Johnson and Starley J50 heating problem

27 Nov 2021
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United Kingdom

I have a Johnson&Statley J50 MAF heater which has started misbehaving lately.

In short the issue is that when I turn the thermista-stat all the way up the main burner only stays on for just over a minute and goes off for 2 and half minutes and then cycles back ON again and so on… effectively it does produces heat but just not quick enough as it used to..

Here is what I’ve tried/measured and observed:

- I’ve replaced the thermista-stat with a brand new one just in case that is the issue but that didn’t fix the issue. When the knob is turned all the way clockwise I get 4.4volts across the thermista-stat terminals

- When the burner is on for just over a minute (and just before it goes off) I see a temperature rise across the heater of more than 80 degrees Celsius. I think that tye manual says this has to be less than 60 degrees… I guess that this triggers the overheat protection

- The fan is working and it fees like it is blowing sufficiently through the vents however when I measured the voltage at the fan terminals whilst the burner is working I only see up to 156volts (AC). Could someone confirm whether I should be getting 200+ volts there if the system is functioning correctly? I was thinking maybe that’s what is causing the high temperature rise across the heater (just not enough air flow)? Considering that I’ve just replaced the thermista-stat with a brand new one would the electronics module be the one to blame next or maybe the air flow sensor - any way I could check and confirm?

The air filter is clean by the way and all vents are wide open…

I would really appreciate your help.

Many thanks,

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