the control board from the metal housing to inspect the rear. Found a slightly brown (due to excessive localised heat over time) mark by one of the pins of the large transistor (the one that’s wrapped in a grey silicon sleeve and clipped to the chassis as a heatsink). It’s a Power MOSFET chip, used to regulate the 240v supply to the main fan.

    On testing it in place with a multimeter, it showed no resistance between pins 2&3 (240v in and out), meaning that as a switch, it was ON, even though there was nothing ‘activating it’ via pin 1 (the control module).

    The chip is marked STW13NK60Z and I picked on up online for £6.39 inc vat and delivery.

    Replaced the chip today and boiler is now functioning perfectly.

    Hope the above helps anyone else with a similar issue.

    PS. Replacement control modules online were anything between £196.00 and £500+!
    Think I did quite well with my £2.00 + vat & del alternative.

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